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Variation in Energy Reserves and Role of Body Size in the Mating System of Anopheles gambiae
Abstract
Anopheles gambiae mates in flight. Males gather at stationary places at sunset and compete for incoming females. Factors that account for male mating success are not known but are critical for the future of any genetic control strategy. The current study explored variations in nutritional reserves (sugars, glycogen, lipids, and proteins) in wild-caught swarming and resting males and evaluated the effect of body size and wing symmetry on male mating success. Our results showed that glycogen and sugar reserves are mobilized for flight. Males consume proportionally 5.9-fold as much energy derived from sugars in swarming activities than when they are at rest. Mated males were on average bigger than unmated ones (P<0.0001). A strong correlation between the left and right wings in both mated and unmated males was found and additional analysis on fluctuating asymmetry did not show any indication of mated males being more symmetrical than unmated ones. The distribution of wing size of mated males was focused around a central value, suggesting that intermediate size of males is advantageous in the An. gambiae mating system. The results are discussed in the context of sexual selection.
... Larvae were also reared under standard conditions in the insectary avoid differences due to biotic and abiotic fluctuations. Both left and right wing lengths were measured as described previously [20]. In short, both wings were removed, dry-mounted on microscope slides, and photographed with a Leica EZ4 D dissection microscope (Leica Microsystems, Suisse). ...
... Length was then measured using the software Image J1.41.0 (Wayne Rasband, National Institutes of Health, U.S.A.) from the annular notch to the end of the radius vein, excluding fringe scales. This length raised to the cube (WL 3 ) was considered an index of mosquito size [20]. ...
... Flight and hearing capabilities, pheromone or sperm production, may also be responsible for the drop in mating efficiency and for lower insemination rates [24,25]. Body size reduction is another trait associated with mosquito mating capacity and competitiveness success [20]. Our results showed that uninfected An. coluzzii offspring were significantly larger than infected ones. ...
Background
Vector control tools are urgently needed to control malaria transmission in Africa. A native strain of Chromobacterium sp. from Burkina Faso was recently isolated and preliminarily named Chromobacterium anophelis sp. nov. IRSSSOUMB001. In bioassays, this bacterium showed a promising virulence against adult mosquitoes and reduces their blood feeding propensity and fecundity. The current study assessed the entomopathogenic effects of C. anophelis IRSSSOUMB001 on larval stages of mosquitoes, as well as its impacts on infected mosquitoes reproductive capacity and trans-generational effects.
Methods
Virulence on larvae and interference with insemination were assayed by co-incubation with C. anophelis IRSSSOUMB001 at a range of 10⁴ to 10⁸ cfu/ml. Trans-generational effects were determined by measuring body size differences of progeny from infected vs. uninfected parent mosquitoes using wing size as a proxy.
Results
Chromobacterium anophelis IRSSSOUMB001 killed larvae of the pyrethroid-resistant Anopheles coluzzii with LT80 of ~ 1.75 ± 0.14 days at 10⁸ cfu/ml in larval breeding trays. Reproductive success was reduced as a measure of insemination rate from 95 ± 1.99% to 21 ± 3.76% for the infected females. There was a difference in wing sizes between control and infected mosquito offsprings from 2.55 ± 0.17 mm to 2.1 ± 0.21 mm in infected females, and from 2.43 ± 0.13 mm to 1.99 ± 0.15 mm in infected males.
Conclusions
This study showed that C. anophelis IRSSSOUMB001 was highly virulent against larvae of insecticide-resistant Anopheles coluzzii, and reduced both mosquito reproduction capacity and offspring fitness. Additional laboratory, field, safety and social acceptance studies are needed to draw firm conclusions about the practical utility of this bacterial strain for malaria vector control.
... Factors aff ecting mating in An. gambiae s.l. include maturity or age, body size, fi tness, food type and food fi nding ability [10,11]. After successful mating, reproduction is guaranteed by three successive biological phases or Beklemischev's phases [12,13]. ...
... gambiae complex is creating controversial conclusion among many researchers whether it has infl uence on mating or not. According to Huho, et al. [42] and Maiga, et al. [11], larger males of An. gambiae s.s are more successful in mating than the smaller one. Indeed, basic mating systems characterized by male territoriality in swarm cause males fi ghting over territory to be in copula with the incoming females [27]. ...
... Moreover, maturation of accessory glands needs another 1-2 days [11]. ...
Anopheles gambiae complex mosquitoes comprise different morphologically identical species and the most medically important malaria vectors in Africa. Understanding An. gambiae complex mosquitoes’ mating behaviour is a clue for prevention and control of malaria. The factors affecting the mating behavior of An. gambiae complex are age, body size, food finding, type of food, host availability and environmental temperature. An. gambiae complex passes through different and unique mating strategies. The fundamental mechanisms underlying mating behavior of An. gambiae complex are male aggregation before copulation which is called swarming, finding female and insemination. Male in the swarm detects the wing beat of female with which he mates by auditory system. The wing beat frequencies of opposite sex of the same genetic form simply harmonize indicating possible hybridization while that of different genetic forms do not harmonize. The successive mating behavior of female An. gambiae mosquitoes includes, locating a male among the swarm for cupola, friend selection and storage of sperm. Successful female An. gambiae complex mosquitoes mating is accomplished by host seeking, blood feeding and digestion, egg development and oviposition. The sequential process of host finding and blood feeding followed by blood digestion and simultaneously egg maturation and accomplished by searching of oviposition site and oviposition is referred to as gonotrophic cycle. The time period between two successive blood feedings or two successive ovipositions is said to be gonotrophic period. The length of gonotrophic period depends on temperature, number of previous gonotrophic cycle, host and breeding site availability. In addition to population net reproduction of the colony, gonotrophic cycle is also used to estimate female mosquitoes’ age by determining the parity. Finally, understandings of mating behavior and gonotrophic cycle of An. gambiae complex mosquitoes help in malaria vector control strategies and their vector competence.
... Females visit these aggregations, find a male and the mating pairs leave the swarm in copula [19][20][21]. Swarming activity lasts about 22 min in the field [22] and consumes about 50% of male's sugar and glycogen reserves [23,24]. Thus, the demand on energy reserves for swarming flight is very high [25,26], and a crucial factor in ensuring reproductive success and survival. ...
... Although the criteria for mate selection within the "lek-like" system in which Anopheles mosquitoes mate are not yet well known [39], it is likely that morphological and physiological criteria are considered for the mate choice. It has been reported that in the swarms of An. gambiae s.l., mated males were on average bigger than un-mated ones [23,26], suggesting that the body size could be an important criterion for choosing a mate in An. gambiae s.l. species. ...
... Glycogen and sugar reserves constitute the main energy fuel for mosquito flight [24]. Swarming activity has an important energy cost and consume about 50% of available energy [23,26,28,41,42]. Males that engage in swarming activities to find mates are assumed to have the required energy for swarming flight and mating. ...
Abstract
Background: It is assumed that malaria vectors feed on locally available nectar sources to obtain energy. Sugar feeding
is energetically critical for the Anopheles male swarming and mating activities. However, little is known about the
impact of local nectar feeding on male physiological development and its consequences on male mosquito life traits
in the malaria control context. This study aimed to evaluate the influence of local fruit juices on the life traits of males
Anopheles coluzzii.
Methods: Swarming characteristics (number of males in swarm, number of mating pairs, and swarm duration) in
semi-field conditions; mating rate and longevity in a laboratory setting were compared between males An. coluzzii fed
exclusively with mango, papaya or banana juices. The trophic preference was investigated in semi-field conditions.
Results: The results of this study showed that in the laboratory, mosquitoes fed with papaya juices lived on average
longer (10 days) than those fed with banana or mango juices (5 days) and had higher a mating rate (53%) than those
fed with banana juice (40%). In the semi-field, the swarm size of mosquitoes fed with banana juice (85 males) was
larger than that of mosquitoes fed with mango juice (60 males). The number of mating pairs formed from banana-fed
male swarms (17 mating pairs) was higher than that formed from mango-fed male swarm (8 mating pairs). There was
no difference in swarming duration between male treatments. Male mosquitoes had a preference for papaya and
banana juices.
Conclusions: The results indicate that the origin of plant-derived feeding is an important factor in the survival and
reproduction of mosquitoes. This calls for further investigations of chemical contents of nectars and their impact on
the physiological development of mosquitoes.
Keywords: Malaria, Mosquito release, Sugar feeding, Trophic preference, Vector control
... Regardless of the cryptic taxa they belong to, there is now strong evidence that females do not simply prefer to mate with larger males. In field studies of natural swarms conducted over several years and different locations in Burkina Faso, males caught in copula were repeatedly found to have significantly larger mean body size than that of other swarming males [16,65] but with a size distribution centred around an intermediate-to-large body size [16,65] (Fig. 5a). Laboratory experiments that experimentally manipulated male size have reported similar results, with mated males typically being larger than unmated ones and intermediate size males securing the majority of copula [63]. ...
... Regardless of the cryptic taxa they belong to, there is now strong evidence that females do not simply prefer to mate with larger males. In field studies of natural swarms conducted over several years and different locations in Burkina Faso, males caught in copula were repeatedly found to have significantly larger mean body size than that of other swarming males [16,65] but with a size distribution centred around an intermediate-to-large body size [16,65] (Fig. 5a). Laboratory experiments that experimentally manipulated male size have reported similar results, with mated males typically being larger than unmated ones and intermediate size males securing the majority of copula [63]. ...
... Importantly, field studies have found that the body sizes of male and female An. gambiae captured in copula are often correlated [65]. The correlation is weak to modest and has also been observed under laboratory conditions (Ekechukwu and Tripet, unpublished) (Fig. 6a,b). ...
Malaria vector control relies heavily on the use of Long-Lasting Insecticidal Nets (LLINs) and Indoor Residual Spraying (IRS). These, together with the combined drug administration efforts to control malaria, have reduced the death toll to less than 700,000 deaths/year. This progress has engendered real excitement but the emergence and spread of insecticide resistance is challenging our ability to sustain and consolidate the substantial gains that have been made. Research is required to discover novel vector control tools that can supplement and improve the effectiveness of those currently available. Here, we argue that recent and continuing progress in our understanding of male mating biology is instrumental in the implementation of new approaches based on the release of either conventional sterile or genetically engineered males. Importantly, further knowledge of male biology could also lead to the development of new interventions, such as sound traps and male mass killing in swarms, and contribute to new population sampling tools. We review and discuss recent advances in the behavioural ecology of male mating with an emphasis on the potential applications that can be derived from such knowledge. We also highlight those aspects of male mating ecology that urgently require additional study in the future.
... As in many lek-based mating, a highly skewed rate of male mating success is observed (Mackenzie et al., 1995). Possible factors accounting for this mating bias are male body size (Yuval et al., 1993;Voordouw and Koella, 2007;Huho et al., 2007;Maïga et al., 2012), age (Chambers and Klowden, 2001;Huho et al., 2006Huho et al., , 2007Sawadogo et al., 2013), genetics (Voordouw and Koella, 2007), sperm length (Voordouw and Koella, 2007) and energetic reserves (Yuval et al., 1994;Huho et al., 2007;Gilles et al., 2011;Damiens et al., 2012). Nutritional reserves accumulated during larval development and from sugar-feeding as adults are critical determinants of adult survival and mating success (Foster, 1995); and flight is a requirement for mosquito mating and this activity is fueled by sugars and glycogen (Yuval et al., 1994;Bargielowski et al., 2012). ...
... Hence, a comprehensive study of energy level variation in mosquitoes at different physiological states and collected at different times of day (early morning (between 06:00 and 07:00 am), late afternoon (between 05:00 and 06:00 pm) and after sunset (around 06:30 pm)) would enable an estimation of the energy allocated to essential activities such as swarming and mating. Our recent study demonstrated variation in energy reserves between indoor resting males of the M form of An. gambiae and those caught in swarms (Maïga et al., 2012). Here we extend this study to both M and S forms of An. gambiae. ...
... Sugar and glycogen concentrations were determined from a standard curve based on glucose (Sigma-Aldrich). Lipid and protein content were not assessed, as they are not used by male mosquitoes to fuel flight (Yuval et al., 1994;Maïga et al., 2012). ...
A better knowledge of the different parameters that account for male mating success in the wild is critical to the development of genetic control strategies. In this study, we measured energy budgets (total sugar and glycogen) as the daily energetic investment in swarming males of An. gambiae s.s. M and S molecular forms from two different field locations, VK7 and Soumousso. We also looked at the difference between energetic reserves in mated males compared to unmated ones, and assessed wing length in both molecular forms to explore whether this phenotypic trait was involved in swarming behavior or mating success. The current study showed that the energetic cost of 25minutes of swarming was around 50% of the male's sugar (M form: 48.5%, S form: 56.2%) and glycogen (M form: 53.1%, S form: 59%) reserves. However, no difference in carbohydrate content was observed between mated and unmated males. Mated males were found to be bigger than unmated ones, while intermediate size of males is advantageous in mating system, both in M and S molecular forms and when collected in two different locations. Regardless of the collection location, no difference in wing size was observed in swarming males collected early or late during a particular swarm. The results are discussed in the context sexual selection in different ecological locations.
... While males can theoretically increase their access to females by maximizing their time in mating swarms, swarming is a costly activity [10]. For example, Maïga et al. [34] estimated that 25 min of swarming consumed 50% of a male Anopheles gambiae's total sugar and glycogen reserves. Observations of Anopheles swarms suggest that the timing of swarming, and not just duration, may also be important for male mating success, with the majority of matings occurring during peak swarming activity [8,35,36]. ...
... While larger, potentially more fecund, females are more likely to be mated [37], male choice is an unlikely explanation for these observations. Time spent searching within the swarm for a preferred female has costs [8,34,38]. Further, with intense scramble competition, males which pass up an encountered female risk failing to mate at all. An alternative explanation for the observed pattern of larger females being mated is that these females are easier to detect, intercept, or inseminate than smaller females [39]. ...
