[Show abstract][Hide abstract] ABSTRACT: The capacity of mosquitoes to resist insecticides threatens the control of diseases such as dengue and malaria. Until alternative control tools are implemented, characterizing resistance mechanisms is crucial for managing resistance in natural populations. Insecticide biodegradation by detoxification enzymes is a common resistance mechanism; however, the genomic changes underlying this mechanism have rarely been identified, precluding individual resistance genotyping. In particular, the role of copy number variations (CNVs) and polymorphisms of detoxification enzymes have never been investigated at the genome level, although they can represent robust markers of metabolic resistance. In this context, we combined target enrichment with high-throughput sequencing for conducting the first comprehensive screening of gene amplifications and polymorphisms associated with insecticide resistance in mosquitoes. More than 760 candidate genes were captured and deep sequenced in several populations of the dengue mosquito Ae. aegypti displaying distinct genetic backgrounds and contrasted resistance levels to the insecticide deltamethrin. CNV analysis identified 41 gene amplifications associated with resistance, most affecting cytochrome P450s overtranscribed in resistant populations. Polymorphism analysis detected more than 30,000 variants and strong selection footprints in specific genomic regions. Combining Bayesian and allele frequency filtering approaches identified 55 nonsynonymous variants strongly associated with resistance. Both CNVs and polymorphisms were conserved within regions but differed across continents, confirming that genomic changes underlying metabolic resistance to insecticides are not universal. By identifying novel DNA markers of insecticide resistance, this study opens the way for tracking down metabolic changes developed by mosquitoes to resist insecticides within and among populations.
[Show abstract][Hide abstract] ABSTRACT: The insecticide susceptibility level of mosquito populations is one of the major factors infl uencing the success of vector control. In this study, Aedes aegypti from seven localities with a current dengue outbreak in Thailand were subjected to synthetic pyrethroid insecticide susceptibility assays. The results revealed that Ae. aegypti from all localities were strongly resistant to bifenthrin, permethrin and deltamethrin. High resistance to lambda-cyhalothrin was detected from all localities with the exception of Ae. aegypti from Bangkok and Uttaradit which demonstrated incipient resistance. However, Ae. aegypti from Bangkok, Phra Nakhon Si Ayutthaya, Sakon Nakhon and Chumphon showed incipient resistance to alpha-cypermethrin whereas Ae. aegypti collected from Uttaradit, Mukdahan and Phatthalung were susceptible. In addition, Ae. aegypti from Bangkok, Phra Nakhon Si Ayutthaya and Uttaradit demonstrated incipient resistance to cypermethrin while susceptibility of Ae. aegypti from Mukdahan, Sakon Nakhon, Phatthalung and Chumphon to cypermethrin was observed. It was concluded that field-collected Ae. aegypti from all localities had developed resistance to the synthetic pyrethroids, with the majority of these being to bifenthrin, permethrin and deltamethrin.
No preview · Article · Jan 2014 · Kasetsart Journal - Natural Science
[Show abstract][Hide abstract] ABSTRACT: Anopheles mosquitoes are highly important insects due to their involvement in the transmission of human malaria and its devastating consequences in endemic countries worldwide. In 2010 alone, malaria was responsible for an estimated 660,000 deaths. As the study of Anopheles species and populations is a key element for reaching the goal of malaria elimination, an enormous amount of information has accumulated over the past century, and together in recent decades with the advent of novel technologies the acquisition of new knowledge has accelerated even further. The originality of this book is to offer the latest compilation on various research, new concepts, paradigms and innovative approaches for the control of anophelines using state-of-the-art methodologies and analysis. The 24 chapters, written by internationally recognized experts from 5 continents, cover the rich landscape for the understanding of Anopheles mosquitoes and the development of more effective weapons to control the vector of malaria.
[Show abstract][Hide abstract] ABSTRACT: A survey of adult anopheline mosquito diversities, collected from September 2009 to August 2010, was conducted in a malaria endemic area of western Thailand. Two anopheline species complexes, Dirus and Minimus, along with the Maculatus group were observed. Of several species documented from within each complex and group, four important malaria vectors were identified, including An. dirus, An. baimaii, An. minimus, and An. sawadwongporni. Information on biting activity and host preference for any single species within the Dirus complex has never been assessed. Using specific molecular identification assays, the trophic behavior and biting activity of each sibling species within the Dirus complex were observed and analyzed for the Kanchanaburi Province, Thailand.
