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Age-Dependent Rates of Infection of Cassava Green Mites by a Fungal Pathogen in Brazil
Abstract
Age-specific effects of invertebrate pathogens on their hosts can greatly influence the population dynamics in such interactions. Explanations for such differences are usually sought within differing intrinsic susceptibilities of the host life stages but we present data which indicate that host size, behaviour and life history may be the overriding factors determining age-specific effects of a fungal pathogen, Neozygitesfloridana (Entomophthorales: Neozygitaceae) on spider mites (Mononychellus tanajoa Bondar, Acari: Tetranychidae). Epizootics of N. floridana in spider mites are characterised by much greater relative mortality of adult females compared with other life stages (ca. 99%), despite similar physiological susceptibilities. We present empirical data that demonstrate encounter rates of mites with N. floridana increasing with life stage during an epizootic on cassava in northeastern Brazil. Estimates of the size, walking speeds and patterns, and life history of different life stages (and adult sexes) were used to calculate expected relative encounter rates which were found not to be different from the observed values (although not testable for larvae). This helps explain the different apparent susceptibility of host life stages in the field. Given the low ecological susceptibility of younger life stages to this pathogen, we predict that the interaction time between host and pathogen, determined by climatic conditions, will be critical in determining the degree of host population control in an epizootic. We further hypothesise that such variation in ecological susceptibility to pathogens can generate selection pressures on basic host traits, contributing to the sessile nature of many microarthropods.
... The measured climatic data were used to estimate, for each sample date, the infection potential of N. tanajoae, expressed as the proportional mortality of pooled nymphs and adult females. Note that the great majority of infection of M. tanajoa occurs in these stages for reasons discussed by Elliot et al. (2002c). Calculations were made of the mortality expected from the introduction of 1, 5 or 25 mummies per 100 cm 2 leaf area (N.B. most leaves were only slightly larger than this and the range of the original regression was 0-28 mummies per 100 cm 2 ). ...
... There was a peak of M. tanajoa densities in May, at over 1,000 adult females and nymphs (the life stages most likely to be infected in the field; Elliot et al. 2002c) per 100 cm 2 on apical leaves and over 200 per 100 cm 2 on median leaves (Figs. 1a and 2a). ...
Monitoring of a population of the phytophagous cassava green mite, Mononychellus tanajoa (Bondar), and its natural enemies was undertaken in central Bahia, Brazil, in mid-1996. In spite of the presence of extremely high densities of the predatory phytoseiid mite Neoseiulus idaeus Denmark & Muma, the phytophagous mite population reached such high densities itself that there was total overexploitation of the cassava plants, leading to total leaf loss. Meanwhile, the mite-pathogenic fungus Neozygites tanajoae Delalibera, Humber & Hajek did not affect the M. tanajoa population in its growth phase as there was no inoculum present, even though we predict from a simple regression model that there was the potential for epizootics at that time. Soon after the M. tanajoa population crashed due to defoliation, there could have been an epizootic but there were simply no mite hosts to infect. These data demonstrate the ineffectiveness of one natural enemy (the predator) in terms of prey population regulation and demonstrate the importance of timing in the possible effectiveness of the other (the pathogen). For the pathogen, this probably explains its sporadic effect on host populations as previously reported. We conclude that the fungus is likely to be most useful as an adjunct to biological control with predatory mites other than N. idaeus.
... For example, sexually immature juveniles are unlikely to encounter sexually transmitted diseases, and we would therefore expect selection to favour a later developmental onset of resistance to these types of diseases. In many organisms, older individuals cover more territory and consume more food, increasing their exposure to ingesting orally transmitted pathogens (Elliot et al., 2002;Garbutt et al., 2014). In social animals, adults often have higher contact rates, and higher network connectivity than juveniles (Carter et al., 2013;Rimbach et al., 2015), potentially increasing the risk of directly transmitted diseases. ...
Juveniles are typically less resistant (more susceptible) to infectious disease than adults, and this difference in susceptibility can help fuel the spread of pathogens in age‐structured populations. However, evolutionary explanations for this variation in resistance across age remain to be tested.
One hypothesis is that natural selection has optimized resistance to peak at ages where disease exposure is greatest. A central assumption of this hypothesis is that hosts have the capacity to evolve resistance independently at different ages. This would mean that host populations have (a) standing genetic variation in resistance at both juvenile and adult stages, and (b) that this variation is not strongly correlated between age classes so that selection acting at one age does not produce a correlated response at the other age.
Here we evaluated the capacity of three wild plant species ( Silene latifolia , S. vulgaris and Dianthus pavonius ) to evolve resistance to their anther‐smut pathogens ( Microbotryum fungi), independently at different ages. The pathogen is pollinator transmitted, and thus exposure risk is considered to be highest at the adult flowering stage.