The field of mosquito mating biology has experienced a considerable expansion in the past decade. Recent work has generated many key insights about specific aspects of mating behavior and physiology. Here, we synthesize these findings and classify swarming mosquito systems as polygynous. Male mating success is highly variable in swarms and evidence suggests that it is likely determined by both scramble competition between males and female choice. Incorporating this new understanding will improve both implementation and long-term stability of reproductive control tools.
... In mosquitoes, resource acquisition in the larval stages determines the energy reserves in adults. Energy reserves can be regarded as a life history trait owing to their correspondence with the longevity and fecundity of an individual mosquito (MOSTOWY & FOSTER 2004, BARGIELOWSKI et al. 2012, MAÏGA et al. 2012, KAUFMANN et al. 2013. Body sizes (pupal weight, adult weight, wing length etc.) of mosquitoes are also dependent on larval feeding, so the food resources of larval habitats play a significant role in the mosquito life cycle (ARRIVILLAGA & BARRERA 2004, MOGI 2010 and mosquito abundance (BANERJEE et al. 2015). ...
... In different species of mosquitoes the correspondence among pupal weight, wing length, longevity and fecundity justifies the use of one or many life history traits as markers of fitness at the individual or population level (BLACKMORE & LORD 2000, PADMANABHA et al. 2011a, 2011b, MUTURI et al. 2012. The amount of energy reserves in an individual mosquito can also be included as an indicator of fitness, owing to the correspondence with multiple life history traits (MOSTOWY & FOSTER 2004, BARGIELOWSKI et al. 2012, MAÏGA et al. 2012, KAUFMANN et al. 2013. The link between life history traits and energy reserves form the basis for using them as an indicator of reproductive success and fitness at individual and population levels. ...
Energy reserves in mosquitoes are an indicator of fitness, linking larval effort in resource acquisition with adult survival and fecundity. In other words, life history strategies and disease transmission potential can be related to the amount of energy reserves. The energy reserves of four mosquitoes – Aedes aegypti, Aedes albopictus, Armigeres subalbatus and Culex quinquefasciatus (Diptera: Culicidae) – were calculated to justify species-specific differences in their life history strategies. Following repeated sampling of pupae from the respective larval habitats, the glycogen, sugar and lipid contents of individual mosquitoes were assessed and corroborated with pupal weight and adult wing length. Discriminant function analysis was used to acquire an initial reflection of the differences of the parameters among the sex and species of the mosquitoes considered in the study. Using logistic regression and ANOVA, the effects of species and sex as contributors to variations in energy reserves could be established. The results indicated that for all the mosquitoes, sex-specific differences were prominent with reference to the energy reserves. Species-specific differences in energy reserves reflect differences in resource acquisition and assimilation in the tissues, and thus the differences in the life history strategies of these four species.
... Larger males have been demonstrated to have greater capacity to produce and transfer sperm to females [33,34]. However, in mating competition experiments both large [14,[35][36][37] and intermediate [38] sized males have been found to be more successful. One published study reported no relationship between body size and male mating success [39]. ...
... Trends in the data show that large males attempting to mate with large females had higher mating success, but mating success of these males was lower when attempting to mate with small females (Fig. 3b). This contrasts with the majority of studies in Anopheles mosquitoes which find that larger males are more likely to form a copula in field swarms [35][36][37]. Consequently, because females from target field populations will contain a mix of body sizes, release males may perform more effectively if reared to have a range of body sizes reflecting this distribution. ...
Background
Several new mosquito control strategies will involve the release of laboratory reared males which will be required to compete with wild males for mates. Currently, the determinants of male mating success remain unclear. The presence of convergence between male and female harmonic flight tone frequencies during a mating attempt have been found to increase male mating success in the yellow fever mosquito, Aedes aegypti. Size has also been implicated as a factor in male mating success. Here, we investigated the relationships among body size, harmonic convergence signalling, and mating success. We predicted that harmonic convergence would be an important determinant of mating success and that large individuals would be more likely to converge. Methods
We used diet to manipulate male and female body size and then measured acoustic interactions during mating attempts between pairs of different body sizes. Additionally, we used playback experiments to measure the direct effect of size on signalling performance. ResultsIn live pair interactions, harmonic convergence was found to be a significant predictor of copula formation. However, we also found interactions between harmonic convergence behaviour and body size. The probability that a given male successfully formed a copula was a consequence of his size, the size of the female encountered, and whether or not they converged. While convergence appears to be predictive of mating success regardless of size, the positive effect of convergence was modulated by size combinations. In playbacks, adult body size did not affect the probability of harmonic convergence responses. Conclusions
Both body size and harmonic convergence signalling were found to be determinants of male mating success. Our results suggest that in addition to measuring convergence ability of mass release lines that the size distribution of released males may need to be adjusted to complement the size distribution of females. We also found that diet amount alone cannot be used to increase male mating success or convergence probability. A clearer understanding of convergence behaviours, their relationship to mating success, and factors influencing convergence ability would provide the groundwork for improving the mating performance of laboratory reared lines.
... Another question facing sterile male programmes is what size male to release. Studies on mating success of different sizes of males of An. gambiae suggest that medium-sized males may have an advantage in mating swarms, perhaps due to a combination of energetic reserves and in-flight manoeuvrability [26,65], although not all studies have confirmed this [27]. To optimize production of sterile males, it may be most efficacious to homogenize the development time and size of the emerging adults [66]. ...
... An outcome of this study is that if size-assortative mating already occurs in a mosquito species, as has been shown recently for An. gambiae [65], or is something that could easily evolve in the face of prolonged sterile male releases, it may be beneficial to release a mixture of large and small males instead, to avoid potential increases in the mosquito population's vectorial capacity resulting from the reduction in larval density-dependent mortality and development. That this unintended increase as a result of sterile male releases was observed only at high levels of size-assortative mating suggests that other mosquito species, sharing the same larval habitat, could potentially be affected in an even stronger manner. ...
The release of genetically-modified or sterile male mosquitoes offers a promising form of mosquito-transmitted pathogen control, but the insights derived from our understanding of male mosquito behaviour have not fully been incorporated into the design of such genetic control or sterile-male release methods. The importance of aspects of male life history and mating behaviour for sterile-male release programmes were investigated by projecting a stage-structured matrix model over time. An elasticity analysis of transient dynamics during sterile-male releases was performed to provide insight on which vector control methods are likely to be most synergistic. The results suggest that high mating competitiveness and mortality costs of released males are required before the sterile-release method becomes ineffective. Additionally, if released males suffer a mortality cost, older males should be released due to their increased mating capacity. If released males are of a homogenous size and size-assortative mating occurs in nature, this can lead to an increase in the abundance of large females and reduce the efficacy of the population-suppression effort. At a high level of size-assortative mating, the disease transmission potential of the vector population increases due to male releases, arguing for the release of a heterogeneously-sized male population. The female population was most sensitive to perturbations of density-dependent components of larval mortality and female survivorship and fecundity. These findings suggest source reduction might be a particularly effective complement to mosquito control based on the sterile insect technique (SIT). In order for SIT to realize its potential as a key component of an integrated vector-management strategy to control mosquito-transmitted pathogens, programme design of sterile-male release programmes must account for the ecology, behaviour and life history of mosquitoes. The model used here takes a step in this direction and can easily be modified to investigate additional aspects of mosquito behaviour or species-specific ecology.
... In addition to inbreeding, assortative mating is another important factor to consider in terms of its effects on gene drive spread. Assortative mating has been observed in many target species; for example, Anopheles gambiae displays assortative mating with regard to body size (Callahan et al., 2018;Diabate & Tripet, 2015;Maïga et al., 2012), and wild rodents exhibit preferential mating behaviour based on olfactory preference (Lenington, 1991;Manser et al., 2015). Under standard Mendelian inheritance, assortative mating alone does not alter allele frequencies over time (Fisher, 1919;Jennings, 1916;Wright, 1921), but only increases the population variance of the property on which the assortment is based (Crow & Felsenstein, 1968;Felsenstein, 1981). ...
Gene drive technology, in which fast‐spreading engineered drive alleles are introduced into wild populations, represents a promising new tool in the fight against vector‐borne diseases, agricultural pests and invasive species. Due to the risks involved, gene drives have so far only been tested in laboratory settings while their population‐level behaviour is mainly studied using mathematical and computational models. The spread of a gene drive is a rapid evolutionary process that occurs over timescales similar to many ecological processes. This can potentially generate strong eco‐evolutionary feedback that could profoundly affect the dynamics and outcome of a gene drive release. We, therefore, argue for the importance of incorporating ecological features into gene drive models. We describe the key ecological features that could affect gene drive behaviour, such as population structure, life‐history, environmental variation and mode of selection. We review previous gene drive modelling efforts and identify areas where further research is needed. As gene drive technology approaches the level of field experimentation, it is crucial to evaluate gene drive dynamics, potential outcomes, and risks realistically by including ecological processes.
... The correlation between pupal and adult body sizes in both sexes have already been already [74]. In addition, size differences in adults have been reported to influence productivity, mating competitiveness, survival, dispersal rate, fecundity and egg production [36,71,[75][76][77][78][79]. Furthermore, the effect of strain colonization on vector competence has been observed [46,47]. ...
The sterile insect technique (SIT) is a biologically based method of pest control, which relies on the mass production, sterilization, and release of sterile males of the target species. Since females can transmit viruses, it is important to develop a mass rearing system to produce a large number of males with a low presence of females. We evaluated the effects of different strains, larval diets and sexing tools on male productivity and residual female presence for the application of SIT against Aedes albopictus. Strains coming from Italy, Germany, Greece, and Montenegro, with different levels of colonization, were reared with three larval diets: IAEA-BY, BLP-B and SLP-BY. Developed pupae were sexed using two different mechanical methods: sieve or Fay-Morlan separator. The results proved that adoption of the Fay-Morlan separator increased the productivity and limited the female presence. The IAEA-BY diet showed the lowest female contamination. Strains with a high number of breeding generations showed a decreased productivity and an increased female presence. Increased female presence was found only in extensively reared strains and only when the sorting operation was conducted with sieves. We hypothesize that extensive colonization may determine a size reduction which limits the sexing tool efficiency itself.
... Indeed, swarming is an activity with a high energy demand, consuming 50% of sugar and glycogen reserves in 25 min (refs. 68,69 ). Mosquitoes probably switched to a more random flight to try to escape and search for a sugar meal to refuel their reserves. ...
Cues involved in mate seeking and recognition prevent hybridization and can be involved in speciation processes. In malaria mosquitoes, females of the two sibling species Anopheles gambiae s.s. and An. coluzzii mate in monospecific male swarms and hybrids are rare. Long-range sex pheromones driving this behaviour have been debated in literature but so far, no study has proven their existence or their absence. Here, we attempted to bring to light their existence. To put all the odds in our favour, we used different chemical ecology methods such as behavioural and electrophysiological assays as well chemical analyses, and we worked with mosquitoes at their optimal physiological mating state that is with swarming males during their natural swarming windows. Despite all our efforts, our results support the absence of long-range sex pheromones involved in swarm detection and recognition by females. We briefly discuss the implications of this finding in ecology, evolution and for control strategies.
... gambiae laboratory swarm studies 4,14,56 , the incubator was programmed with a 12 h light: 12 h dark photoperiod (two 17-W fluorescent tube bulbs), including 1 h of crepuscular lighting (one 5.5-W LED bulb) at dawn and dusk. Because flight tone frequencies can vary by adult size 25,28,30,36 , rearing methods were designed to obtain field-sized adult mosquitoes 6,8,[57][58][59] of equivalent sizes between trials. Male and female wing lengths from a subset of individuals from each experimental group were measured as a proxy for overall size as previously described 60 (Supplementary Fig. S1; Supplementary Table S1). ...
The mosquito Anopheles gambiae is a major African malaria vector, transmitting parasites responsible for significant mortality and disease burden. Although flight acoustics are essential to mosquito mating and present promising alternatives to insecticide-based vector control strategies, there is limited data on mosquito flight tones during swarming. Here, for the first time, we present detailed analyses of free-flying male and female An. gambiae flight tones and their harmonization (harmonic convergence) over a complete swarm sequence. Audio analysis of single-sex swarms showed synchronized elevation of male and female flight tones during swarming. Analysis of mixed-sex swarms revealed additional 50 Hz increases in male and female flight tones due to mating activity. Furthermore, harmonic differences between male and female swarm tones in mixed-sex swarms and in single-sex male swarms with artificial female swarm audio playback indicate that frequency differences of approximately 50 Hz or less at the male second and female third harmonics (M2:F3) are maintained both before and during mating interactions. This harmonization likely coordinates male scramble competition by maintaining ideal acoustic recognition within mating pairs while acoustically masking phonotactic responses of nearby swarming males to mating females. These findings advance our knowledge of mosquito swarm acoustics and provide vital information for reproductive control strategies.
... A male mosquito can mate several times during its lifetime , and energy consumption during swarming flight is exceptionally high (Gary et al. 2009. Swarming behavior in male Anopheles uses approximately 50% of the reserves of glycogen and sugar, which is the primary source of energy used in flight (Maïga et al. 2012, Yahouédo et al. 2014). An. gambiae s.s. has 6% higher sugar and glycogen contents than An. ...
Effective management of insect disease vectors requires a detailed understanding of their ecology and behavior. In Anopheles gambiae sensu lato (s.l.) (Diptera: Culicidae) mating occurs during swarming, but knowledge of their mating behavior under natural conditions is limited. Mosquitoes mate in flight over specific landmarks, known as swarm markers, at particular locations. Swarms consist of males; the females usually approach the swarm and depart following copulation. The number of mating pairs per swarm is closely associated with swarm size. The shape and height of swarm markers vary and may depend on the environmental conditions at the swarm's location. Male-male interactions in mosquito swarms with similar levels of attractive flight activity can offer a mating advantage to some individuals. Flight tone is used by mosquitoes to recognize the other sex and choose a desirable mate. Clarifying these and other aspects of mosquito reproductive behavior can facilitate the development of population control measures that target swarming sites. This review describes what is currently known about swarming behavior in Anopheles gambiae s.l., including swarm characteristics; mating within and outside of swarms, insemination in females, and factors affecting and stimulating swarming.