Adult female mosquitoes were collected for two consecutive nights each month during a one year period. Three collection methods, human landing indoor (HLI), human landing outdoor (HLO), and cattle baited collections (CBC) were applied. Each team of collectors captured mosquitoes between 1800 and 0600 h.
From a total of 9,824 specimens, 656 belong to the Dirus complex (An. dirus 6.09% and An. baimaii 0.59%), 8,802 to the Minimus complex (An. minimus 4.95% and An. harrisoni 84.65%) and 366 to the Maculatus group (An. maculatus 2.43% and An. sawadwongporni 1.29%). Both An. dirus and An. baimaii demonstrated exophagic and zoophilic behaviors. Significantly greater numbers of An. dirus and An. baimaii were collected from cattle as compared to humans (P = 0.003 for An. dirus and P = 0.048 for An. baimaii).
Significantly greater numbers of An. dirus and An. baimaii were collected from cattle baited traps as compared to human landing collections (P < 0.05), demonstrating that both species show a strong zoophilic behavior. Knowledge of host-seeking behavior helps to define a species' capacity to acquire and transmit malaria and its contribution to the overall risk for disease transmission in the human population, as well as, assisting in the design and implementation of appropriate vector prevention and control strategies.
[Show abstract][Hide abstract] ABSTRACT: Host feeding patterns of Anopheles minimus in relation to ambient environmental conditions were observed during a 2-year period at Tum Sua Village, located in Mae Sot District, Tak Province, in western Thailand, where An. minimus is found in abundance and regarded as the most predominant malaria vector species. Detailed information on mosquito behavior is important for understanding the epidemiology of disease transmission and developing more effective and efficient vector control methods.
Adult mosquitoes were collected every 2 months for two consecutive nights from 1800 to 0600 hrs. Three collection methods were used; indoor human-landing collections (HLC), outdoor HLC, and outdoor cattle-bait collections (CBC).
A total of 7,663 female Anopheles mosquitoes were collected of which 5,392 were identified as members of 3 different species complexes, the most prevalent being Anopheles minimus complex (50.36%), followed by Anopheles maculatus complex (19.68%) and Anopheles dirus complex (0.33%). An. minimus s.s. comprised virtually all (> 99.8 percent) of Minimus Complex species captured. Blood feeding behavior of An. minimus was more pronounced during the second half of the evening, showing a slight preference to blood feed outdoors (~60%) versus inside structures. Significantly (P < 0.0001) more An. minimus were collected from human-baited methods compared with a tethered cow, indicating a more anthropophilic feeding behavior. Although a significant difference in total number of mosquitoes from the HLC was recorded between the first and second year, the mean biting frequency over the course of the evening hours remained similar.
The Human landing activity of An. minimus in Tum Sua Village showed a stronger preference/attraction for humans compared to a cow-baited collection method. This study supports the incrimination of An. minimus as the primary malaria vector in the area. A better understanding of mosquito behavior related to host preference, and the temporal and spatial blood feeding activity will help facilitate the design of vector control strategies and effectiveness of vector control management programs in Thailand.
Preview · Article · Jun 2012 · Parasites & Vectors
[Show abstract][Hide abstract] ABSTRACT: Establishing baseline insecticide discriminating doses is crucial in accurately determining susceptibility status and changing temporal patterns of physiological response in mosquito populations. Pyrethroids are the predominant chemicals used for controlling adult Aedes aegypti and Ae. albopictus, both vectors of dengue viruses, in Thailand. Presently, only 2 pyrethroids, permethrin and lambda-cyhalothrin, have published diagnostic dose rates for monitoring Ae. aegypti. This study established the diagnostic lethal concentrations for 6 different pyrethroids available in Thailand for dengue vector control. United States Department of Agriculture insecticide-susceptible strain of Ae. aegypti was used to establish the baseline concentrations for subsequent susceptibility testing of field populations. Our findings showed lower discriminating concentrations for lambda-cyhalothrin and permethrin than those recommended by the World Health Organization (WHO), at 2.5- and 1.7-fold lower dosing, respectively. The susceptibility status of 3 different geographical populations of field-collected Ae. aegypti were tested using the standard WHO procedures. All 3 field strains demonstrated varying levels of physiological resistance to each compound. We conclude that establishing the baseline diagnostic concentration of an insecticide is of paramount importance in accurately determining the susceptibility status in field-collected mosquitoes. If possible, discriminating doses should be established for all insecticides and test assays run concurrently with a known susceptible strain for more accurate monitoring of resistance in mosquito populations in Thailand.