Within each species we grew families to different ages, inoculated individuals with anther smut, and evaluated the effects of age, family and their interaction on infection.
In two of the plant species, S. latifolia and D. pavonius , resistance to smut at the juvenile stage was not correlated with resistance to smut at the adult stage. In all three species, we show there are significant age × family interaction effects, indicating that age specificity of resistance varies among the plant families.
Synthesis . These results indicate that different mechanisms likely underlie resistance at juvenile and adult stages and support the hypothesis that resistance can evolve independently in response to differing selection pressures as hosts age. Taken together our results provide new insight into the structure of genetic variation in age‐dependent resistance in three well‐studied wild host–pathogen systems.
... The larvae of the Indian meal moth Plodia interpunctella, a commonly studied host organism in host-parasite interactions, suffers high mortality rates when infected by a granulosis virus, whereas adults do not appear to be infected at all 8 . Alternatively, some diseases only affect adults, such as in the high mortality of adult female cassava mites (Mononychelles tanajoa) infected with a fungal pathogen, Neozygites floridana, which leaves other life stages unharmed 9 . Host life stage may be an important variable when analyzing infection dynamics and host defense. ...
Host-parasite research often focuses on a single host life stage, yet different life stages may exhibit different defenses. The nematode Caenorhabditis elegans has an alternate dispersal life stage, dauer. Despite dauer’s importance in nature, we know little of how it responds to parasites. Previous research indicates that non-dauer C. elegans prefer to consume the virulent bacterial parasite, Serratia marcescens, when given a choice between the parasite and benign Escherichia coli. Here, we compared the preferences of dauer individuals from six strains of C. elegans to the preferences of other life stages. We found that dauer individuals exhibited reduced preference for S. marcescens, and dauers from some strains preferred E. coli to S. marcescens. In addition to testing food preference, a mechanism of parasite avoidance, we also measured host mortality rates after direct parasite exposure to determine if life stage also altered host survival. Overall, dauer individuals exhibited reduced mortality rates. However, dauer versus non-dauer larvae mortality rates also varied significantly by host strain. Collectively, we found evidence of dauer-induced parasite avoidance and reduced mortality in the presence of a parasite, but these effects were strain-specific. These results demonstrate the importance of host life stage and genotype when assessing infection dynamics.
... For instance, the larvae of the Indian meal moth Plodia interpunctella are highly susceptible to infection by a lethal granulosis virus, while adults do not appear to acquire infection (Sait et al. 1994, Boots 1998. In contrast, the fungus Neozygites floridana, a pathogen of cassava mites Monoychellus tanajoa, kills only adult females and spares other life stages (Elliot et al. 2002). In the confused flour beetle Tribolium confusum -protozoan parasite Gregarina minuta system, both larval and adult beetles can contract the parasites, but larvae support much higher parasite loads than adults, shed larger numbers of infectious protozoa, and suffer stage-specific development and mortality costs when infected (Detwiler andJanovy 2008, Thomas andRudolf 2010). ...
The immune response of a host can have important impacts on host-pathogen interactions, but investment in immunity often changes dynamically across the life history of a host. One form of investment involves the induction of a primed immune response against previously encountered pathogens that protects the host from re-infection. In addition to providing immediate protective effects, immune priming can also provide two types of ‘delayed’ protection against pathogens: priming across life stages (ontogenic priming) and priming across generations (trans-generational priming). Consequently both types of immune priming have the potential to mediate life history variability in host–pathogen interactions, which could have important consequences for disease prevalence and dynamics as well as for the demographic structure of the host population. Here we develop a stage-structured SIRS model for an invertebrate host to explore the relative and combined impact of ontogenic priming and trans-generational priming on infection prevalence, host population size, and population age structure. Our model predicts that both types of immune priming can dramatically reduce disease prevalence at equilibrium, but their individual and combined effects on population size and age structure depend on the magnitude of tradeoffs between immune protection and reproduction as well as on the symmetry of infection parameters between life stages. This model underscores the potential importance of life-history based immune investment patterns for disease dynamics and highlights the need for wide-spread empirical estimation of parameters that represent the maintenance of immune priming in insects.
... (Delalibera, Moraes, Lapointe, Silva, and Tamai 2000; Elliot et al. 2002). However, resting spores are rarely encountered in field-collected cadavers. ...
To determine the effect of storage on fungal survival, mummified cadavers of the cassava green mite pathogen, Neozygites tanajoae were placed at different conditions of temperature and relative humidity. The best condition for long-term preservation was −10°C. At this condition, the fungus retained viability for 10 years when the experiment was terminated, with a decrease in sporulation with time. Cadavers placed at 4°C and 5% RH sporulated for 2 years, while the fungus survived for only 7 days at 25°C and 50% RH.