... Harmonic convergence, the adjustment of wingbeat frequencies between a male and female, is observed leading up to close-range interactions in Anopheles like in other mosquito species, but there is no evidence that it increases the likelihood of successful copulation in A. gambiae [8][9][10] . Other studies have striven to understand whether male fitness, reflected by body size, determines mating outcomes, with unclear and conflicting conclusions 5,[11][12][13] . When a swarming male approaches a female, there is substantial contact between their legs and abdomens after which, in successful mating events, the male grasps the female and completes copulation 14 . ...
Anopheles coluzzii females, important malaria vectors in Africa, mate only once in their lifetime. Mating occurs in aerial swarms with a high male-to-female ratio, where traits underlying male mating success are largely unknown. Here, we investigated whether cuticular hydrocarbons (CHCs) influence mating success in natural mating swarms in Burkina Faso. As insecticides are widely used in this area for malaria control, we also determined whether CHCs affect insecticide resistance levels. We find that mated males have higher CHC abundance than unmated controls, suggesting CHCs could be determinants of mating success. Additionally, mated males have higher insecticide resistance under pyrethroid challenge, and we show a link between resistance intensity and CHC abundance. Taken together, our results suggest that CHC abundance may be subject to sexual selection in addition to selection by insecticide pressure. This has implications for insecticide resistance management, as these traits may be sustained in the population due to their benefits in mating even in the absence of insecticides. In this study, Adams et al. investigate the effect of cuticular hydrocarbons on mating success in natural mosquito mating swarms. These hydrocarbons confer both higher mating success and increased resistance to pyrethroid, suggesting sexual selection for insecticide resistance in this population secondary to mating success.
... gambiae s.s. population in Burkina Faso included age and body size [301][302][303]. ...
The Joint FAO/IAEA Centre (formerly called Division) of Nuclear Techniques in Food and Agriculture was established in 1964 and its accompanying laboratories in 1961. One of its subprograms deals with insect pest control, and has the mandate to develop and implement the sterile insect technique (SIT) for selected key insect pests, with the goal of reducing the use of insecticides, reducing animal and crop losses, protecting the environment, facilitating international trade in agricultural commodities and improving human health. Since its inception, the Insect Pest Control Laboratory (IPCL) (formerly named Entomology Unit) has been implementing research in relation to the development of the SIT package for insect pests of crops, livestock and human health. This paper provides a review of research carried out between 2010 and 2020 at the IPCL. Research on plant pests has focused on the development of genetic sexing strains, characterizing and assessing the performance of these strains (e.g., Ceratitis capitata), elucidation of the taxonomic status of several members of the Bactrocera dorsalis and Anastrepha fraterculus complexes, the use of microbiota as probiotics, genomics, supplements to improve the performance of the reared insects, and the development of the SIT package for fruit fly species such as Bactrocera oleae and Drosophila suzukii. Research on livestock pests has focused on colony maintenance and establishment, tsetse symbionts and pathogens, sex separation, morphology, sterile male quality, radiation biology, mating behavior and transportation and release systems. Research with human disease vectors has focused on the development of genetic sexing strains (Anopheles arabiensis, Aedes aegypti and Aedes albopictus), the development of a more cost-effective larvae and adult rearing system, assessing various aspects of radiation biology, characterizing symbionts and pathogens, studying mating behavior and the development of quality control procedures, and handling and release methods. During the review period, 13 coordinated research projects (CRPs) were completed and six are still being implemented. At the end of each CRP, the results were published in a special issue of a peer-reviewed journal. The review concludes with an overview of future challenges, such as the need to adhere to a phased conditional approach for the implementation of operational SIT programs, the need to make the SIT more cost effective, to respond with demand driven research to solve the problems faced by the operational SIT programs and the use of the SIT to address a multitude of exotic species that are being introduced, due to globalization, and established in areas where they could not survive before, due to climate change.
... This result is interesting because for SIT release programme to be successful, sterile males must be of sufficient quality to disperse into the environment, survive long enough to locate and attract wild females, in competition with wild counterparts, and copulate with as many as wild females as possible. The size of adult male mosquitoes is thought by many to be an important predictor of their mating success [15,[32][33][34]. Pupae of the same age and adult males of the same size were used here in order to minimize any effect of adult size and age on the mating competition between males from the SRS and MRS treatments. ...
BACKGROUND:
The success of the sterile insect technique depends, among other things, on continuous releases of sexually competitive sterile males within the target area. Several factors (including high rearing density and physical manipulation, such as larvae and pupae separation) can influence the quality of males produced in mass-rearing facilities. The different steps in mass production in the laboratory may modify the behaviour of mosquitoes, directly or through loss of natural characters as a result of adaptation to lab rearing, and lead to the competitiveness of sterile male being reduced. In the present study, the objective was to evaluate the effect of mass-rearing conditions on sterile male sexual competitiveness in semi-field cages compared to routine small scale laboratory rearing methods.
METHODS:
Anopheles arabiensis immature stages were reared both on a large scale using a rack and tray system developed by the FAO/IAEA (MRS), and on a small scale using standard laboratory rearing trays (SRS). Mosquito life history traits such as pupation rate, emergence rate, adult size as well as the effect of irradiation on adult longevity were evaluated. Moreover, 5-6 day old mosquitoes were released into field cages and left for two nights to mate and the mating competitiveness between sterile mass-reared males and fertile males reared on a small scale when competing for small scale reared virgin females was investigated. Resulting fertility in a treatment ratio of 1:1:1 (100 irradiated males: 100 non-irradiated males: 100 virgin females) was compared to control cages with 0:100:100 (non-irradiated control) and 100:0:100 (irradiated control).
RESULTS:
No significant differences in life history parameters were observed between rearing methods. The competitiveness index of mass reared males (0.58) was similar to males reared on a small scale (0.59). A residual fertility rate of 20% was observed in the irradiated control (100:0:100), measured as the percentage of eggs collected from the cages which developed to adulthood. No significant difference was observed (t = 0.2896, df = 4, P = 0.7865) between the rearing treatments (MRS and SRS) in the fertility rate, a measure of mating competitiveness.
CONCLUSIONS:
The results showed that the FAO/IAEA mass-rearing process did not affect mosquito life history parameters or the mating competitiveness of males.
... Specimens fed on the IAEA 2 diet exhibited higher percentage of pupation and larger males compared with those fed on Khan's diet, but no evidence indicated reduced mating capacity among small mosquitoes fed on Khan's diet. The mating competitiveness of male mosquitoes at various test temperatures and diet regimes is of interest, because diet regime was always size-dependent [67]. The protein levels of males fed on Khan's diet and the IAEA 2 diet also warrant further investigation. ...
Background:
Producing high quality sterile males is vital in Aedes aegypti rear-and-release birth control strategies. Larval diets, rearing temperatures, and their interactions determine the accumulation rates of essential nutrients in larvae, but these factors have been understudied in relation to mass-rearing techniques for producing eminent males.
Methods:
We compared the effects of two larval diets, a cereal-legume-based diet (Khan's diet) and a standard larval diet developed in the FAO/IAEA Insect Pest Control Laboratory (IAEA 2 diet). Diets were tested at selected temperatures for both larval and male adult life history traits, adult extreme temperature tolerance, and mating capacity relative to energy reserves of reared male adult Ae. aegypti.
Results:
Khan's diet resulted in shorter immature development time at each test temperature (except for 25 °C) than an IAEA 2 diet. Larvae reared at 28 °C and 32 °C with Khan's diet demonstrated low pupation rates (c.80%). We accounted for these phenomena as secondary sex ratio manipulation, because a higher proportion of male adults emerged at 28 °C and 32 °C than that for the IAEA 2 diet. In general, the pupal development time shortened as temperature increased, resulting in higher teneral energy reserves in male mosquitoes. High energy reserves allowed male mosquitoes reared with Khan's diet to have higher adult longevity (5-6 days longer when sugar-fed and 2-3 days longer when water-fed) and tolerance of heat stress than those fed on the IAEA 2 diet. The IAEA 2 diet produced larger male mosquitoes than Khan's diet did: mosquitoes fed on Khan's diet were 1.03-1.05 times smaller than those fed on the IAEA 2 diet at 28 °C and 32 °C. No evidence indicated reduced mating capacity for small mosquitoes fed on Khan's diet.
Conclusions:
Larvae reared at 28 °C and 32 °C with Khan's diet were characterized by shorter immature development time compared with those fed on the IAEA 2 diet. Adult mosquitoes produced from that larval rearing condition exhibited a significant male bias, long lifespan, and better endurance against extreme temperatures relative to energy reserves. Thus, the larval diet at rearing temperature of 28 °C and 32 °C optimized rearing techniques for the sterile insect programmes. However, mating competitiveness and flight performance of adult males require further investigation.
... Male mosquitoes showed higher mortality rates compared to females, which is likely to be, among other factors, due to mating-related stress. It is well known that mating is costly in terms of energy [44,45] and a recent study pointed out that an increase in female survival could be due to male-derived seminal fluid molecules transferred to the female [46]. ...
... Male mosquitoes showed higher mortality rates compared to females, which is likely to be, among other factors, due to mating-related stress. It is well known that mating is costly in terms of energy [44,45] and a recent study pointed out that an increase in female survival could be due to male-derived seminal fluid molecules transferred to the female [46]. ...
Introduction
The widespread emergence of resistance to insecticides used to control adult Aedes mosquitoes has made traditional control strategies inadequate for the reduction of various vector populations. Therefore, complementary vector control methods, such as the Sterile Insect Technique, are needed to enhance existing efforts. The technique relies on the rearing and release of large numbers of sterile males, and the development of efficient and standardized mass-rearing procedures and tools is essential for its application against medically important mosquitoes.
Methods
In the effort to reduce the cost of the rearing process, a prototype low-cost plexiglass mass-rearing cage has been developed and tested for egg production and egg hatch rate in comparison to the current Food and Agriculture Organization/International Atomic Energy Agency (FAO/IAEA) stainless-steel cage. Additionally, an adult-index was validated and used as a proxy to estimate the mosquito survival rates by counting the number of male and female mosquitoes that were resting within each of the 6 squares at a given point of time each day in the cage.
Results
The study has shown that the prototype mass-rearing cage is cheap and is as efficient as the FAO/IAEA stainless-steel cage in terms of egg production, with even better overall egg hatch rate. The mean numbers of eggs per cage, after seven cycles of blood feeding and egg collection, were 969,789 ± 138,101 and 779,970 ± 123,042, corresponding to 81 ± 11 and 65 ± 10 eggs per female over her lifespan, in the prototype and the stainless-steel-mass-rearing cages, respectively. The longevity of adult male and female mosquitoes was not affected by cage type and, the adult-index could be considered as an appropriate proxy for survival. Moreover, the mass-rearing cage prototype is easy to handle and transport and improves economic and logistic efficiency.
Conclusion
The low-cost mass-rearing prototype cage can be recommended to produce Ae. aegypti in the context of rear and release techniques. The proposed adult-index can be used as a quick proxy of mosquito survival rates in mass-rearing settings.
... Our findings support the notion that male mating performance in Aedes is more nuanced than "bigger is better" [45], as appears to be the case for Anopheles mosquitoes [86,87]. Recent analyses of mating in Ae. aegypti have indicated that success may be influenced by a relative size-assortative relationship [44,45]. ...
Background:
Evaluating and improving mating success and competitive ability of laboratory-reared transgenic mosquito strains will enhance the effectiveness of proposed disease-control strategies that involve deployment of transgenic strains. Two components of the mosquito rearing process, larval diet quantity and aquatic environment - which are linked to physiological and behavioural differences in adults - are both relatively easy to manipulate. In mosquitoes, as for many other arthropod species, the quality of the juvenile habitat is strongly associated with adult fitness characteristics, such as longevity and fecundity. However, the influence of larval conditioning on mating performance is poorly understood. Here, we investigated the combined effects of larval diet amount and environmental water source on adult male mating success in a genetically modified strain of Aedes aegypti mosquitoes in competition with wild-type conspecifics. Importantly, this research was conducted in a field setting using low generation laboratory and wild-type lines.
Results:
By controlling larval diet (high and low) and rearing water source (field-collected and laboratory water), we generated four treatment lines of a genetically modified strain of Ae. aegypti tagged with fluorescent sperm. Laboratory reared mosquitoes were then competed against a low generation wild-type colony in a series of laboratory and semi-field mating experiments. While neither food quantity nor larval aquatic environment were found to affect male mating fitness, the transgenic lines consistently outperformed wild-types in laboratory competition assays, an advantage that was not conferred to semi-field tests.
Conclusions:
Using a model transgenic system, our results indicate that differences in the experimental conditions of laboratory- and field-based measures of mating success can lead to variation in the perceived performance ability of modified strains if they are only tested in certain environments. While there are many potential sources of variation between laboratory and field lines, laboratory adaptation - which may occur over relatively few generations in this species - may directly impact mating ability depending on the context in which it is measured. We suggest that colony-hybridization with field material can potentially be used to mitigate these effects in a field setting. Release programs utilising mass-produced modified laboratory strains should incorporate comparative assessments of quality in candidate lines.
... Mosquitoes reared in the laboratory are not under high selective pressures compared to wild mosquitoes. Consequently, some of the laboratory mosquitoes, those that would have never survived or mated in the wild, may not possess enough reserves to compete for females in this very demanding behavior [38,39]. Moreover, flight ability is also controlled by the thoracic muscles which are formed during metamorphosis. ...