No preview · Article · Mar 2012 · Journal of the American Mosquito Control Association
[Show abstract][Hide abstract] ABSTRACT: Determining the diagnostic dose is crucial in evaluating the susceptibility of a mosquito population to certain insecticides. Presently, the diagnostic doses of only two pyrethroids (permethrin and λ-cyhalothrin) have been published for monitoring insecticide susceptibility of Aedes aegypti. Therefore, the objective of this study was to establish the diagnostic doses for two synthetic pyrethroids-bifenthrin and α-cypermethrin-currently used in dengue vector control, using the United State Department of Agriculture (USDA) standard strain of Aedes aegypti. These diagnostic doses were subsequently used to evaluate the susceptibility status in field collected specimens of Ae. aegypti. Based on the baseline susceptibility levels, all three field strains demonstrated varying levels of physiological resistance to each compound. Strong resistance to bifenthrin was seen in one strain from Nong Khai resulting in 14.14% mortality whereas complete mortality (100%) was observed in the USDA standard strain when exposed to the established diagnostic doses of these two synthetic pyrethroids. These diagnostic doses could be applied for monitoring the susceptibility status in mosquito populations across Thailand.
No preview · Article · Jan 2012 · Kasetsart Journal - Natural Science
[Show abstract][Hide abstract] ABSTRACT: Behavioral responses of female mosquitoes representing two species in the Minimus Complex exposed to an operational field dose of bifenthrin or DEET (N,N-diethyl-m-toluamide) were described using an excito-repellency test system. Two test populations of An. minimus, one from the field (Tak Province, western Thailand), the other from a long-established laboratory colony, and Anopheles harrisoni collected from Kanchanaburi Province, western Thailand, were used. Results showed that all test populations rapidly escaped after direct contact with surfaces treated with either bifenthrin or DEET compared to match-paired untreated controls. Greater escape response by exposed females to bifenthrin and DEET were observed in the An. minimus colony compared to the two field populations. Field-collected An. minimus demonstrated a more rapid escape response to DEET than to bifenthrin, whereas An. harrisoni showed a converse response. Although fewer females escaped from test chambers without direct contact with treated surfaces compared to contact tests, the spatial repellency response was significantly pronounced in all test populations compared to match-paired controls (P < 0.05). DEET was found to perform as both a contact stimulant and moderate spatial repellent.
Full-text · Article · Dec 2011 · Journal of Vector Ecology
[Show abstract][Hide abstract] ABSTRACT: Excito-repellency responses of 3 test populations, representing 2 sibling species within the Minimus Complex, Anopheles minimus and An. harrisoni, were characterized for contact irritant and noncontact repellent actions of chemicals during and after exposure to alpha-cypermethrin at half the recommended field (0.010 g/m2), the recommended field (0.020 g/m2), and double the recommended field concentration (0.040 g/m2), using an excito-repellency escape chamber system. Two field populations of An. minimus and An. harrisoni collected from the malaria-endemic areas in Tak and Kanchanuburi provinces in western Thailand, respectively, were tested along with a laboratory population of An. minimus maintained since 1993. Females of all 3 test populations rapidly escaped after direct contact with treated surfaces for each concentration. In general, increased escape responses in the An. minimus test populations were proportionate to increased insecticide dosages. The greatest escape response for An. harrisoni was observed at the operational field concentration of alpha-cypermethrin. The noncontact repellency response to alpha-cypermethrin was comparatively weak for all 3 test populations, but significantly different from each paired contact test and respective noncontact controls. We conclude that strong contact irritancy is a major action of alpha-cypermethrin, whereas noncontact repellency plays no role in the escape responses of 2 species in the Minimus Complex in Thailand.