... However, reliance on horizontal transmission implies that these fungi are dependent on host population density for survival and dispersal, which means that their eYcacy may be compromised at low host densities (Fuxa 1987 ). Intrinsic diVerences in mite susceptibility to N. tanajoae and N. Xoridana is associated with the mite life stages, size and behaviour, as well as age (Elliot et al. 2002). Host death caused by these fungi normally occurs at night, when relative humidity is high, favorable for sporulation (Hajek and St Leger 1994). ...
The spider mites Tetranychus urticae Koch and Tetranychus evansi Baker and Pritchard are important pests of horticultural crops. They are infected by entomopathogenic fungi naturally or experimentally. Fungal pathogens known to cause high infection in spider mite populations belong to the order Entomophthorales and include Neozygites spp. Studies are being carried out to develop some of these fungi as mycoacaricides, as stand-alone control measures in an inundative strategy to replace the synthetic acaricides currently in use or as a component of integrated mite management. Although emphasis has been put on inundative releases, entomopathogenic fungi can also be used in classical, conservation and augmentative biological control. Permanent establishment of an exotic agent in a new area of introduction may be possible in the case of spider mites. Conservation biological control can be achieved by identifying strategies to promote any natural enemies already present within crop ecosystems, based on a thorough understanding of their biology, ecology and behaviour. Further research should focus on development of efficient mass production systems, formulation, and delivery systems of fungal pathogens.
According to the density-dependent hypothesis (DDP), hosts living at high densities suffer greater risk of disease and so invest more in immunity. Although there is much empirical support for this, especially from invertebrate systems, there are many exceptions, notably in social insects. We propose that (A) density is not always the most appropriate population parameter to use when considering the risks associated with disease and (B) behavioral defenses should be given a greater emphasis in considerations of a host's repertoire of immune defenses. We propose a complementary framework stressing the connectivity between and within populations as a starting point and emphasizing the costs represented by disease above the risk of disease per se. We consider the components of immune defense and propose that behaviors may represent lower-cost defenses than their physiological counterparts. As group-living and particularly social animals will have a greater behavioral repertoire, we conclude that with group living comes a greater capacity for behavioral immune defense, most particularly for social insects. This may escape our notice if we consider physiological parameters alone.
Most fungal pathogens lack the capacity to search for their host but rather develop sit-and-wait strategies that favour contact with them. The success of these strategies depends upon the interactions of the pathogen with its host, the host plant and the environmental conditions, which altogether determine its transmissibility. Given the limited success that has characterized application of sustainable microbial control, particularly using Entomophthorales, interaction studies have been conducted with the entomophthoralean fungus Neozygites tanajoae, pathogenic to the cassava green mite (CGM), Mononychellus tanajoa, to help understand differences observed between laboratory and field performances of this pathogen. Reciprocal pathogen-host interactions as well as tritrophic interactions involving the host plant were studied. It was found that herbivory triggers the release of volatiles that promote sporulation of isolates of N. tanajoae, whereas the host mite avoids haloes of spores of this pathogen. However, the host mite does not avoid the pathogen when inside the mummified fungus-killed cadaver. The status of microbial control of CGM in Africa is reviewed and implications of these interactions are discussed for prospective application of microbial control using Entomophthorales.
The mite-pathogenic fungus Neozygites floridana Fisher (Entomophthorales: Neozygitaceae) is considered to have potential for the biological control of the cassava green mite, Mononychellus tanajoa (Bondar). However, its activity is sporadic and laboratory data suggest a strong dependence on night-time saturation deficits for transmission. We report on an epizootic of this fungus in a mite population in northeastern Brazil. During the epizootic, host populations appeared to he limited by a combination of the pathogen and a predatory mite Neoseiulus idaeus (Acari: Phytoseiidae). When temperatures increased, the epizootic finished and the host population began to grow. Abiotic conditions could not explain the variation in host mortality following pickup of infective propagules in this epizootic. However, night-time saturation did help to explain the variation in transmission from infective cadavers to newly killed hosts. This supports laboratory observations that horizontal transmission between hosts is determined mainly by saturation deficits, while the process of infection is little affected by abiotic conditions. A further field observation was the near-absence of resting spores in dead mites (ca. 0.1% of cadavers), suggesting that the pathogen population was unsuccessful in producing inoculum to infect future M. tanajoa populations. The implications are that this pathogen will only be effective as a biological control agent in periods of high relative humidity, and establishment in new areas may be limited by resting spore formation.