Background
The sterile insect technique (SIT) aims at suppressing or decreasing insect pest populations by introducing sterile insects into wild populations. SIT requires the mass-production of insects and their sterilization through, for example, radiation. However, both mass-rearing and radiation can affect the life history traits of insects making them less competitive than their wild counterparts. In the malaria mosquito Anopheles arabiensis, some progress has been made to improve the mating competitiveness of mass-reared irradiated males. However, to date, no study has explored the relative effects of colonization and irradiation on important reproductive traits in this species. Such data may help to focus research efforts more precisely to improve current techniques.
Methods
Two strains of An. arabiensis originating from the same locality were used: one reared in the laboratory for five generations and the second collected as late larval instars in the field prior to experimentation. Pupae were irradiated with 95 Gy and some adult reproductive traits, including insemination rate, fecundity, oviposition behavior, fertility and male survivorship, were assessed in different mating combinations.
Results
Our study revealed the different effects of mosquito strain and irradiation on reproductive processes. The insemination rate was higher in field (67.3%) than in laboratory (54.9%) females and was negatively affected by both female and male irradiation (un-irradiated vs irradiated: 70.2 vs 51.3% in females; 67.7 vs 53.7% in males). Irradiated females did not produce eggs and egg prevalence was lower in the field strain (75.4%) than in the laboratory strain (83.9%). The hatching rate was higher in the field strain (88.7%) than in the laboratory strain (70.6%) as well as in un-irradiated mosquitoes (96.5%) than in irradiated ones (49%). Larval viability was higher in the field strain (96.2%) than in the laboratory strain (78.5%) and in un-irradiated mosquitoes (97.6%) than irradiated ones (52%). Finally, field males lived longer than laboratory males (25.1 vs 20.5 days, respectively).
Conclusions
Our results revealed that both irradiation and colonization alter reproductive traits. However, different developmental stages are not equally affected. It is necessary to consider as many fitness traits as possible to evaluate the efficacy of the sterile insect technique.
Electronic supplementary material
The online version of this article (10.1186/s13071-018-3228-3) contains supplementary material, which is available to authorized users.
... However, a large body size may also require the consumption of more nutrients and may reduce flexibility of movement. Some research has shown that intermediate body size correlated to male mating success [39,41]. In this study both diet concentration and larval density were shown to strongly affect wing length, which increased with diet availability, and decreased with larval density, as has previously been observed [21,24,42]. ...
Background:
In a context of increasing resistance of both vectors toward main classes of insecticides used in public health and parasites toward anti-malarial drugs, development of new and complementary molecules or control approaches is fundamental to achieve the objective of controlling or even eliminating malaria. Concerning vector control, the sterile insect technique and other genetic control approaches are among promising complementary tools in an integrated management strategy for malaria control. These approaches rely not only on a good understanding of vector biology (especially during larval stages), but also on the availability of adequate supplies and protocols for efficient mosquito rearing. The aim of this study was to assess the factors impacting the life history of Anopheles coluzzii mosquitoes at the larval stage, in the context of genetic and sterile insect approaches to control malaria vectors.
Methods:
The effect of different larval diets and larval rearing densities on the development of An. coluzzii were evaluated in the laboratory. Emergence rate (ER), pre-imaginal developmental time (DT) and adult wing length (WL) were measured under different food regimes. Four diets were tested among which three were provided by the Insect Pest Control Laboratory (IPCL) of the FAO/IAEA Joint division.
Results:
Data showed significant differences in the quality of the different diets and suggested a negative density dependence in all three life history parameters measured under tested rearing conditions. ER and WL increased with food availability, but decreased with increasing larval density. Conversely DT was shortened with increasing food availability but increased with larval density. These data demonstrates intraspecific larval competition modulated by food amount and space availability. Of the four diets tested, the one made of a mix of tuna meal, bovine liver powder, brewer's yeast, squid liver powder and vitamin mix (diet 2) yielded the best results as it produced a good balance between ER, DT and WL. Food availability for optimal development (highest survival at shortest time) was in the range of 180-400 µg/larvae/day for the three diets provided by the IPCL.
Conclusion:
There is an interaction between diet type, diet concentration and larval density. Best results in terms of optimal larvae development parameters happen when moderately high values of those three variables are observed.
... Mosquitoes reared in the laboratory are not under high selective pressures compared to wild mosquitoes. Consequently, some of the laboratory mosquitoes, those that would have never survived or mated in the wild, may not possess enough reserves to compete for females in this very demanding behavior [38,39]. Moreover, flight ability is also controlled by the thoracic muscles which are formed during metamorphosis. ...
... It is well known that genitalia rotation is essential for mating to take place [20]. This automatically deduce the conclusion that newly emerged or young males will not participate in swarms in order to reserve their energy reserves until they are physically able to find a mate [15,[21][22][23]. Swarming of young immature males will also expose them unnecessarily to the threat to predation [24]. ...
Background
Anopheles funestus has been recognized as a major malaria vector in Africa for over 100 years, but knowledge on many aspects of the biology of this species is still lacking. Anopheles funestus, as with most other anophelines, mate through swarming. A key event that is crucial for the An. funestus male to mate is genitalia rotation. This involves the 135° to 180° rotation of claspers, which are tipped with claws. This physical change then enables the male to grasp the female during copulation. The aim of this investigation was to molecularly characterize wild An. funestus swarms from Zambia and examine the degree of genitalia rotation within the swarm. Methods
Anopheles funestus swarms were collected from Nchelenge, northern Zambia, during dusk periods in May 2016. All the adults from the swarm were analysed morphologically and identified to species level using a multiplex PCR assay. Anopheles funestus s.s. specimens were molecularly characterized by restriction fragment length polymorphism type and Clade type assays. The different stages of genitalia rotation were examined in the adult males. ResultsA total of six swarms were observed during the study period and between 6 and 26 mosquitoes were caught from each swarm. Species analysis revealed that 90% of the males from the swarms were An. funestus s.s. MW-type, with 84% belonging to clade I compared to 14% clade II and 2% failed to amplify. Very few specimens (3.4%) were identified as Anopheles gambiae s.s. Eighty percent of the males from the swarm had complete genitalia rotation. Conclusions
This is the first time that An. funestus swarms have been molecularly identified to species level. Anopheles funestus swarms appear to be species-specific with no evidence of clade-type differentiation within these swarms. The An. funestus swarms consist mainly of males with fully rotated genitalia, which strongly suggests that swarming behaviour is triggered primarily when males have matured.
... The reduction of energy reserves was the result of the deficit between nutrient reward and energy consumption, which could be attributed either to the insufficient nectar production or inadequate amounts of host plants available for mosquitoes. Sugar is the main fuel used in flight (Nayar and Van Handel 1971;Yuval et al. 1994), and a male mosquito could use up more than 50% of the sugar reserves for flight and swarming activities (Maïga et al. 2012(Maïga et al. , 2014. During flight, mosquitoes consume crop sugar firstly, when The content values of metabolic reserves were presented as mean ± standard error; different letters in the same column indicate the significant differences at α = 0.05 by ANOVA test The content values of metabolic reserves were presented as mean ± standard error; different letters in the same column indicate the significant differences at α = 0.05 by ANOVA test mosquitoes are flown to exhaustion, recovery involves renewed accumulation of glycogen in the flight muscles (Nayar and Van Handel 1971;Foster 1995). ...
Mosquito adults usually need to ingest sugar from nectar host plants to sustain their metabolic needs. Mosquitoes could be differentially attracted by various flowering plant species, and the volatiles were thought to be important factors attributed to the differential attractiveness. However, whether mosquitoes’ preference for host plants correlates with their nutritional rewards from sugar sources remains unclear. In the present study, the preference of newly emerged Culex pipiens pallens to three kinds of flowering plants (Ligustrum quihoui, Abelia chinensis, and Nerium indicum) was determined in the olfactometer. Besides, when the newly emerged mosquitoes were provided with these flowering plants as sugar sources, the content of their metabolic reserves (glycogen, lipid, and protein) was determined. The results revealed that Cx. pipiens pallens could be differentially attracted by the odors emitted by the inflorescences of the tested flowering plants, and the nutritional rewards of mosquitoes were significantly affected by feeding on different inflorescences. The present study demonstrated that feeding on nectar host plants with differential attraction could affect the energy reserves of Cx. pipiens pallens.
... This result is interesting because for SIT release programme to be successful, sterile males must be of sufficient quality to disperse into the environment, survive long enough to locate and attract wild females, in competition with wild counterparts, and copulate with as many as wild females as possible. The size of adult male mosquitoes is thought by many to be an important predictor of their mating success [15,[32][33][34]. Pupae of the same age and adult males of the same size were used here in order to minimize any effect of adult size and age on the mating competition between males from the SRS and MRS treatments. ...
Background
The success of the sterile insect technique depends, among other things, on continuous releases of sexually competitive sterile males within the target area. Several factors (including high rearing density and physical manipulation, such as larvae and pupae separation) can influence the quality of males produced in mass-rearing facilities. The different steps in mass production in the laboratory may modify the behaviour of mosquitoes, directly or through loss of natural characters as a result of adaptation to lab rearing, and lead to the competitiveness of sterile male being reduced. In the present study, the objective was to evaluate the effect of mass-rearing conditions on sterile male sexual competitiveness in semi-field cages compared to routine small scale laboratory rearing methods.
Methods
Anopheles arabiensis immature stages were reared both on a large scale using a rack and tray system developed by the FAO/IAEA (MRS), and on a small scale using standard laboratory rearing trays (SRS). Mosquito life history traits such as pupation rate, emergence rate, adult size as well as the effect of irradiation on adult longevity were evaluated. Moreover, 5–6 day old mosquitoes were released into field cages and left for two nights to mate and the mating competitiveness between sterile mass-reared males and fertile males reared on a small scale when competing for small scale reared virgin females was investigated. Resulting fertility in a treatment ratio of 1:1:1 (100 irradiated males: 100 non-irradiated males: 100 virgin females) was compared to control cages with 0:100:100 (non-irradiated control) and 100:0:100 (irradiated control).
Results
No significant differences in life history parameters were observed between rearing methods. The competitiveness index of mass reared males (0.58) was similar to males reared on a small scale (0.59). A residual fertility rate of 20% was observed in the irradiated control (100:0:100), measured as the percentage of eggs collected from the cages which developed to adulthood. No significant difference was observed (t = 0.2896, df = 4, P = 0.7865) between the rearing treatments (MRS and SRS) in the fertility rate, a measure of mating competitiveness.
Conclusions
The results showed that the FAO/IAEA mass-rearing process did not affect mosquito life history parameters or the mating competitiveness of males.
... One study demonstrated that male An. gambiae mosquitoes lose over 50% of the carbohydrate reserves during swarming (Maiga et al. 2012). Another possibility is that males may imbibe less sugar while feeding than do females, resulting in a more rapid depletion of their energy reserves and thus not detecting the color dye from the non-attractive sugar solution. ...
We evaluated how the presence of sugar sources impacted the distribution of Aedes aegypti in different habitats in Durán, Ecuador. Land cover and normalized difference vegetation index maps were used to guide a random point sampling routine to select study grids (30 m × 30 m) in low vegetation (LV) and high vegetation (HV). Five individual plants, at one home in the LV and HV grid, were treated with a different colored, non-attractive, 60% sucrose solution to determine mosquito feeding and movement. Sugar alone is not attractive to mosquitoes, so spraying vegetation with a dyed sugar solution can be used for visual determination of sugar feeding. Outdoor collections using BG sentinel traps and indoor collections using aspirators were conducted at the treatment home and with collection points at 20, 40, and 60 m surrounding the treatment home for three consecutive days. A total of 3,245 mosquitoes in two genera, Aedes and Culex, was collected. The proportion of stained Ae. aegypti females was 56.8% (510/898) and 0% for males. For Culex, 63.9% (248/388) females and 36.1% (140/388) males were collected stained. Aedes aegypti and Culex spp. were found up to 60 m stained in both LV and HV grids. Significantly more stained females Ae. aegypti were found inside homes compared to females and males of Culex spp. in both habitats. This study identifies that outdoor sugar feeding is a common behavior of Ae. aegypti and can be targeted as a control strategy in urban habitats in Latin America.
... freeborni can mate more often with females than the smaller male. Similarly, Maïga et al. [62] found that mated male Anopheles gambiae was significantly bigger than non-mated ones. Comparative measurements of wings in adult females and males between the stenogamous and eurygamous species showed that the wings of males were statistically significant difference. ...
Establishment of laboratory colony is essential for mosquito-borne-disease research. Mating behavior of stenogamous Anopheles peditaeniatus and seven eurygamous species (Anopheles argyropus, Anopheles crawfordi, Anopheles nigerrimus, Anopheles nitidus, Anopheles paraliae (=An. lesteri), Anopheles pursati and Anopheles sinensis), were investigated and compared in this study. The self-mating success of adult mosquitoes in different size cages at two density resting surface (DRS) values, 3.6 and 7.2, was statistically significant between stenogamous and eurygamous species. The results obtained from comparative measurements of specific characters in adult females (maxillary palpomere and antennal sensilla characters) and males (wing and genitalia) indicate those characters might influence the mating success of An. peditaeniatus in a small cage. The gonostylus of An. peditaeniatus was shorter than the eurygamous species. Additionally, the lower frequency of clasper movement and shorter mating time could be important mechanisms that control the stenogamous behavior of An. peditaeniatus. Interestingly, for the first time, a cluster of large sensilla coeloconica was recorded on the antenna of An. argyropus and An. peditaeniatus females. There was no statistically significant difference in the mean number per female of those large antennal sensilla coeloconica among six of the eurygamous species.
... A photograph of each wing was taken under a dissecting microscope (Leica). Wing length was measured from the tip of the wing (excluding fringe) to the distal end of the alula [30,31] using analysis ® FIVE software. Wing length is considered to be a proxy for mosquito body size [32]. ...
Background
There is growing interest in applying the sterile insect technique (SIT) against mosquitoes. Mass production of mosquitoes for large-scale releases demands a huge amount of water. Yet, many arid and/or seasonally arid countries face the difficulties of acute water shortage, deterioration of water quality and environmental constraints. The re-use of water to rear successive generations of larvae is attractive as a way to reduce water usage and running costs, and help to make this control method viable.
