Identifying the fitness costs of a pyrethroid-resistant genotype in the major arboviral vector Aedes aegypti

Insecticide Resistance in Aedes aegypti Mosquitoes: Possible Detection of kdr F1534C, S989P, and V1016G Triple Mutation in Benin, West Africa

Epidemics of arboviruses in general, and dengue fever in particular, are an increasing threat in areas where Aedes (Ae.) aegypti is present. The effectiveness of chemical control of Ae. aegypti is jeopardized by the increasing frequency of insecticide resistance. The aim of this study was to determine the susceptibility status of Ae. aegypti to public health insecticides and assess the underlying mechanisms driving insecticide resistance. Ae. aegypti eggs were collected in two study sites in the vicinity of houses for two weeks using gravid Aedes traps (GATs). After rearing the mosquitoes to adulthood, female Ae. aegypti were exposed to diagnostic doses of permethrin, deltamethrin and bendiocarb, using Centers for Disease Control and Prevention (CDC) bottle bioassays. Unexposed, un-engorged female Ae. aegypti were tested individually for mixed-function oxidase (MFO), glutathione-S-transferase (GST) and α and β esterase activities. Finally, allele-specific PCR (AS-PCR) was used to detect possible kdr mutations (F1534C, S989P, and V1016G) in the voltage-gated sodium channel gene in insecticide-exposed Ae. aegypti. Most traps were oviposition positive; 93.2% and 97% of traps contained Ae. aegypti eggs in the 10ème arrondissement of Cotonou and in Godomey-Togoudo, respectively. Insecticide bioassays detected resistance to permethrin and deltamethrin in both study sites and complete susceptibility to bendiocarb. By comparison to the insecticide-susceptible Rockefeller strain, field Ae. aegypti populations had significantly higher levels of GSTs and significantly lower levels of α and β esterases; there was no significant difference between levels of MFOs. AS-PCR genotyping revealed the possible presence of 3 kdr mutations (F1534C, S989P, and V1016G) at high frequencies; 80.9% (228/282) of the Ae. aegypti tested had at least 1 mutation, while the simultaneous presence of all 3 kdr mutations was identified in 13 resistant individuals. Study findings demonstrated phenotypic pyrethroid resistance, the over-expression of key detoxification enzymes, and the possible presence of several kdr mutations in Ae. aegypti populations, emphasizing the urgent need to implement vector control strategies targeting arbovirus vector species in Benin.

Molecular surveillance of insecticide resistance in Phlebotomus argentipes targeted by indoor residual spraying for visceral leishmaniasis elimination in India

Molecular surveillance of resistance is an increasingly important part of vector borne disease control programmes that utilise insecticides. The visceral leishmaniasis (VL) elimination programme in India uses indoor residual spraying (IRS) with the pyrethroid, alpha-cypermethrin to control Phlebotomus argentipes the vector of Leishmania donovani, the causative agent of VL. Prior long-term use of DDT may have selected for knockdown resistance (kdr) mutants (1014F and S) at the shared DDT and pyrethroid target site, which are common in India and can also cause pyrethroid cross-resistance. We monitored the frequency of these marker mutations over five years from 2017-2021 in sentinel sites in eight districts of north-eastern India covered by IRS. Frequencies varied markedly among the districts, though finer scale variation, among villages within districts, was limited. A pronounced and highly significant increase in resistance-associated genotypes occurred between 2017 and 2018, but with relative stability thereafter, and some reversion toward more susceptible genotypes in 2021. Analyses linked IRS with mutant frequencies suggesting an advantage to more resistant genotypes, especially when pyrethroid was under-sprayed in IRS. However, this advantage did not translate into sustained allele frequency changes over the study period, potentially because of a relatively greater net advantage under field conditions for a wild-type/mutant genotype than projected from laboratory studies and/or high costs of the most resistant genotype. Further work is required to improve calibration of each 1014 genotype with resistance, preferably using operationally relevant measures. The lack of change in resistance mechanism over the span of the study period, coupled with available bioassay data suggesting susceptibility, suggests that resistance has yet to emerge despite intensive IRS. Nevertheless, the advantage of resistance-associated genotypes with IRS and under spraying, suggest that measures to continue monitoring and improvement of spray quality are vital, and consideration of future alternatives to pyrethroids for IRS would be advisable.