Methods for measuring prevalence of Neozygites floridana in a Tetranychus urticae population collected from strawberries were developed and compared. T. urticae were extracted from leaves using a soapy water solution (0.5 ml washing detergent : 8 L water) and then placed into 80% alcohol for use in Methods 1 and 2. Method 1: N. floridana-sporulating T. urticae cadavers were observed and quantified under a compound microscope (40-80x). Method 2: Adult females were mounted in lactophenol cotton blue and observed for the presence or absence of N. floridana hyphal bodies under a microscope (200-400x). Method 3: Live T. urticae females were incubated at 25 degrees C and 75% RH and observed for mortality and N. floridana infection under a compound microscope (6.4-40x). Method 1 was the most time-efficient method and it also allows processing of samples as time permits. Method 2 quantified significantly higher fungal prevalence than Methods 1 and 3, but Method 2 is not considered to be reliable because hyphal bodies are difficult to detect. No significant differences were found between Methods 1 and 3.
Two Brazilian isolates and one Benin (indigenous) isolate of Neozygites floridana were released against the cassava green mite, Mononychellus tanajoa , in January 1999 in the Adjohoun district, Ouémé administrative region, Republic of Benin. Post-release monitoring conducted 8, 14, 22 and 36 weeks later showed very low mean infection rates on M. tanajoa by isolate (0.03-0.4%). However, 48 weeks after releases, mean infection rates increased noticeably to between 2.3 and 18.7%, and higher infection rates were observed for the Brazilian isolates compared with the indigenous one. The highest infection rate for the indigenous isolate was 4.5% while it reached over 30% for the Brazilian isolates (36.5 and 34.0%). Observations made to study dispersal from inoculated plants showed the absence of infected mites at 4 m from the inoculated plants in all fields 8 weeks after the releases, while they were already present on those at 2 m away. From the next monitoring, 14 weeks after the releases, infection was found at all three sampling positions (inoculated plants and plants at 2 and 4 m away). Only four mites with resting spores were found in over 460 000 mites examined. The highest infection levels were observed in December during 'harmattan' a period characterized by hot days and cool nights with high relative humidity.
Seasonal occurrence and abundance of redlegged earth mite, Halotydeus destructor (Tucker), was measured by weekly sampling in grazed annual pastures near Keysbrook (1990–1992) and Narrogin (1991–1992) in southwestern Australia. Mites were active for 27 weeks from the late autumn (May) to mid-spring (October), completing three generations at approximately 8 week intervals. The summer is spent as diapause eggs in the cadavers of adult female mites. In 1991 and 1992, active H. destructor was on average twice as abundant at Keysbrook (mean 11,300 mites/m2), as at Narrogin (mean 6400 mites/m2). Three times more eggs were laid at Keysbrook than Narrogin (mean 8500 and 2900 eggs/m2 respectively). Rainfall at Keysbrook was twice that at Narrogin, and temperatures were higher. We suggest that the rate of oviposition was less at Narrogin, probably because of resource limitation. The mature adult sex ratio was female biased, but was lower at Keysbrook (0.669) than at Narrogin (0.813). Neozygites acaridis, a fungal disease, was detected in less than 4% of the population, mainly in adult mites in late spring. Rainfall and temperatures were lower in the spring of 1992 than 1991 at both sites. Pasture was grazed considerably shorter in the spring of 1992, and numbers of H. destructor were lower, but numbers of eggs laid in the two years were similar. We suggest that active mite mortality was greater in spring of 1992, probably due to lower relative humidity. There were fewer adult mites in spring, and fewer diapausing eggs in summer at both sites in 1992 (36,600 diapause eggs/m2) than in 1991 (148,000 diapause eggs/m2). It is proposed that controlling mites in spring should lead to lower numbers of diapause eggs in summer and of mites emerging in autumn.
The survival of Neozygites cf. floridana (Weiser and Muma) as dry hyphal bodies in mummified cassava green mites, Mononychellus tanajoa (Bondar), at 5.0% RH in the dark was affected by storage temperature. Survival of the fungus in mummies kept at 241.0C could be demonstrated for 6–7 months. When stored at 4C, the fungus sporulated from 90% of the mummies liberating an average of 186.9 primary conidia per mummy even after a storage period of 16 months, when the experiment was terminated. The temperature, humidity and light condition significantly affected the viability of primary conidia. The percent viability across all factors dropped from 98.4% after 0 h (beginning of the experiment) to 23.4% after a 1 h exposure to the conditions tested. Lower temperatures maintained higher viabilities with 86.3% of the conidia surviving after 18 h at 18C, whereas almost all conidia died after 12 h at 33C. Conidia survived less than 1 h when exposed to SDs (saturation deficit) of 2.0 mm Hg or higher at any tested temperature.