Methods
To determine whether dirty larval water was a suitable rearing medium for Anopheles arabiensis, in place of the ‘clean’ dechlorinated water routinely used, a series of three experiments was carried out to evaluate the effect of dirty water or mixed clean and dirty water on several parameters of insect quality. Batches of 100 fresh eggs were distributed in dirty water or added to clean water to test the effect of dirty water on egg hatching, whereas first-instar larvae were used to determine the effect on immature development time, pupation, adult emergence, body size, and longevity. Moreover, to assess the effect of dirty water on larval mortality, pupation rate, adult emergence, and longevity, L4 larvae collected after the tilting or larvae/pupae separation events were returned either to the dirty water or added to clean water.
Results
Results indicated that reusing dirty water or using a 50:50 mix of clean and dirty water did not affect egg hatching. Moreover, no difference was found in time to pupation, larval mortality or sex ratio when first-instar larvae were added to clean water, dirty water, or a 75:25, 50:50 or 25:75 mix of clean and dirty water and reared until emergence. When late-instar larvae were put back into their own rearing water, there was no effect on pupation rate, emergence rate or female longevity, though male longevity was reduced. When reared from first-instar larvae, however, dirty water decreased pupation rate, emergence rate, body size, and adult longevity.
Conclusions
Re-used larval-rearing water has no impact on egg hatching, development time or mortality of the immature stages of An. arabiensis. However, dirty water is not suitable for the production of high quality adult mosquitoes. Recycling processes to improve water quality and increase insect quality will be investigated, since it may have important implications for the implementation of the SIT in areas where clean water is a scarce or costly resource.
... Adult wing length was used as a proxy for body size [28][29][30][31][32]. The length of both wings was measured and their arithmetic mean taken as the 'size' of each mosquito. ...
Abstract Background Anopheles mosquito life-history parameters and population dynamics strongly influence malaria transmission, and environmental factors, particularly temperature, strongly affect these parameters. There are currently some studies on how temperature affects Anopheles gambiae s.s. survival but very few exist examining other life-history traits. We investigate here the effect of temperature on population dynamics parameters. Methods Anopheles gambiae s.s. immatures were reared individually at 23 ± 1 °C, 27 ± 1 °C, 31 ± 1 °C, and 35 ± 1 °C, and adults were held at their larval temperature or at one of the other temperatures. Larvae were checked every 24 h for development to the next stage and measured for size; wing length was measured as a proxy for adult size. Females were blood fed three times, and the number of females feeding and laying eggs was counted. The numbers of eggs and percentage of eggs hatched were recorded. Results Increasing temperatures during the larval stages resulted in significantly smaller larvae (p = 0.005) and smaller adults (p
... The nature of the available sugar source was found to affect survival, flight ability and mating competitiveness in An. gambiae s.s. (Maïga et al., 2012). A pupal release device providing access to sugar post-emergence was developed and tested in Italy (Bellini et al., 2014). ...
Mosquito-borne pathogens are transmitted mainly during blood feeding by infective female mosquito vectors. Blood meals’ digestion stages reveal necessary entomological information for retrospective assessment and future plan for vector control. This study was done to assess the blood digestion stages of Anopheles gambiae and Culex quinquefasciatus mosquitoes and their relationships with climatic conditions in school dormitories in Egor, Oredo and Ikpoba Okha Local Government Areas (LGAs) of Benin City, Edo State. Indoor mosquitoes were collected between 08:00 and 11:00 hours from randomly selected dormitories bi-monthly for 6 months, using sweep net and mechanical aspirator. Female mosquitoes were identified and grouped into various blood digestion stages using relevant keys. A total of 439 adult female mosquitoes identified as An. gambiae (n=28) and Cx. quinquefasciatus (n=411) were examined for gonotrophic status. Total gonotrophic stages of both vector species in the collective LGAs were highly significant (p<0.01). Only Cx. quinquefasciatus revealed significant variation (p<0.01) in monthly mean percentage abundance of the gonotrophic stages across the study months in the combined LGA data. Regarding each LGA, there was significant variation in monthly mean percentage abundance of Cx. quinquefasciatus gonotrophic stages across the study months (p<0.05) in Egor and Ikpoba Okha except Oredo. As for An. gambiae, no significant difference (p>0.05) was observed per LGA. Correlations between gravid Cx. quinquefasciatus vs relative humidity, unfed Cx. quinquefasciatus vs rainfall and An. gambiae vs rainfall were the only associations that correlated significantly (p<0.05). The results indicate the need for deployment of thorough and effective vector management strategy
The mosquito Anopheles gambiae is a major African malaria vector, transmitting parasites responsible for significant mortality and disease burden. Malaria declines have stagnated recently due to widespread insecticide resistance among vector populations. Flight acoustics are essential to mosquito mating biology and represent promising alternative targets for mosquito control. However, mosquito swarm acoustics data are limited. Here, for the first time, we present detailed analyses of free-flying male and female An. gambiae flight tones and their harmonization (harmonic convergence) over a complete swarm sequence. Audio analysis of single-sex swarms showed elevated male or female flight tone frequencies and amplitudes during swarming flight with gradual declines to pre-swarm levels over an approximately 35-min period. Analysis of mixed-sex swarms revealed additional increases in flight tone frequencies and amplitudes due to mating activity. Data from mixed-sex swarms suggest harmonic convergence during swarming enhances the efficiency of female detection by synchronizing male and female baseline swarm tones. Further, data from experiments using female swarm tone playbacks to males indicate that harmonic convergence during mating interactions coordinates male scramble competition by acoustically masking mating couple flight tones. These findings advance our knowledge of mosquito swarm acoustics, providing vital information for reproductive control strategies.
Background
Aedes albopictus is currently the most widespread invasive mosquito species in the world. It has paramount medical importance since females are efficient vectors of important viruses affecting humans. The development of alternative control strategies to complement control measures has become an imperative and involves the Sterile Insect Technique (SIT). Research to improve the productivity of mass-rearing, as well as the quality of mass-reared males is of essential importance for the success of SIT.
Methods
This study compared the influence of three differently sized cages for Ae. albopictus mass-rearing on wing length, adult survival and egg production during 20 generations of colonization. Plexiglas cages of 40x40x40 cm (C1), 100x20x100 cm (C2) and 100x65x100 cm (C3) were loaded with equal adult density, and sex ratio of 1:1. An open source image processing and analysis programme (ImageJ) was used for the wing measurement and egg counting.
Results
In all tested cages, we identified two periods separated by the generation showing the minimum value of each considered parameter (wing length, adult survival and egg production). The wing length and adult survival passed through the phases of initial decrease to about intermediate colonization time, and increased afterwards. Fecundity was steady during the first period and increased in the second one. Cage C1 demonstrated not only the best values for all parameters but also the smallest decrease in the initial phase. Recovering of the caged mosquitoes in the second half of the study was higher in cages C1 and C2, than in C3.
Conclusions
C1 provided the least negative selection pressure on wing length, adult survival and egg production for reared Ae. albopictus. Anyhow, since maximising mosquito density by exploiting the minimum space is a priority in mosquito mass-rearing, C2 might be a better choice for better fitting the space of mass-rearing rooms.
The study evaluated the effects of R-limonene, in sublethal concentration, on the histology, histochemistry, biochemistry, and carbohydrates and proteins levels in the third instar Aedes aegypti larvae. The R-limonene (LC50 of 27 ppm) and control groups were analyzed 12 and 24h after the beginning of treatments. The midgut of the control larvae was composed of cylindrical and elongated cells with a spherical and central nucleus and regenerative cells with a pyramidal shape. After 12 hours of treatment, columnar cells, protuberances, and cytoplasmic vacuolization were found. However, 24 hours after treatment, complete disorganization of the epithelium was observed. There was a positive reaction in all treatments for the presence of glycogen. However, the midgut of larvae treated with R-limonene showed higher levels. For the total protein, positive marking occurred in all groups evaluated, with higher levels in treatments and the lowest in control 12h. The levels of total protein and glycogen increased in the treated larvae compared to the 12h control. Besides, a reduction in total sugar levels was observed in the treated larvae compared to controls 12 and 24h, being more evident in the last one. Therefore, these results demonstrate that R-Limonene caused pathological changes in the epithelium of the A. aegypti midgut at histophysiological and biochemical levels.
Par MAIGA Hamidou Etude de la bio-écologie des mâles d'Anopheles gambiae s.l. et optimisation des méthodes d'élevage dans une perspective de développement de la lutte génétique
Despite the importance of mosquito mating biology to reproductive control strategies, a mechanistic understanding of individual mating interactions is currently lacking. Using synchronised high-speed video and audio recordings, we quantified behavioural and acoustic features of mating attempts between tethered female and free-flying male Aedes aegypti. In most couplings, males were actively displaced by female kicks in the early phases of the interaction, while flight cessation prior to adoption of the pre-copulatory mating pose also inhibited copulation. Successful males were kicked at a reduced rate and sustained paired contact-flight for longer than those that were rejected. We identified two distinct phases of acoustic interaction. Rapid frequency modulation of flight tones was observed in all interactions up to acceptance of the male. Harmonic convergence (wingbeat frequency matching) was detected more often in successful attempts, coinciding with the transition to stabilised paired flight and subsequent genital contact. Our findings provide a clearer understanding of the relationship between acoustic interactions and mating performance in mosquitoes, offering insights which may be used to target improvements in laboratory reared lines.
Studies of variation in male mating success within populations have been instrumental in shaping perspectives of the role of female choice in sexual selection. We present simple models to quantify the potential contributions to such variation from a variety of processes: average male mating success (equivalent to the number of female matings per male), variation in male and territory quality, male attendance times at leks, and degree of female synchrony, aggregation, and copying. If matings occur by chance, we expect variance in male mating success to increase proportionately with mean mating success. Increasing average mating success results in a superproportionate increase in variance when males or their territories differ in quality, when males differ in time spent at the lek, when females copy each other, or when females aggregate and mate as a group. In contrast, when females arrive in synchrony there is a subproportionate increase in variance with respect to mean male mating success. A review of 36 field studies from 20 species indicates that all studied leks exhibit greater variance in male mating success than expected by chance. Furthermore, the slope of a regression between variance and mean success is significantly greater than unity, as predicted by the models of variation in male quality, attendance at leks, female aggregation, and female copying. In studies of four leks in which we were able to partition the component of variance attributable to differences among males in attendance at leks, approximately 60% of the variance remained unexplained and was attributable to female copying, aggregation, or variation in male quality. The framework presented here allows a quantified breakdown of variation among males in mating success and facilitates comparisons among studies of the potential for sexual selection.
Mating behaviour in the Anopheles gambiae complex was studied in the laboratory, using an infra-red TV system to make observations in the ‘dark’. Maximum fertilization occurred when both sexes were at least 2 days old. In LD 12:12 most mating took place in the first hour of the dark phase. At this time, the males had erect fibrillae on the antennae and showed the maximum response to an artificial female flight tone, approaching, landing-on and attempting to clasp the sound source. The cycle of fibrillar erection and of responsiveness to sound continued in constant dark. Males did not appear to discriminate on contact, between their own and other species. Previously mated females actively resisted copulation, but, when tethered, were unable to prevent the deposition of further internal mating plugs. Free-flying, previously inseminated, females prevented the males from depositing internal plugs, but not from ejaculating and depositing external plugs. The differences between A.gambiae and the better known Stegomyia mosquitoes appear to be related to their different habits, A.gambiae males being less likely to encounter females of other species or inseminated females of their own species at the restricted time and place of mating.
Large size often confers a fitness advantage to female insects because fecundity increases with body size. However, the fitness benefits of large size for male insects are less clear. We investigated the mating behavior of the mayfly Baetis bicaudatus to determine whether the probability of male mating success increased with body size. Males formed mating aggregations (swarms) ranging from a few to hundreds of individuals, 1-4 m above the ground for about 1.5-2 h in the early morning. Females that flew near swarms were grabbed by males, pairs dropped to the vegetation where they mated and then flew off individually. Some marked males returned to swarms 1, 2 or 3 days after marking. Larger males swarmed near spruce trees at the edges of meadows, but the probability of copulating was not a function of male body size (no large male advantage). Furthermore, the potential fitness advantage of mating with larger, more fecund females was not greater for large males (no size-assortative mating). However, the sizes of copulating males were significantly less variable than those of non-mating males collected at random in swarms. Intermediate male size may be optimal during mating because of trade-offs between flight agility and longevity or competitive ability. Results of this study are consistent with the hypotheses that there is stabilizing selection on adult male body size during mating, and that male body size in this species may be influenced more by selection pressures acting on larvae than on adults.
We examined the accumulation of lipids in adult females of the mosquito, Aedes aegypti. Females emerged with about 100 μg lipid in the fat body. With access to sugar water lipids increased over seven days to 300 μg. After a blood meal on day five, sugar-fed females accumulated 120–140 μg of lipids in their ovaries within 2 days. At the same time the lipid content of the fat body decreased by 100 μg, indicating transfer of lipids from fat body to oocytes. Experiments in which fat body lipids were prelabelled support this conclusion. Label was transferred to oocytes: in mature oocytes the specific radioactivity of lipids was 80% of the specific radioactivity of prelabeled fat body lipids. Components of blood meals are also used to synthesize oocyte lipids. Fat bodies of females starved for four days had only 27 μg of lipids left. When these females were given a blood meal, they matured oocytes, although the number of ooyctes was reduced and ovaries contained only half the amount of lipids found in ovaries of females which had first fed on sugar water. Fat body lipids of these females had only slightly increased to 36 μg. This demonstrates that female Ae. aegypti use sugar to synthesize lipids, but they can also use components of blood for this purpose.