Full-text · Article · Sep 2011 · Journal of the American Mosquito Control Association
[Show abstract][Hide abstract] ABSTRACT: Thirty-two Aedes aegypti populations collected throughout Thailand and five populations of Aedes albopictus from southern Thailand were subjected to standard WHO contact bioassays to assess susceptibility to three commonly used synthetic pyrethroids: permethrin, deltamethrin, and lambda-cyhalothrin. A wide degree of physiological response to permethrin was detected in Ae. aegypti, ranging from 56.5% survival (Lampang, northern Thailand) to only 4% (Kalasin in northeastern and Phuket in southern Thailand). All 32 populations of Ae. aegypti were found to have evidence of incipient resistance (62.5%) or levels of survival deemed resistant (37.5%) to permethrin. Four populations of Ae. albopictus were found with incipient resistance (97 - 80% mortality) and one with resistance (< 80%) to permethrin. The majority of Ae. aegypti populations (68.7%) was susceptible (> 98% mortality) to deltamethrin, with incipient resistance (observed 97-82% mortality) in other localities. In contrast, all populations of Ae. aegypti were completely susceptible (100% mortality) to the recommended operational dosage of lambda-cyhalothrin. All five populations of Ae. albopictus were found completely susceptible to both deltamethrin and lambda-cyhalothrin. Evidence of defined incipient or resistance to synthetic pyrethroids mandates appropriate response and countermeasures to mitigate further development and spread of resistance. In light of these findings, we conclude that routine and comprehensive susceptibility monitoring of dengue mosquito vectors to synthetic pyrethroids should be a required component of resistance management policies and disease control activities.
Full-text · Article · Jun 2011 · Journal of Vector Ecology
[Show abstract][Hide abstract] ABSTRACT: The insecticidal activities of essential oil extracted from Litsea salicifolia to Sitophilus zeamais and Tribolium castaneum were investigated under laboratory conditions. Essential oil was extracted by hydro-distillation method and then analyzed by GC/MS. The major components of L. salicifolia were (E)-citral and (Z)-citral. Litsea salicifolia had repellency effect on both insect species even at the lowest application rate (0.16 µg/cm 2). In addition, it exhibited fumigant toxicity to S. zeamais, contact toxicity to both species and high antifeedant toxicity to T. castaneum compared to S. zeamais. Hence, L. salicifolia might be considered as a grain protectant to major stored product insects.
Preview · Article · Oct 2010 · Pakistan journal of zoology
[Show abstract][Hide abstract] ABSTRACT: Litsea cubeba is found in many parts of Thailand and has medicinal properties. Mature fruit of L. cubeba was collected from Doi Ang-khang, in the Fang District of Chiang Mai Province, Thailand in June 2007 and the essential oil was extracted by a water-distillation method. The insecticidal properties of L. cubeba were evaluated under laboratory conditions. The results showed that the essential oil of L. cubeba strongly repelled Sitophilus zeamais and Tribolium castaneum even at low concentrations, but its repellency was more marked toward T. castaneum. Of interest was the fact that the repellency against T. castaneum was fairly consistent over the 5 h period of the experiment. Moreover, it showed both contact and fumigant toxicities against the tested species. Probit analysis showed that S. zeamais was more susceptible than T. castaneum in both the fumigant and contact bioassays. Hence, the essential oil of L. cubeba might be used as an alternative for grain protection against stored-grain insects. Further studies are needed prior to its commercial use.
[Show abstract][Hide abstract] ABSTRACT: Melaleuca cajuputi (Cajuput tree) which can be found in Myanmar (Burma) and Thailand was examined for insecticidal properties against Sitophilus zeamais and Tribolium castaneum. The results showed that M. cajuputi leaf essential oil had repellency, fumigant and contact toxicities against these two insects. The 100% repellency was only occurred in T. castaneum at 2 h and 5 h. Probit analysis showed that S. zeamais adults were more susceptible than T. castaneum. In fumigant assays, LC 50 value for S. zeamais was 178.23 µL L -1 and for T. castaneum was 213.17 µL L -1 . The LD 50 values for S. zeamais and T. castaneum in contact toxicities were 0.062 and 0.143 µL insect -1 . Melaleuca cajuputi leaf essential oil could be used as an alternative grain protectant for stored-product insects and further investigation should be done for other stored-product insects.