Leucine to tryptophane substitution in the pore helix IIP1 confer sodium channel resistance to pyrethroids and DDT

Aedes aegypti is responsible for transmitting a variety of arboviral infectious diseases such as dengue and chikungunya. Insecticides, particularly pyrethroids, are used widely for mosquito control. However, intensive used of pyrethroids has led to the selection of kdr mutations on sodium channels. L982W, locating in the PyR1 (Pyrethroid receptor site 1), was first reported in Ae. aegypti populations collected from Vietnam. Recently, the high frequency of L982W was detected in pyrethroid-resistant populations of Vietnam and Cambodia, and also concomitant mutations L982W + F1534C was detected in both countries. However, the role of L982W in pyrethroid resistance remains unclear. In this study, we examined the effects of L982W on gating properties and pyrethroid sensitivity in Xenopus oocytes. We found that mutations L982W and L982W + F1534C shifted the voltage dependence of activation in the depolarizing direction, however, neither mutations altered the voltage dependence of inactivation. L982W significantly reduced channel sensitivity to Type I pyrethroids, permethrin and bifenthrin, and Type II pyrethroids, deltamethrin and cypermethrin. No enhancement was observed when synergized with F1534C. In addition, L982W and L982W + F1534C mutations reduced the channel sensitivity to DDT. Our results illustrate the molecular basis of resistance mediates by L982W mutation, which will be helpful to understand the interacions of pyrethroids or DDT with sodium channels and develop molecular markers for monitoring pest resistance to pyrethroids and DDT.

Concomitant knockdown resistance allele, L982W + F1534C, in Aedes aegypti has the potential to impose fitness costs without selection pressure

The Aedes aegypti mosquito, is an arbovirus vector that can spread dengue, chikungunya, Zika, and yellow fever. Pyrethroids are widely used to control mosquitoes. The voltage-gated sodium channel (Vgsc) is the target of pyrethroids, and amino acid substitutions in this channel attenuate the effects of pyrethroids. This is known as knockdown resistance (kdr). Recently, we found that Ae. aegypti with concomitant Vgsc mutations L982W + F1534C exhibit extremely high levels of pyrethroid resistance. L982 is located in a highly conserved region of Vgsc in vertebrates and invertebrates. This study aimed to evaluate the viability of Ae. aegypti, with concomitant L982W + F1534C mutations in Vgsc. We crossed a resistant strain (FTWC) with a susceptible strain (SMK) and reared it up to 15 generations. We developed a rapid and convenient genotyping method using a fluorescent probe (Eprobe) to easily and accurately distinguish between three genotypes: wild-type and mutant homozygotes, and heterozygotes. As generations progressed, the proportion of wild-type homozygotes increased, and only 2.9% of mutant homozygotes were present at the 15th generation; the allele frequencies of L982W + F1534C showed a decreasing trend over generations. These observations show that these concomitant mutations have some fitness costs, suggesting that mosquitoes can potentially recover pyrethroid susceptibility over time without pyrethroid selection pressure in the field.

Insecticide resistance compromises the control of Aedes aegypti in Bangladesh

BACKGROUND

With no effective drugs or widely available vaccines, dengue control in Bangladesh is dependent on targeting the primary vector Aedes aegypti with insecticides and larval source management. Despite these interventions, the dengue burden is increasing in Bangladesh, and the country experienced its worst outbreak in 2019 with 101 354 hospitalized cases. This may be partially facilitated by the presence of intense insecticide resistance in vector populations. Here, we describe the intensity and mechanisms of resistance to insecticides commonly deployed against Ae. aegypti in Dhaka, Bangladesh.

RESULTS

Dhaka Ae. aegypti colonies exhibited high-intensity resistance to pyrethroids. Using CDC bottle assays, we recorded 2-24% mortality (recorded at 24 h) to permethrin and 48-94% mortality to deltamethrin, at 10× the diagnostic dose. Bioassays conducted using insecticide-synergist combinations suggested that metabolic mechanisms were contributing to pyrethroid resistance, specifically multi-function oxidases, esterases, and glutathione S-transferases. In addition, kdr alleles were detected, with a high frequency (78-98%) of homozygotes for the V1016G mutation. A large proportion (≤ 74%) of free-flying and resting mosquitoes from Dhaka colonies survived exposure to standard applications of pyrethroid aerosols in an experimental free-flight room. Although that exposure affected the immediate host-seeking behavior of Ae. aegypti, the effect was transient in surviving mosquitoes.