Several studies have focused on understanding the biochemistry and morphology of the fat body of the hematophagous mosquito Aedes aegypti (L.) (Diptera: Culicidae). In contrast, few studies, if any, have focused on morphological characters of the fat body in other mosquitoes, especially non-hematophagous taxa such as the culicid Toxorhynchites. Larvae of Toxorhynchites prey upon the larvae of other mosquito species and are used in vector mosquito control. We investigated aspects of the fat body trophocytes, including the morphometric analyses of the lipid droplets, protein granules and nuclei, during Toxorhynchites theobaldi (Dyar & Knab) post-embryonic development. Following the body weight increase from larval stage L2 to L4, the size of lipid droplets within the trophocytes also increase, and are likely the result of lipogenesis. Lipid droplets decrease in size during L4 to the female pupal stage and increase once again during the period from newly-emerged to mature adult females. Protein granules are observed for the first time in female pupae, and their appearance might be related to protein storage during metamorphosis. The size of the nucleus of trophocytes also increases during larval development, followed by a decrease during metamorphosis and an additional increase as adult female ages. In conclusion, the morphology of the fat body of T. theobaldi changes according to the developmental stage. Our study provides for the first time important insights into T. theobaldi fat body development and contributes to understand this species biology.
Anopheles gambiae mates in flight at particular mating sites over specific landmarks known as swarm markers. The swarms are composed of males; females typically approach a swarm, and leave in copula. This mating aggregation looks like a lek, but appears to lack the component of female choice. To investigate the possible mechanisms promoting the evolution of swarming in this mosquito species, we looked at the variation in mating success between swarms and discussed the factors that structure it in light of the three major lekking models, known as the female preference model, the hotspot model, and the hotshot model.
We found substantial variation in swarm size and in mating success between swarms. A strong correlation between swarm size and mating success was observed, and consistent with the hotspot model of lek formation, the per capita mating success of individual males did not increase with swarm size. For the spatial distribution of swarms, our results revealed that some display sites were more attractive to both males and females and that females were more attracted to large swarms. While the swarm markers we recognize help us in localizing swarms, they did not account for the variation in swarm size or in the swarm mating success, suggesting that mosquitoes probably are attracted to these markers, but also perceive and respond to other aspects of the swarming site.
Characterizing the mating system of a species helps understand how this species has evolved and how selective pressures operate on male and female traits. The current study looked at male mating success of An. gambiae and discussed possible factors that account for its variation. We found that swarms of An. gambiae conform to the hotspot model of lek formation. But because swarms may lack the female choice component, we propose that the An. gambiae mating system is a lek-like system that incorporates characteristics pertaining to other mating systems such as scramble mating competition.
In the Sahel, the Anopheles gambiae complex consists of Anopheles arabiensis and the M and S molecular forms of A. gambiae sensu stricto. However, the composition of these malaria vectors varies spatially and temporally throughout the region and is thought to be linked to environmental factors such as rainfall, larval site characteristics and duration of the dry season. To examine possible physiological divergence between these taxa, we measured metabolic rates of mosquitoes during the wet season in a Sahelian village in Mali. To our knowledge, this study provides the first measurements of metabolic rates of A. gambiae and A. arabiensis in the field. The mean metabolic rate of A. arabiensis was higher than that of M-form A. gambiae when accounting for the effects of female gonotrophic status, temperature and flight activity. However, after accounting for their difference in body size, no significant difference in metabolic rate was found between these two species (whilst all other factors were found to be significant). Thus, body size may be a key character that has diverged in response to ecological differences between these two species. Alternatively, these species may display additional differences in metabolic rate only during the dry season. Overall, our results indicate that changes in behavior and feeding activity provide an effective mechanism for mosquitoes to reduce their metabolic rate, and provide insight into the possible strategies employed by aestivating individuals during the dry season. We hypothesize that female mosquitoes switch to sugar feeding while in dormancy because of elevated metabolism associated with blood digestion.
An improved knowledge of mosquito life history could strengthen malaria vector control efforts that primarily focus on killing mosquitoes indoors using insecticide treated nets and indoor residual spraying. Natural sugar sources, usually floral nectars of plants, are a primary energy resource for adult mosquitoes but their role in regulating the dynamics of mosquito populations is unclear. To determine how the sugar availability impacts Anopheles sergentii populations, mark-release-recapture studies were conducted in two oases in Israel with either absence or presence of the local primary sugar source, flowering Acacia raddiana trees. Compared with population estimates from the sugar-rich oasis, An. sergentii in the sugar-poor oasis showed smaller population size (37,494 vs. 85,595), lower survival rates (0.72 vs. 0.93), and prolonged gonotrophic cycles (3.33 vs. 2.36 days). The estimated number of females older than the extrinsic incubation period of malaria (10 days) in the sugar rich site was 4 times greater than in the sugar poor site. Sugar feeding detected in mosquito guts in the sugar-rich site was significantly higher (73%) than in the sugar-poor site (48%). In contrast, plant tissue feeding (poor quality sugar source) in the sugar-rich habitat was much less (0.3%) than in the sugar-poor site (30%). More important, the estimated vectorial capacity, a standard measure of malaria transmission potential, was more than 250-fold higher in the sugar-rich oasis than that in the sugar-poor site. Our results convincingly show that the availability of sugar sources in the local environment is a major determinant regulating the dynamics of mosquito populations and their vector potential, suggesting that control interventions targeting sugar-feeding mosquitoes pose a promising tactic for combating transmission of malaria parasites and other pathogens.
Our research tests the hypothesis that the inability to sugar-feed reduces the insemination rate in mosquito populations. To test this, we measured the effects of sugar availability on cumulative insemination performance of male Anopheles gambiae Giles s.s. (Diptera: Culicidae) during 10-d periods of continual emergence of equal numbers of both sexes, and we evaluated the implications at the population level with a matrix population model. On each day of each of four replicates, 20 newly emerged mosquitoes of each sex were recruited into the populations within two mesocosms, large walk-in enclosures with simulated natural conditions. Each mesocosm contained a cage to replicate the experiment on a small scale. Scented sucrose was absent or present (control). A human host was available nightly as a bloodmeal source in both mesocosms. Sugar availability and enclosure size significantly influenced female insemination. In the mesocosms, with sugar 49.7% of the females were inseminated, compared with 10.9% of the females without sugar. In the small cages, the insemination rates were 76.0 and 23.5%, respectively. In the mesocosms, cumulative survival of females after 10 d was 51.6% with sugar and 25.6% without sugar. In the cages, female survival was 95 and 73%, respectively. Sensitivity analysis of the population projection matrix shows that both reduced male survival and reduced mating capability due to a lack of sugar contributed to lower insemination rates in females, and in the absence of sugar the insemination rate was lowered to an extent that led to population decline.
Anopheles gambiae, the major malaria vector in Africa, can be divided into two subgroups based on genetic and ecological criteria. These two subgroups, termed the M and S molecular forms, are believed to be incipient species. Although they display differences in the ecological niches they occupy in the field, they are often sympatric and readily hybridize in the laboratory to produce viable and fertile offspring. Evidence for assortative mating in the field was recently reported, but the underlying mechanisms awaited discovery. We studied swarming behaviour of the molecular forms and investigated the role of swarm segregation in mediating assortative mating. Molecular identification of 1145 males collected from 68 swarms in Donéguébougou, Mali, over 2 years revealed a strict pattern of spatial segregation, resulting in almost exclusively monotypic swarms with respect to molecular form. We found evidence of clustering of swarms composed of individuals of a single molecular form within the village. Tethered M and S females were introduced into natural swarms of the M form to verify the existence of possible mate recognition operating within-swarm. Both M and S females were inseminated regardless of their form under these conditions, suggesting no within-mate recognition. We argue that our results provide evidence that swarm spatial segregation strongly contributes to reproductive isolation between the molecular forms in Mali. However this does not exclude the possibility of additional mate recognition operating across the range distribution of the forms. We discuss the importance of spatial segregation in the context of possible geographic variation in mechanisms of reproductive isolation.
Anopheles gambiae plant-sugar feeding was thought to be rare and physiologically optional. Unlike adult females, males have no alternative source of energy and soon die with only water, yet they might be competent to inseminate all females within their brief lifespan. This study was designed to detect sugar's effect, if any, on male performance.
Males with and without 20% sucrose were evaluated at two body sizes and two temperatures, 23 degrees and 27 degrees C. Survival was recorded twice daily, and sexual behaviour was recorded each night after adult emergence. Insemination at a 2:1 male:female ratio was examined in three cage sizes, including walk-in mesocosms.
Without sugar, males of both sizes lived longer at 23 degrees than 27 degrees C, and large males lived longer at each temperature. Survival of large males at low temperature averaged 3.7 days, small males at high temperature, 1.9 days. With sugar, males in all four treatments suffered minimal mortality. With sugar, in small cages, large males at 27 degrees C matured most rapidly. A few erected fibrillae and inseminated females on night 1. On night 2, maximal proportions erected fibrillae and swarmed, and over one-third of females became inseminated. Small sugar-fed males at 23 degrees C matured most slowly but had achieved nearly maximal levels of swarming by night 3. By night 5, small males had inseminated more than half the females, and large males had inseminated nearly all of them. Without sugar, large males progressed similarly during the first two nights. On night 3, however, the proportion erecting fibrillae and swarming declined precipitously at 27 degrees C, and to a lesser degree at 23 degrees C. Cumulative insemination never reached high levels. Small males never achieved high levels of fibrillar erection or swarming and inseminated few females, even at 23 degrees C. In larger cages and under more semi-natural conditions, regardless of body size, without sugar male insemination capacity was virtually nonexistent.
Under some conditions, a limited number of sugar-deprived males can survive long enough to inseminate females. However, in nature males that cannot obtain sugar at frequent intervals will not be competitive with those that can, suggesting that male performance is closely tied to plant communities.
Mating in Anopheles gambiae has been observed only in outdoor swarms. Here we evaluate whether mating also occurs indoors. Mark-release-recapture of virgin males and females in natural houses showed that mating occurred over a single day even when mosquitoes can leave the house through exit traps and without adaptation to laboratory conditions. In these experiments, insemination rate in the M molecular form of An. gambiae (and An. arabiensis) was higher than that of the S form (15 versus 6%). Under these conditions, smaller females of the M form mated more frequently than larger females of that form. Sampling mosquitoes throughout the day showed that both sexes enter houses around sunrise and leave around sunset, staying indoors together from dawn to dusk. In an area dominated by the M form, the daily rate of insemination in samples from exit traps was approximately 5% higher than in those from entry traps, implying that mating occurred indoors. Importantly, frequency of cross mating between the molecular forms was as high as that between members of the same form, indicating that, indoors, assortative mating breaks down. Altogether, these results suggest that indoor mating is an alternative mating strategy of the M molecular form of An. gambiae. Because naturally occurring mating couples have not yet been observed indoors, this conclusion awaits validation.
For malaria control, the utility of transgenic vector Anopheles mosquitoes (Diptera: Culicidae) refractory to Plasmodium transmission, will depend on their interbreeding with the wild vector population. In many species, larger males are more successful in obtaining mates. In São Tomé island, we determined that size did not affect mating success of male Anopheles gambiae Giles sensu stricto, the main malaria vector in tropical Africa. Also we showed that larval intraspecific competition is probably insignificant in this population of An. gambiae. Thus, the potential success of transgenic An. gambiae is unlikely to be affected by size selection under field conditions.
Potential applications for reducing transmission of mosquito-borne diseases by releasing genetically modified mosquitoes have been proposed, and mosquitoes are being created with such an application in mind in several laboratories. The use of the sterile insect technique (SIT) provides a safe programme in which production, release and mating competitiveness questions related to mass-reared genetically modified mosquitoes could be answered. It also provides a reversible effect that would be difficult to accomplish with gene introgression approaches. Could new technologies, including recombinant DNA techniques, have improved the success of previous mosquito releases? Criteria for an acceptable transgenic sterile mosquito are described, and the characteristics of radiation-induced sterility are compared with that of current transgenic approaches. We argue that SIT using transgenic material would provide an essentially safe and efficacious foundation for other possible approaches that are more ambitious.
A longitudinal study based on mosquitoes sampled by larvae prospecting and adult catches on humans was carried out during 1999 in the rice field area of the Kou Valley (South-West Burkina Faso) to evaluate the malaria transmission level. Two sites were studied: VK5 located in the rice field centre and VK7 in the periphery. Irrigation is sub-permanent and two crops are grown each year: from February to June during the dry season and from July to November during the rainy season. A man sleeping in VK5 without any protection is exposed to more than 60,000 mosquito bites/year. Two majors vectors are present: Anopheles gambiae sl. and An. funestus. The An. gambiae M form which breeds in rice fields constitutes the majority of the complex. At the periphery of the rice fields, we observed a mix of M and S forms during the rainy season, the latter form coming from classical breeding sites created by rainfall (residual puddles). An. arabiensis is rare in this environment while we can find it in sympathy with An. gambiae in the surrounding savannah. An. gambiae remains present throughout the year. Its dynamics depends closely on both season and rice cycle. There are two density peaks, in March (210 bites/man/night) and in July (306 b/m/n), corresponding to the moment when rice is planted. The latter peak is more important due to additional productivity of residual puddles created by rainfall. These large and very young populations can not transmit malaria. Growing rice reduces larvae productivity: transmission occurs when the adult population decreases and becomes older. Variations in parity rate and sporozoitic index are inversely related to mosquito density. Finally, the An. gambiae population is lowest during the dry season in January when irrigation stops. An. funestus does not develop in rice fields but rather in drains made of dirt, in addition to the classical natural breeding sites at the end of the rainy season. The density of An. funestus is 25 to 30 times smaller than that of A. gambiae, as observed during 8 months in VK5 and 10 months in VK7, with a peak of 16 b/m/n in January. Its mean parity rate and sporozoitic index is higher than those of An. gambiae. An. funestus plays an additional role in malaria transmission at the same period as An. gambiae but more shortly and less intensively. Finally, malaria transmission is due mainly to An. gambiae (90%) and, to a lesser extent, to An. funestus (10%). It occurs in VK5 during two periods of four month search : from May to August and from November to February. In VK7, transmission is also bimodal but it occurs more extensively throughout the year with only July and Septemberas exemptions of transmission. The annual inoculation rate amounts to 697 and 515 infected bites per man. This value is higher than those obtained in 1984/1985 : 50 and 60 infected bites per man per year. This increase is due to both the growth of aggressive vectorial density and the sporozoitic index. Growth of adult density can be explained by the environmental modification and/or better adaptation of vectorial species. The sporozoitic index of vectors has been obtained using Elisa, which is twice as sensitive as the microscopic observations used in 1984/1985. This can partly explain the increase in the sporozoitic index. Other hypotheses in relation to human parasitological inputs have to be evaluated more accurately. The level and rhythm of transmission in the rice field in 1999 are more important than those registered in the surrounding savannah the same year. However, this important transmission is not proportionally related to the high vectorial density found in rice fields. Probably, it does not imply more malaria diseases, but its intensity and rhythm could concentrate malaria cases in young people with an early acquisition of premunition. Pyrethroid impregnated bednets should be used to prevent malaria especially since high mosquito nuisance drives local populations to use it and since the An. gambiae M form is particularly susceptible to this class of insecticides.