The effect of temperature, humidity and photoperiod on the development of Neozygites cf. floridana (Weiser and Muma) in the cassava green mite, Mononychellus tanajoa (Bondar) was studied in the laboratory. Dead infected mites began to appear 2.5 days after inoculation. At 33 and 28C peak mortalities were higher and occurred earlier (after 2.5 days), than at 23 and 18C. Mean LT50 (time for half the infected mites to die) decreased with increasing temperature as follows: 3.9, 3.0, 2.9 and 2.5 days at 18, 23, 28 and 33C, respectively. When placed under conditions of high relative humidity for a period of 24 h, the percentage of dead infected mites from which the fungus sporulated was highest at 28C (51.4%) and lowest at 33C (6.5%). The development of the fungus inside the mite was not significantly affected by ambient humidity or photoperiod. No significant interactions between tested factors were found.
The fungus, Neozygitis cf. floridana is parasitic on the cassava green mite, Mononychellus tanajoa (Bondar) (Acari: Tetranychidae)
in South America and may be considered for classical biological control of cassava green mites in Africa, where cassava is
an important subsistence crop, cassava green mites are an imported pest and specific natural enemies are lacking. Spider mites
generally have a viscous structure of local populations, a trait that would normally hamper the spread of a fungus that is
transmitted by the contact of susceptible hosts with the halo of capilliconidia surrounding an infectious host. However, if
infected mites search and settle to produce capilliconidia on sites where they are surrounded by susceptible mites before
becoming infectious, then the conditions for maximal transmission in a viscous host population are met. Because the ratio
between spider mites and the leaf area they occupy is constant, parasite-induced host searching behaviour leads to a constant
per capita transmission rate. Hence, the transmission rate only depends on the number of infectious hosts. These assumptions
on parasite-induced host search and constant host density lead to a simple, analytically tractable model that can be used
to estimate the maximal capacity of the fungus to decimate local populations of the cassava green mite. By estimating the
parameters of this model (host density, per capita transmission rate and duration of infected and infectious state) it was
shown that the fungal pathogen can reduce the population growth of M. tanajoa, but cannot drive local mite populations to
extinction. Only when the initial ratio of infectious to susceptible mites exceeds unity or the effective growth rate of the
mite population is sufficiently reduced by other factors than the fungus (e.g. lower food quality of the host plant, dislodgement
and death by rain and wind and predation), will the fungal pathogen be capable of decimating the cassava green mite population.
Under realistic field conditions, where all of these growth-reducing factors are likely to operate, there may well be room
for effective control by the parasitic fungus.
The cassava green mite, Mononychellus tanajoa (Bondar), is an exotic pest in Africa and is the target of a classical biological control programme. Field data from the Neotropics, where it is indigenous, are presented for the first time, charting the variation in abundance of M. tanajoa over several seasons. This was highly variable, with a characteristic trough mid-year and a peak at the turn of the year. This pattern corresponded positively with rainfall levels, appearing to fit a phenology also characteristic of African studies, where rainfall at the start of the wet season promotes a leaf flush and so growth in M. tanajoa populations. Analyses implied some impact of leaf-inhabiting predatory mites (predominantly Neoseiulus idaeus Denmark & Muma) and a considerable impact of the fungal pathogen Neozygites floridana Fisher on M. tanajoa populations. This pathogen was not observed in the host population for several (generally dry) periods implying survival outside the host, perhaps as resting spores. This is a particularly desirable characteristic of a biological control agent. It is therefore proposed that N. floridana might be of particular use in drier cassava-growing areas where rainfall at the outset of the wet season is not sufficiently intense to cause heavy M. tanajoa mortality but may be sufficient to stimulate epizootics of the fungal pathogen, protecting the flush of new cassava growth.
An overview is given of studies on diseases of mites. Knowledge of diseases of mites is still fragmentary but in recent years more attention has been paid to acaropathogens, often because of the economic importance of many mite species. Most research on mite pathogens concerns studies on fungal pathogens of eriophyoids and spider mites especially. These fungi often play an important role in the regulation of natural mite populations and are sometimes able to decimate populations of phytophagous mites. Studies are being conducted to develop some of these fungi as commercial acaricides. Few bacteria have been reported as pathogens of the Acari but in recent years research has been concentrated on intracellular organisms such as Wolbachia that may cause distorted sex ratios in offspring and incompatibility between populations. The role of these organisms in natural populations of spider mites is in particular discussed. The effect of Bacillus thuringiensis on mites is also treated in this review, although its mode of action in arthropods is mainly due to the presence of toxins and it is, therefore, not considered to be a pathogen in the true sense of the word. Microsporidia have been observed in several mite species especially in oribatid mites, although other groups of mites may also be affected. In recent years, Microsporidia infections in Phytoseiidae have received considerable attention, as they are often found in mass rearings of beneficial arthropods. They affect the efficacy of these predators as biological control agent of insect and mite pests. Microsporidia do not seem to have potential for biological control of mites.