CONCLUSION

The intense resistance characterized in this study is likely compromising the operational effectiveness of pyrethroids against Ae. aegypti in Dhaka. Switching to alternative chemical classes may offer a medium-term solution, but ultimately a more sustainable and effective approach to controlling dengue vectors is required. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

A comprehensive survey of permethrin resistance in human head louse populations from northwest Iran: ex vivo and molecular monitoring of knockdown resistance alleles

BACKGROUND

Head louse infestation is an important public health problem, and expanding resistance to permethrin is a major challenge to its control. The mapping and detection of pyrethroid resistance are essential to the development of appropriate treatments and ensure the effectiveness of current measures. The aim of this study was to present the phenotypic and genotypic basis of permethrin resistance and identify knockdown resistance (kdr) mutations in head louse populations in northwestern Iran.

METHODS

Adult head lice were collected from 1059 infested girls in Ardebil, East Azerbaijan, West Azerbaijan and Zanjan Provinces, northwestern Iran. The toxicity of permethrin and the possible synergistic effect of piperonyl butoxide (PBO) on this toxicity were assessed using bioassays. Fragments of voltage-sensitive sodium channels (vssc) and cytochrome b (cytb) genes were amplified and analyzed for the detection of knockdown resistance (kdr) mutations and mitochondrial groups. Moreover, genotypes of the two hot spot regions of the vssc gene were determined by melting curve analysis of amplicons.

RESULTS

A total of 1450 adult head lice were collected during 2016-2021. Live lice were exposed to a dose of 1% permethrin for 12 h, and the median lethal time (LT₅₀) and time to achieve 90% mortality (LT₉₀) were determined to be 6 and 14.8 h, respectively. Topical application of 2 and 16 ng permethrin per louse resulted in 25% and 42.11% mortality, respectively. Pre-exposure of samples to 3% piperonyl butoxide had no synergistic effect on the effects of permethrin. Analysis of the 774-bp vssc gene fragment showed the presence of the M815I, T917I and L920F mutations, wild-type and T917I mutation, in 91.6%, 4.2% and 4.2% of samples, respectively. Investigation of the mitochondrial cytb gene demonstrated the predominance of clade B. The frequency of domain II segment 4 (S4)-S5 kdr genotypes in mitochondrial groups was identical, and heterozygotes were present in 93.5% of samples. A significant difference was detected in the frequency of domain IIS1-S3 kdr genotypes, and the frequency of resistant alleles and heterozygotes was higher in clade B than in clade A.

CONCLUSIONS

The presence of kdr mutations in the vssc gene and the non-synergist effect of PBO indicate that pyrethroid target site insensitivity is the main resistance mechanism. This phenomenon and the high frequency of resistant alleles necessitate that new pediculosis management programs be developed. Further studies need to be conducted to identify all factors contributing this resistance and to develop alternative pediculicides.

Insecticide resistance in malaria and arbovirus vectors in Papua New Guinea, 2017-2022

BACKGROUND

Insecticide resistance (IR) monitoring is essential for evidence-based control of mosquito-borne diseases. While widespread pyrethroid resistance in Anopheles and Aedes species has been described in many countries, data for Papua New Guinea (PNG) are limited. Available data indicate that the local Anopheles populations in PNG remain pyrethroid-susceptible, making regular IR monitoring even more important. In addition, Aedes aegypti pyrethroid resistance has been described in PNG. Here, Anopheles and Aedes IR monitoring data generated from across PNG between 2017 and 2022 are presented.

METHODS

Mosquito larvae were collected in larval habitat surveys and through ovitraps. Mosquitoes were reared to adults and tested using standard WHO susceptibility bioassays. DNA from a subset of Aedes mosquitoes was sequenced to analyse the voltage-sensitive sodium channel (Vssc) region for any resistance-related mutations.

RESULTS

Approximately 20,000 adult female mosquitoes from nine PNG provinces were tested. Anopheles punctulatus sensu lato mosquitoes were susceptible to pyrethroids but there were signs of reduced mortality in some areas. Some Anopheles populations were also resistant to DDT. Tests also showed that Aedes. aegypti in PNG are resistant to pyrethroids and DDT and that there was also likelihood of bendiocarb resistance. A range of Vssc resistance mutations were identified. Aedes albopictus were DDT resistant and were likely developing pyrethroid resistance, given a low frequency of Vssc mutations was observed.

CONCLUSIONS

Aedes aegypti is highly pyrethroid resistant and also shows signs of resistance against carbamates in PNG. Anopheles punctulatus s.l. and Ae. albopictus populations exhibit low levels of resistance against pyrethroids and DDT in some areas. Pyrethroid-only bed nets are currently the only programmatic vector control tool used in PNG. It is important to continue to monitor IR in PNG and develop proactive insecticide resistance management strategies in primary disease vectors to retain pyrethroid susceptibility especially in the malaria vectors for as long as possible.