Evidence continues to accumulate showing that the malaria parasites (Plasmodium spp.) reduce the survival and fecundity of their mosquito vectors (Anopheles spp.). Our ability to identify the possible epidemiological and evolutionary consequences of these parasite-induced fitness reductions has been hampered by a poor understanding of the physiological basis of these shifts. Here, we explore whether the reductions in fecundity and longevity are the result of a parasite-mediated depletion or reallocation of the energetic resources of the mosquito. Mosquitoes infected with Plasmodium chabaudi were expected to have less energetic resources than uninfected mosquitoes, and energy levels were predicted to be lowest in mosquitoes infected with the most virulent parasite genotypes. Not only was there no evidence of a parasite-mediated reduction in the overall energetic budget of mosquitoes, but Plasmodium was actually associated with increased levels of glucose, a key insect nutritional and energetic resource. The data strongly suggest the existence of an increase in sugar feeding in mosquitoes infected with Plasmodium. We suggest different adaptive explanations for an enhanced sugar uptake in infected mosquitoes and call for more studies to investigate the physiological role of glucose in the Plasmodium-mosquito interaction.
Charles Darwin's theory of natural selection has at its focal point the mating success of organisms. Among male animals, large body size is widely seen as the principal determinant of mating success. However, where mating takes place in a three-dimensional arena such as water, the arboreal habitat or air, small size with its concomitant aerobatic advantages might be advantageous. Despite considerable interest, the relationship between aerobatic ability and mating success has not yet been demonstrated in a single animal species. Here, we test the hypothesis that the known mating success of small male midges is due to their greater aerobatic ability. To do this, male midges collected from leks in the wild were flown and their flight paths in free flight were recorded on high-speed cameras in the laboratory. Four flight parameters that would seem relevant to male mate acquisition in flight, i.e. acceleration, maximum speed, tortuosity and turn-rate, were analysed with respect to body size. We show that, while in terms of maximum speed there was no detectable difference between small and large males, small males outperformed larger ones with respect to acceleration, tortuosity and turn-rate. We conclude that the hypothesis that small males gain their mating advantage through aerobatic superiority is consistent with the observations reported here.
We compared the development of the molecular forms of Anopheles gambiae s.s. in different larval habitats. First stage larvae (L1s) of wild-caught females were placed into cages in natural habitats of the M form (rice fields) or the S form (puddles/ quarries). Each cage was covered with cloth, allowing exchange of water, solutes, and small particles, including microorganisms, and was seeded with 100 L1s of a single form (M or S) or by a mixture of 50:50 of M and S forms. Emergence success of both forms in puddles and quarries was three-fold higher than in the rice fields. The emergence rate of the S form was higher than that of the M form in both habitats, but the form x habitat interaction was not significant. In temporary larval sites such as puddles, emergence success of the M form was lower in mixed cages than in single form cages, whereas the reverse was true for the S form, suggesting competition between the forms. The median developmental time was not significantly different between forms. Although these findings demonstrate differences between forms, they do not suggest that their spatial segregation is determined by differences in their exploitation of the physical and chemical conditions in these environments. These results should be regarded with caution because small numbers of first stage larvae could pass through the cloth of the cages.
The success of sterile or transgenic Anopheles for malaria control depends on their mating competitiveness within wild populations. Current evidence suggests that transgenic mosquitoes have reduced fitness. One means of compensating for this fitness deficit would be to identify environmental conditions that increase their mating competitiveness, and incorporate them into laboratory rearing regimes.
Anopheles gambiae larvae were allocated to three crowding treatments with the same food input per larva. Emerged males were competed against one another for access to females, and their corresponding longevity and energetic reserves measured.
Males from the low-crowding treatment were much more likely to acquire the first mating. They won the first female approximately 11 times more often than those from the high-crowding treatment (Odds ratio = 11.17) and four times more often than those from the medium-crowding treatment (Odds ratio = 3.51). However, there was no overall difference in the total number of matings acquired by males from different treatments (p = 0.08). The survival of males from the low crowding treatment was lower than those from other treatments. The body size and teneral reserves of adult males did not differ between crowding treatments, but larger males were more likely to acquire mates than small individuals.
Larval crowding and body size have strong, independent effects on the mating competitiveness of adult male An. gambiae. Thus manipulation of larval crowding during mass rearing could provide a simple technique for boosting the competitiveness of sterile or transgenic male mosquitoes prior to release.
Volatile odors are important sensory inputs that shape the behaviour of insects, including agricultural pests and disease vectors. Anopheles gambiae s.s. is a highly anthropophilic mosquito and is the major vector for human malaria in sub-Sahara Africa, while Anopheles quadriannulatus, largely due to its zoophilic behaviour, is considered a non-vector species in the same region. Careful studies of olfaction in these sibling species may lead to insights about the mechanisms that drive host preference behaviour. In the present study, the external anatomy of the antenna, the principle olfactory organ in the female mosquito of both species, was examined as an initial step toward more detailed comparisons.
Scanning electron and light microscopy were used to examine the antennae ultrastructures of adult female An. gambiae s.s. and An. quadriannulatus. Sensory structures, called sensilla, were categorized and counted; their distributions are reported here as well as densities calculated for each species.
Both An. gambiae s.s. and An. quadriannulatus bear five classes of sensilla on their antennae: chaetica (bristles), trichodea (hairs), basiconica (pegs), coeloconica (pitted pegs), and ampullacea (pegs in tubes). Female An. quadriannulatus antennae have approximately one-third more sensilla, and a proportionally larger surface area, than female An. gambiae s.s. antennae.
The same types of sensilla are found on the antennae of both species. While An. quadriannulatus has greater numbers of each sensilla type, sensilla densities are very similar for each species, suggesting that other factors may be more important to such olfactory-driven behaviours as host preference.
The M and S molecular forms of Anopheles gambiae sensu stricto Giles are thought to be reproductively isolated through premating barriers. However, the exact mechanisms of recognition of conspecific partners are unknown. Because mating in An. gambiae occurs in swarms, one might expect swarming behavior between the M and S forms to be different and that this probably reduces the risk of contact between males and females of the different forms in areas where they are sympatric. We report the occurrence of four mixed swarms, containing males of M and S forms, out of a total of 26 swarms sampled in Soumousso, a typical savannah village of Burkina Faso, West Africa. However, the frequency of mixed swarms was lower than that expected by chance. This observation suggests partial segregation between the swarms of the molecular forms, which may contribute to their isolation. Because the frequency of mixed swarms seems too high to explain the low frequency of cross-mating and hybrids, we suggest that mate recognition in a swarm is more important than swarm segregation.
Release of genetically-modified (GM) or sterile male mosquitoes for malaria control is hampered by inability to assess the age and mating history of free-living male Anopheles.
Age and mating-related changes in the reproductive system of male Anopheles gambiae were quantified and used to fit predictive statistical models. These models, based on numbers of spermatocysts, relative size of sperm reservoir and presence/absence of a clear area around the accessory gland, were evaluated using an independent sample of mosquitoes whose status was blinded during the experiment.
The number of spermatocysts in male testes decreased with age, and the relative size of their sperm reservoir increased. The presence of a clear area around accessory glands was also linked to age and mating status. A quantitative model was able to categorize males from the blind trial into age groups of young (< or = 4 days) and old (> 4 days) with an overall efficiency of 89%. Using the parameters of this model, a simple table was compiled that can be used to predict male age. In contrast, mating history could not be reliably assessed as virgins could not be distinguished from mated males.
Simple assessment of a few morphological traits which are easily collected in the field allows accurate age-grading of male An. gambiae. This simple, yet robust, model enables evaluation of demographic patterns and mortality in wild and released males in populations targeted by GM or sterile male-based control programmes.
For Anopheline mosquitoes, the vectors of human malaria, genetic variation in male reproductive success can have important consequences for any control strategy based on the release of transgenic or sterile males.
A quantitative genetics approach was used to test whether there was a genetic component to variation in male reproductive success in a laboratory population of Anopheles gambiae. Swarms of full sibling brothers were mated with a fixed number of females and their reproductive success was measured as (1) proportion of ovipositing females, (2) proportion of ovipositing females that produced larvae, (3) proportion of females that produced larvae, (4) number of eggs laid per female, (5) number of larvae per ovipositing female and (6) number of larvae per female.
The proportion of ovipositing females (trait 1) and the proportion of ovipositing females that produced larvae (trait 2) differed among full sib families, suggesting a genetic basis of mating success. In contrast, the other measures of male reproductive success showed little variation due to the full sib families, as their variation are probably mostly due to differences among females. While age at emergence and wing length of the males were also heritable, they were not associated with reproductive success. Larger females produced more eggs, but males did not prefer such partners.
The first study to quantify genetic variation for male reproductive success in A. gambiae found that while the initial stages of male reproduction (i.e. the proportion of ovipositing females and the proportion of ovipositing females that produced larvae) had a genetic basis, the overall reproductive success (i.e. the mean number of larvae per female) did not.
The great importance of mosquitoes lies in their role as transmitters of pathogens and parasites, and in their use as experimental animals well suited to laboratory investigations into aspects of biochemistry, physiology and behaviour. The largest part of this latest volume of The Biology of Mosquitoes concerns interactions between mosquitoes and viruses and the transmission of arboviruses to their vertebrate hosts, while the remainder concerns symbiotic interactions between mosquitoes and bacteria. The introduction provides a timely review of the first major development in mosquito taxonomy for several decades. Further chapters describe the interactions between mosquitoes and the viruses that infect them, the transmission and epidemiology of seven very important arboviruses, and the biology of bacteria that are important control agents or of great biological interest. Like the earlier volumes, Volume 3 combines recent information with earlier important findings from field and laboratory to provide the broadest coverage available on the subject.
Males and females of Aedes sollicitans and Aedes taeniorhynchus were flown on a flight mill and analysed for duration and distance of flight, and for utilization of triglycerides, glycogen, and trehalose. Unfed, blood-fed, and glucose-fed mosquitoes were flown either to exhaustion or for 1 hr.Triglycerides and other lipids were not used for flight. Utilization of carbohydrate was 0·08 to 0·10 cal/1000 m or 27 to 37 cal/hr per g, whether calculated from the utilization of glycogen (in unfed and blood-fed mosquitoes) or from the production of carbon dioxide (in glucose-U-14C fed mosquitoes). The maximum metabolic rate during flight was four to five times that of non-fed controls. In sugar-fed mosquitoes, glycogen and triglycerides accumulated during flight. Glycogen was not utilized as a flight substrate as long as sugar was available. Starved mosquitoes and mosquitoes flown to exhaustion could resume vigorous flight immediately after a sugar meal. Trehalose did not change during vigorous flight and made a negligible contribution to exhaustive flight. In unfed mosquitoes, the flight following a rest period after exhaustion was sustained by residual glycogen; no increase in glycogen took place while resting. The significance of these results to migration and dispersal in mosquitoes is discussed.
The objective of this study was to determine, in the field, the energetic costs of swarming for male Anopheles freeborni (Diptera: Culicidae). By comparing the caloric contents of resting males to marked males captured after swarming, we established when sugar feeding takes place, what energy source is used to fuel swarming flight, and how much energy is invested in this activity.
Sugar‐feeding takes place sometime during the night after swarming is concluded. Nectar sugars are therefore not immediately available to fuel flight. Stored sugars (trehalose and glucose) and glycogen are the primary sources of energy for flight. Lipids are not used to fuel flight but may be used in resting metabolism.
Male size is not related to feeding success. For males of all sizes, swarming consumes more than 50% of available calories. Accordingly, the ability of an individual to find and exploit nectar sources will greatly affect reproductive success.
1. The mating system of Mediterranean fruit flies ( Ceratitis capitata , Diptera: Tephritidae), is based on male leks that form on the foliage of trees. Following observations that not all males participate in leks, the hypotheses that (a) small males are absent from leks and (b) males with inadequate nutrient reserves are unable to participate in leks were examined.
2. The size and weight of lekking males ( n = 183) and resting males captured at the same time in the vicinity of leks ( n = 148) were established. In addition, using biochemical techniques, the amounts of sugar, glycogen, lipid and protein in each individual were established quantitatively.
3. There was no significant size difference between lekking and resting males. However, lekking males were significantly heavier and contained significantly more sugars and protein than resting males.
4. In conclusion, leks are exclusive, and only males with adequate nutritional reserves may join. Thus, in this species, reproductive success is closely linked to foraging success.