Factors determining residual pesticide toxicity to insects were investigated using a published stochastic model which simulates the encounter and transfer of insecticide from treated plant surfaces to lepidopteran larvae and predicts the proportion of insects responding. The walking velocity of an insect, the proportion of pesticide transferred per encounter and its area of contact with the leaf surface have important influences on mortality following residual exposure. Formulae were tested against data sets describing the response of larvae of Spodoptera littoralis exposed to cypermethrin and deltamethrin ECs and adults of Coccinella septempunctata exposed to dimethoate EC. The prediction of residual uptake underestimated the observed transfer, but only at high concentrations of active ingredient. Concentration-dependent behavioral response were implicated in the shortfall. A simple hazard index based on contact area, track width, walking speed and tolerance to pesticide correctly predicted the ranking of susceptibility of three coleopteran species to residual deposits of insecticide. -from Authors
Few insects are susceptible to pathogens at all stages of their life cycle. The population dynamic consequences of introducing age and stage structure are examined through the use of models of insect-pathogen interactions in which hosts are assumed to contract the disease by ingesting infectious particles in the environment. The models are phrased as delay-differential equations that assume constant demographic parameters within age or stage classes. The first and simplest model assumes that only the adult insect is susceptible to infection, while the second model assumes a juvenile susceptible stage. Two further Variants of the second model are considered: one in which the disease does not have a fixed incubation period and one that assumes that a reservoir for pathogen particles exists in the environment. Both linear and nonlinear transmission processes are examined. The introduction of explicit time delays has little effect on model equilibria but strongly influences dynamics. Multigenerational host-pathogen cycles are predicted by all models, although the parameter space in which they occur is markedly model specific. A type of population dynamics Previously observed in host-parasitoid models, but not in insect-pathogen models, was also found: cycles with a period of one host generation or of a fraction of a host generation. This dynamic behavior arises through the interaction of unequal time delays in the two populations. The absence of a fixed incubation period and the presence of a pathogen reservoir both tend to be stabilizing. The models developed in this article are most applicable to insect-pathogen interactions in tropical or subtropical environments, where populations are not synchronized by a severe winter.
A survey of the pathogenic fungi associated with mites on cassava in Benin, West Africa, revealed both geographical and seasonal variation in the presence of Neozygites cf. floridana (Weiser and Muma) and Hirsutella thompsonii Fisher on Mononychellus tanajoa (Bondar) and Oligonychus gossypii (Zacher). Few dead and infected mites were found during the dry season, regardless of vegetation zone. In three of 30 surveyed sites, N. floridana was found infecting 1% of the dead M. tanajoa and 2% of the dead O. gossypii, while H. thompsonii was observed infecting 20% of the dead M. tanajoa in a single site. The frequency of sites having infected mites during the wet season was 3.5 times greater than that seen during the dry season. N. floridana infected 10% of the dead M. tanajoa and 19% of the dead O. gossypii on young leaves. Mites infected with N. floridana were found either in the coastal Southern Forest Mosaic (SFM) or in the Northern Guinea Savanna vegetation zones. N. floridana was rare in the low mite densities associated with mature leaves. H. thompsonii was found on 19% and 29% of the dead M. tanajoa on young and mature leaves respectively. All M. tanajoa infected with H. thompsonii on young leaves and mature leaves (75%) were found in the SFM. A single M. tanajoa was the only infected mite found in the Southern Guinea Savanna. Relatively few O. gossypii were infected with H. thompsonii. N. floridana and H. thompsonii were found together in three sites, but never on the same host. Phytoseiids were never found infected with either pathogen. In a regression analysis, the number of dead mites was significantly estimated from the total number of mites for both species, regardless of leaf age. The numbers of dead M. tanajoa on mature leaves were also estimated from the proportion infected with H. thompsonii. The numbers of infected mites on young leaves were estimated from their association with the SFM for M. tanajoa infected with H. thompsonii, and from total mites for O. gossypii infected with N. floridana. On mature leaves, infected mite numbers were estimated from the numbers of dead M. tanajoa infected with H. thompsonii. The merit of introducing more virulent or better adapted isolates of N. floridana to control M. tanajoa in Africa is discussed.
Reviews insect disease models by comparing them to general epidemiological models and models of parasitism, and by discussing their contributions to ecological and epidemiological theory. There is discussion of how epizootiological modeling can be linked with empirical studies to provide insight into the dynamics of host and pathogen communities. -from Authors
Effects of temperature, season (wet versus dry), plant age, and leaf age on developmental time, fecundity, survivorship, and intrinsic rate of increase (rm) of Mononychellus tanajoa (Bondar) on cassava, Manihot esculenta Crantz, were evaluated using age-specific life table statistics. Preadult developmental times were similar regardless of treatment; however, adult periods, fecundity, and rm were enhanced on young leaves of young plants. Patterns of survivorship and fecundity were similar at 24, 27, and 31°C on a degree-day basis but lower at 20 and 34°C, whereas estimates of rm were similar at all temperatures studied except at 34°C. The lower thermal threshold for development was estimated to be 14.4°C. A simple population model based on empirical rm values predicted larger mite populations on host plants during the wet season than commonly observed. However, when rainfall-induced mortality was added, the large mite populations predicted on wet-season host plants collapsed to levels similar to those observed in the field.
During the summer of 1968, a study was conducted in Alabama to determine the importance of Entomophthora sp. as a natural control factor in field populations of the two-spotted spider mite, Tetranychus urticae Koch. Studies on the distribution of this fungus revealed its presence in 14 of the 15 counties where collections were made. Average infection by Entomophthora was 25%.
Five epizootics of the pathogenic fungus were observed in two-spotted spider mite populations in Lee County, Ala. Each epizootic was characterized by a high degree of infection by Entomophthora accompanied by a rapid decline in mite numbers.
The pick-up of pesticide by lepidopterous larvae walking across ultra-low volume pesticide treated surfaces has been stochastically modelled and simulated using a computer. The duration of the random walk was considered as a series of small, discrete time intervals during which the mode of pick-up varied, depending upon the behavioural state (resting, feeding or walking) of a larva. Estimates can be obtained for the total mass picked up by the larva (gross) and the net amount eliminated, the internal exposure to insecticide, and the proportion of insects knocked down and killed with increasing time. The results of the simulation of Spodoptera littoralis larvae confined to permethrin-treated surfaces were in agreement with experimental data, and suggested that a high pick-up of insecticidal droplets resulted from adhesion of the pesticide to larvae following encounter. However, the availability of drops at the leaf surface decreased with time, probably as a result of droplet spread and subsequent uptake by the cuticular waxes. Droplet stability on leaf surfaces may be related to droplet size.
The germination of capilliconidia of Neozygites floridana on the cassava green mite, Mononychellus tanajoa, and the viability of these conidia collected on glass coverslips were studied at different temperatures, humidities (expressed as saturation deficit (SD), measured in mg Hg), and light conditions. Germination began within 2 hr of their attachment to the mites, at which time more than 66.0% of the conidia maintained at between 18 and 28°C, at SDs 0 and 0.2, and in the dark produced germ tubes. Percent germination pooled across all factors significantly increased from 15.7% after 2 hr to 28.2% after 6 hr and 45.0% after 14 hr. Germination at the extreme temperatures tested, 24.5% at 13°C and 14.8% at 33°C, was significantly lower than 40.0, 47.2, and 36.5% recorded at 18, 23, and 28°C, respectively. Percent germination increased with increasing humidity from 2.8% at SD 1.2 to 60.9% at SD 0. Although germination was observed when both light and dark conditions were tested, the 44.9% germination recorded under continuous darkness after 6 hr was significantly higher than 20.3% under continuous light. Rate of germination of capilliconidia declined with increasing storage time from 93.0% before storage to a pooled mean of 81.0% after 1 day and 3.5% after 10 days. Higher germination rates were maintained at lower temperatures. Whereas germination after 7 days did not exceed 8.0% at 28 and 33°C, it was 85.9 at SD 10 in the dark at 18°C. Germination at SD 10 was significantly higher (43.3%) than at SDs 6, 2, and 0 (between 31.34 and 34.95%). Continuous light reduced viability as an average of 47.1% of the conidia maintained in the dark germinated, compared to 23.5% of those in the light.
Models of insect-pathogen interactions in highly seasonal environments are developed. The models apply to insects such as many temperate forest pests that have a single generation per year and which are susceptible to viral disease only during their larval period. The disease kills the hosts after a fixed time period when infectious pathogen particles are released into the environment. Depending on the time taken to kill the host, one to many cycles of pathogen replication may occur during the portion of the year when susceptible hosts are present. A baseline model with linear disease transmission is always unstable although a stable equilibrium can be achieved if there is sufficient density dependence in the transmission process. Persistent, long-period cycles are virtually never observed. The release of pathogen particles prior to host death contributes towards stability although it does not result in limit cycles. Long-period cycles were found in two other extensions of the baseline model, one in which some hosts carry a sublethal infection which is transmitted to their offspring; and a second which includes a reservoir where pathogen particles are relatively long lived although unable to cause new infections. The relationship between this work and previous host-pathogen and host-parasitoid models is discussed.