We examined whether body size affects the swarming behavior and mating success of male Anopheles freeborninear California rice fields. Swarms formed after dusk and persisted for approximately 30 min. The proportion of males in 33 swarms sampled n=6028
ranged from 100 to 92% but decreased over time (r=0.73, t=6.03, POn average, swarming males (n=1058) were larger than males sampled from the resting population (n=735, H=35.6, Pindicating that some males never swarm at all. Males swarming early were significantly smaller than those swarming during the peak (H=6.71, P=0.009)or final minutes of the swarm (H=4.86, P=0.002). Mated males returned to the swarm after mating, and larger males enjoyed greater mating success than did smaller ones (n=398, H=16.1, P=0.0005).
Male Chironomus plumosus most successful at acquiring mates showed lower levels of fluctuating asymmetry in length of wing than their rivals. These males were not of the most common size, but were the smallest in the population. These results are consistent with the prediction that where there is directional selection for small individuals, fluctuating asymmetry will be positively correlated with size. However, for a species that mates on the wing, selection may act upon symmetry per se rather than body size. Uncoupling the effects of size from those of asymmetry suggested that fluctuating asymmetry might, on its own, account for the observed mating success of C. plumosus. It is suggested that the success of the more symmetrical males is due to their improved aerobatic ability. Chironomus plumosus provides an example of the importance of fluctuating asymmetry in male characters that are not purely ornamental and these results are therefore more readily interpreted in terms of natural rather than sexual selection.
One of the greatest paradoxes in evolutionary biology is the continued maintenance of genetic variation for phenotypic traits that appear to confer strong fitness advantages. Of these traits, body size is perhaps the one that has been most consistently linked to increased longevity and reproductive success in males. We investigated two hypotheses for how events occurring during mating in the mosquito Anopheles gambiae could impede directional selection on male body size: (1) whether male copulation probability is random with respect to body size in aerial swarms, and (2) whether large males are less competitive during mating than smaller, shorter-lived rivals. By manipulation of larval nutritional conditions (low, intermediate and high food allocation), we generated cohorts of male A. gambiae mosquitoes that differed in adult body size and energy reserves (body size and energy reserves being positively correlated with larval nutrition). When competing against one another in aerial swarms, males from the intermediate food treatment were six and two times more successful at acquiring mates than those from the high and low food treatments, respectively. The median survival of males from this most sexually competitive group was approximately 13% lower than that of the larger males with high larval nutrition. We conclude that phenotypic determinants of long-term survival and mating success may not be correlated in this system, and thus that stabilizing selection as well as environmental condition-dependent expression of traits could account for the maintenance of variation in male body size in this species and in other swarming insects.
Sugar feeding by male mosquitoes is critical for their success in mating competition. However, the facets of sugar source finding under natural conditions remain unknown. Here, evidence obtained in Western Burkina Faso indicated that the distribution of An. gambiae s.s. (M and S molecular forms) males across different peri-domestic habitats is dependent on the availability of potential sugar sources from which they obtain more favorable sites for feeding or resting. Among field-collected anophelines, a higher proportion of specimens containing fructose were found on flowering Mangifera indica (Anacardiaceae), Dolonix regia (Fabaceae), Thevetia neriifolia (Apocynaceae), Senna siamea, and Cassia sieberiana (both Fabaceae) compared to that recorded on other nearby plants, suggesting that some plants are favored for use as a sugar source over others. Y-tube olfactometer assays with newly-emerged An. gambiae s.s. exposed to odors from individual plants and some combinations thereof showed that males use odor cues to guide their preference. The number of sugar-positive males was variable in a no-choice cage assay, consistent with the olfactory response patterns towards corresponding odor stimuli. These experiments provide the first evidence both in field and laboratory conditions for previously unstudied interactions between males of An. gambiae and natural sugar sources.
In four Anopheles species, An. albimanus, An. gambiae, An. stephensi, and An. quadrimaculatus Say, total protein, lipid, and carbohydrate present at eclosion, after feeding on sucrose, and after extreme starvation were quantified to study the effect of teneral and maximal reserves on subsequent fecundity and to judge the extent of reserve mobilization and the minimal irreducible amounts required for survival. All parameters were regressed on body size, presented as the cubic value of wing length. Teneral reserves were isometric with body size, were considerably lower than previously reported for Aedes aegypti (L.) and were sexually dimorphic with respect to reserves and body size, all being slightly reduced in males. On the average, up to 70% of the teneral female lipids and up to 50% of their teneral protein could be mobilized during nutritive stress. Conversely, access to sucrose for a few days led to a pronounced glycogenesis (up to 509%) and lipogenesis (up to 450% of the teneral values), depending on the species. In absolute terms, lipogenesis prevailed over glycogenesis. On a caloric basis, up to 30% of the blood meal protein was utilized for synthesis of yolk protein and lipid, and another 15% was deposited as an extra-ovarian, maternal protein and lipid store, perhaps compensating for the limited teneral reserves. A complete nitrogen budget revealed that in a given class of body size, roughly 80% of the blood meal protein was catabolized and excreted through the three major pathways of uricotely, ureotely, and ammonotely. Quantification of the hematin in fecal samples allowed a stoichiometrical determination of the amount of blood ingested. Eco-physiological aspects of larval feeding, teneral reserves, blood meal utilization, and possible behavioral adaptations to these physiological constraints are discussed and compared with previous data on culicine mosquitoes, stressing the invalidity of generalizations among these taxa.
Total lipids in individual mosquitoes can be determined by extraction with chloroform-methanol followed by reaction with sulfuric acid and a vanillin-phosphoric acid reagent. By subtracting the lipids determined in a starved population, the method is suitable to establish variations of lipid reserves in field populations.
A method is presented for rapid determination of glycogen and sugars in individual mosquitoes. It is suitable for large scale field studies of correlations between nutritional status and behavior. The method also describes a one step procedure for total carbohydrates and the determination of nectar sugars and the hemolymph sugar trehalose.
Sugar feeding is a fundamental characteristic of mosquito life. Most evidence indicates frequent ingestion by both sexes and all ages of mosquitoes of plant sugar, usually as floral and extrafloral nectar and honeydew. Energetically, sugar and blood are interchangeable; females of some species have evolved independence from one or the other, but most need blood to develop eggs and sugar to survive, to fly, and to enhance reproduction. Mosquitoes' commitment to sugar is further illustrated by a wealth of behavioral, structural, and physiological specializations for finding, feeding on, and processing it. Blood and sugar feeding activities are antagonistic and mutually exclusive, owing to conflicting demands, yet they support the same goals and often share the same activity period. The rules by which females make food-choice decisions have been inadequately explored, and we still lack convincing evidence that sugar availability in nature varies sufficiently to affect mosquito populations.
Female Aedes aegypti of small and large body sizes were fed ad libitum from eclosion with eight different concentrations of sucrose from 0.1% to 50%; females were also starved with access to water. For each experiment we determined the survivorship of such populations. The 90%, 50%, 10%, and maximal survivorships followed linear regressions with the logarithm of the sucrose concentration. For each condition we measured the extent of synthesis of glycogen and lipid reserves. There was a critical sucrose concentration of 0.5% for both size classes: lower concentrations were of no nutritive effect, and all higher concentrations extended survivorship and allowed reserve synthesis. With respect to the teneral value, and normalized for body size, the maximal amounts of glycogen increased 2-3-fold within one week, whereas lipogenesis increased 3-5-fold requiring two weeks. Solid sugar cubes could also be utilized as long as drinking water was available, but synthesis of additional reserves failed. Flight mill experiments revealed the temporal flight pattern, its maturation after eclosion, and the maximal flight performances. Flights shorter than 1000 m per female per night were considered as low activities, whereas flights lower than 1000 m represented strong vigorous flights. Maximal distances were from 11-18 km/female/night. Periods of continuous flights lasted between 2-9 hr per female (mean 2.2 hr). Maximal flight performances were gradually reached within the first and third day of eclosion. Mean caloric energy consumption during flight was 33% to 44% of the pre-flight glycogen, accompanied by lipid reductions of 9%. Evidently, feeding on carbohydrates allows extended flight activities of this species and is essential for survival in the absence of blood meals.
Female Anopheles gambiae were more likely to oviposit when they mated with males that were two days old than when they mated with older males. The females' age at the time of mating and blood feeding did not influence ovipositional patterns. Females were no less likely to oviposit when they mated after blood feeding and maturing eggs. Sperm were present in the seminal vesicles of males 28 hours after eclosion, and their accessory glands were replete by 76-100 hours. The mean life span of sucrose-fed unmated females and males was 22 days.
The close association of Anopheles gambiae Giles with humans and its females' ability to live on human blood alone suggest that females may ignore sources of sugar in favour of human blood as a source of energy. They have limited energy reserves at emergence, and at 27 degrees C both sexes generally die if they do not feed during night 1, 24-36 h after emergence. Food preferences during this critical period were tested by measuring responses to volatiles from honey and soiled socks, which served as surrogates for nectar-related and human-related volatiles in a wind-tunnel olfactometer. Both sexes responded more strongly to honey than to human volatiles, and given a choice, preferred honey over human volatiles. After 5 days of sugar access and maturation, males continued to prefer honey volatiles, whereas females changed behaviour, responding almost exclusively to human volatiles. Night 1 experiments also demonstrated that: (i). females previously having had sugar during the night of emergence responded more strongly to human volatiles; (ii). large-bodied mosquitoes of both sexes responded more strongly to honey than small-bodied ones; and (iii). females were equally responsive to honey in both early and late scotophase but were slightly more responsive to human volatiles in late scotophase. These results indicate that for a female's first meal, sugar is a viable option and is preferred when nectar-related stimuli are strong. This supports field evidence that sugar-feeding is a significant component of A. gambiae female behaviour.
When exposed to a human host, Anopheles gambiae started probing 4 h post-eclosion, but 95% successfully blood-fed by 16-20 h with maximal blood volumes of 5- 10 microl per female. When fed sugar, the 95% feeding was not observed until 36-40 h post-eclosion; sugar meals appeared to interfere with blood meals. Similarly in An. atroparvus, maximum volumes were 10 microl when starved but only 6 microl when fed sugar. This species did not bite before 2 d, and 95% biting was by 4 d. Given single blood meals to water-kept An. gambiae, a threshold body size for oogenesis was detected. With wing lengths below 2.8 mm, eggs never matured, but when sugar-fed, females of all sizes matured eggs including the synthesis of maternal deposits. Although sugar feeding interfered with blood feeding, more lipid was transferred to the yolk. In water-kept An. atroparvus only 5% of the females produced eggs. When sugar-fed for 4 d, all females matured eggs, so in this species sugar feeding appeared to be essential for oogenesis. An. gambiae always took multiple blood meals, tested at any time after the first ones, leading to 120 mature eggs/female. Yolk composition was 3.9 mcal protein and 3.8 mcal lipid/oocyte when kept on water, but 2.8 meal protein and 4.3 mcal lipid/oocyte with intermittent sugar meals, thus marking a surprising flexibility in synthesis of yolk protein and lipid that strongly depends on additional carbohydrates sources. Only 80% of water-fed An. atroparvus re-fed 2 d after a first blood meal with small females taking three blood meals but they still showed reduced fecundity. Only the large water-fed females matured eggs, with blood volumes higher than 9-12 microl. When fed sugar, the blood meal input was reduced, but oogenesis was possible, whereas water-fed females required three blood meals to reach the caloric level comparable to pre-feeding sugar-fed females. Water-fedAn. gambiae could survive on daily blood meals alone, but survival was further extended by intermittent sugar meals. When offered a blood donor daily, there was a behavioral difference. Females maintained alone showed a more or less regular 3 d feeding and oviposition activity, while females kept in groups fed daily followed a daily oviposition pattern, suggesting gonotrophic discordance.
In Anopheles gambiae, as in most species of mosquitoes, mating is initiated in flight. The males aggregatein aerial swarms and conspecific females
individually fly to these swarms where they mate with males. In this study, we investigated the swarming behaviour of A. gambiae and conducted 2 surveys in the rice field area of the Vallée du Kou in Burkina Faso in 1999 and 2002. A high number of anopheline
mosquitoes were observed in this area and both molecular M and S forms of A. gambiae were found in sympatry. Swarms formed a few minutes after sunset in different places and no obvious markers were associated
with their occurrence. However, swarms occurred close to cow herds generally in open flat areas, 2–3 m above the ground. Overall,
2829 anopheline mosquitoes were collected from 21 swarms composed primarily of males. A few specimens of Culex quinquefasciatus were collected from 3 swarms. Although both molecular M and S forms were found in sympatry in the village, swarms were composed
almost exclusively of the molecular M form. This suggests that there are alternative swarming habits for both molecular M
and S forms of A. gambiae in nature.
The recent development of transgenic mosquitoes that are resistant to infection by the Plasmodium malarial parasite is a promising new tool in the fight against malaria. However, results of large-scale field releases of alternatively modified mosquitoes carried out during the 1970s and 1980s suggest that this approach could be difficult to implement in the field. These past attempts to control mosquito populations largely floundered as a result of our insufficient understanding of the behavioural ecology of released males. In spite of this, contemporary research on genetic control strategies has concentrated predominantly on molecular aspects, with little progress being made toward resolving key ecological uncertainties, male mosquito ecology being the most important. Here, we review the state of knowledge of male mosquito ecology, and highlight priorities for further research. Case studies of two crop pests, the Mediterranean fruit fly and melon fly, are given as examples of how knowledge of male ecology facilitates successful control in other species. Unless similar information becomes available for mosquitoes, any future genetic control strategy will risk failure.
Floral nectar sugars provide energy for mosquito activities. We presumed that scarce flowering trees in arid areas are attractive, central sugar sources and tested the assumption in the southern desert of Israel. In traps baited with flowers of Acacia raddiana, Tamarix nilotica or Ochradenus baccatus the catch of Anopheles sergentii was approximately 35-75 times greater than with baits of flowerless branches. At a small isolated oasis, a spray of sugar and food-dye solution on the few flowering A. raddiana trees dye-labelled 80-90% of A. sergentii. At a similar oasis, this spray with addition of oral insecticide, virtually eliminated the local mosquitoes.