The production and germination of primary conidia of Neozygites floridana as affected by temperature, humidity, and photoperiod was studied in the laboratory. All tested factors significantly affected the two processes studied. Production increased with increasing temperature between 13 and 23°C, with means of 35.5 and 55.4 conidia, respectively, discharged from each mummified mite cadaver. No conidia were produced at 28 or 33°C. Conidial production dropped significantly from 96.1/mummy in a moisture-saturated environment [saturation deficit (SD) 0] to 33.9/mummy at SD 0.2. Very few conidia were produced at SD 0.7 (0.6/mummy) and none at SD 1.2. Significantly fewer conidia were produced under continuous light (11.2/mummy) than under continuous darkness (40.1/mummy) or 12L:12D (46.7/mummy) photoperiods. Between 82 and 100% of the conidia produced under 12L:12D photoperiod were released in the dark phase. Germination of primary conidia started within 2 hr and increased with decreasing temperature between 13 and 28°C. Percentage germination of 20.1% at 13°C and 17.6% at 18°C was significantly higher than 11.2% at 28°C. There was no germination at 33°C. High humidities (>95%) were necessary to effect germination. Germination at SD 0 (27.2%) and 0.2 (23.4%) was significantly higher than at SD 0.7 (0.4%), where germination began after 6 hr and was observed only at 13, 18, and 23°C. No germination was observed at SD 1.2. There was no germination among conidia maintained under continuous light.
The control of insect pests by using insect pathogens as dynamic biological control agents is a recent effort. Model studies on insect-pathogen relations can help in the development of biocontrol programs. Except for the work of Briggs and Godfray [1], insect-pathogen models ignore the stage-specific susceptibility of insects. Moreover most models do not incorporate insect self-regulation. We develop stage-structured models of insect-pathogen relations incorporating insect-density dependence and disease transmitted through direct contact between susceptible and infective individuals. The models are analyzed by using steady-state and stability analysis. Numerical solutions are used as sources of further insight into the dynamics of the insect-pathogen systems. It is shown that there are major differences in the dynamics of adult- and juvenile-infecting diseases. Moreover, the interplay between insect-density dependence and stage-specific susceptibility has important consequences for the dynamics of insect-pathogen systems.
How the pathogen inoculum size affects the pathogenicity of Neozygites floridana to Mononychellus tanajoa was studied at 28°C, 12L:12D photoperiod and 90-100% RH. All inoculum sizes tested (one, two, four, six, and eight capilliconidia per mite) were lethal. Dead mites started to appear 48 hr after exposure to the conidia. Inoculum size significantly affected time to mortality. All mites exposed to six or eight capilliconidia died of mycosis 57.1 and 62.9 hr, respectively, after infection. At smaller doses, infection rates were lower, dead mites started to appear later, and time to mortality was significantly longer (between 68.7 and 89.4 hr). Most mites died within the first 6 hr of the dark phases of the photoperiod.
The mite-pathogenic fungus Neozygites floridana Fisher (Entomophthorales: Neozygitaceae) is considered to have potential for the biological control of the cassava green mite, Mononychellus tanajoa (Bondar). However, its activity is sporadic and laboratory data suggest a strong dependence on night-time saturation deficits for transmission. We report on an epizootic of this fungus in a mite population in northeastern Brazil. During the epizootic, host populations appeared to he limited by a combination of the pathogen and a predatory mite Neoseiulus idaeus (Acari: Phytoseiidae). When temperatures increased, the epizootic finished and the host population began to grow. Abiotic conditions could not explain the variation in host mortality following pickup of infective propagules in this epizootic. However, night-time saturation did help to explain the variation in transmission from infective cadavers to newly killed hosts. This supports laboratory observations that horizontal transmission between hosts is determined mainly by saturation deficits, while the process of infection is little affected by abiotic conditions. A further field observation was the near-absence of resting spores in dead mites (ca. 0.1% of cadavers), suggesting that the pathogen population was unsuccessful in producing inoculum to infect future M. tanajoa populations. The implications are that this pathogen will only be effective as a biological control agent in periods of high relative humidity, and establishment in new areas may be limited by resting spore formation.
Survival of pathogens during long periods of unfavorable conditions can be critical to their ecology and to their use in biological control. In northeastern Brazil, the mite pathogen Neozygites floridana must survive hot and dry conditions between wet seasons when it infects the cassava green mite Mononychellus tanajoa. We report on large numbers of mite cadavers bearing resting spores towards the end of epizootics in mid-1995. High within-leaf variability indicated that local factors may be important in determining resting spore formation. These spores remain in the host cadaver on a leaf until the cadaver breaks up, whereupon the spores fall freely to the soil, there to remain dormant. Laboratory simulation of field conditions led to ca. 25% of mycosed individuals bearing resting spores. Mummies (without resting spores) kept in hot and dry conditions showed little or no viability within 2 months, implying no role for survival over extended dry periods. It is proposed that resting spores form the principal means by which this pathogen survives the dry season in the study area. This has implications for its introduction to new areas in classical biological control.
Age-specific and inter-sexual susceptibility of twospotted spider mite, Tetranychus urticae Koch, to Neozygites floridana Weiser and Muma
- F X Susilo
- G L Nordin
- G C Brown