Multiple insecticide resistance and first evidence of V410L kdr mutation in Aedes (Stegomyia) aegypti (Linnaeus) from Burkina Faso

The response to recent dengue outbreaks in Burkina Faso was insecticide-based, despite poor knowledge of the vector population's susceptibility to the insecticides used. Here, we report on the susceptibility to the main insecticide classes and identify important underlying mechanisms in Aedes aegypti populations in Ouagadougou and Banfora, in 2019 and 2020. Wild Ae. aegypti were tested as adults in WHO bioassays and then screened in real time melting curve qPCR analyses to genotype the F1534C, V1016I, and V410L Aedes kdr mutations. Ae. aegypti showed moderate resistance to 0.1% bendiocarb (80-95% survival post-exposure), 0.8% Malathion (60-100%), 0.21% pirimiphos-methyl (75% - 97%), and high resistance to 0.03% deltamethrin (20-70%). PBO pre-exposure partially restored pyrethroid susceptibility. Genotyping detected high frequency of 1534C allele (0.92) and moderate 1016I (0.1-0.32). The V410L mutation was detected in Burkina Faso for the first time (frequency 0.1-0.36). Mosquitoes surviving 4 h exposure to 0.03% deltamethrin had significantly higher frequencies of the F1534C mutation than dead mosquitoes (0.70 vs. 0.96, p < 0.0001) and mosquitoes surviving 2 - 4 h exposure had a significantly reduced life span. Ae. aegypti from Burkina Faso are resistant to multiple insecticide classes with multiple mechanisms involved, demonstrating the essential role of insecticide resistance monitoring within national dengue control programmes.

Differential Hatching, Development, Oviposition, and Longevity Patterns among Colombian Aedes aegypti Populations

Simple Summary

Aedes aegypti is a mosquito that transmits viruses responsible for several diseases in humans, such as dengue, Zika, and chikungunya. It is crucial to study mosquito populations from different countries and regions because control of disease transmission with insecticides can be more effective if adjusted to each population’s characteristics. For this reason, we determined several features of mosquitoes captured in different cities of Colombia: Neiva, Bello, Itagüí, and Riohacha. These included the length of their lifespan, the number of eggs they lay, and the stages in which they die. We found specific patterns for each population. This knowledge will help control programs determine the optimal times to apply insecticides and make surveillance, as well as the type of insecticide used.

Abstract

Dengue, Zika, and chikungunya are arboviral diseases for which there are no effective therapies or vaccines. The only way to avoid their transmission is by controlling the vector Aedes aegypti, but insecticide resistance limits this strategy. To generate relevant information for surveillance and control mechanisms, we determined life cycle parameters, including longevity, fecundity, and mortality, of Colombian Ae. aegypti populations from four different geographical regions: Neiva, Bello, Itagüí, and Riohacha. When reared at 28 °C, Bello had the shortest development time, and Riohacha had the longest. Each mosquito population had its own characteristic fecundity pattern during four gonotrophic cycles. The survival curves of each population were significantly different, with Riohacha having the longest survival in both males and females and Bello the shortest. High mortality was observed in mosquitoes from Neiva in the egg stage and for Bello in the pupae stage. Finally, when mosquitoes from Neiva and Bello were reared at 35 °C, development times and mortality were severely affected. In conclusion, each population has a unique development pattern with an innate trace in their biological characteristics that confers vulnerability in specific stages of development.

Fitness Cost of Sequential Selection with Deltamethrin in Aedes aegypti (Diptera: Culicidae)

In Mexico, Aedes aegypti (L.) is the primary dengue vector, chikungunya, and Zika viruses. The continued use of synthetic pyrethroids has led to the development of resistance in target populations, which has diminished the effectiveness of vector control programs. Resistance has been associated with disadvantages that affect the biological parameters of resistant mosquitoes compared to susceptible ones. In the present study, the disadvantages were evaluated by parameters related to survival and reproduction ('fitness cost') after selection with deltamethrin for five generations. The parameters analyzed were the length of the development cycle, sex ratio, survival, longevity, fecundity, egg viability, preoviposition, oviposition and postoviposition periods, and growth parameters. In the deltamethrin-selected strain, there was a decrease in the development cycle duration, the percentage of pupae, the oviposition period, and eggs viability. Although mean daily fecundity was not affected after the selection process, this, together with the decrease in the survival and fecundity levels by specific age, significantly affected the gross reproductive rate (GRR), net reproductive rate (Ro), and intrinsic growth rate (rm) of the group selected for five generations with deltamethrin compared to the group without selection. Identifying the 'cost' of resistance in biological fitness represents an advantage if it is desired to limit the spread of resistant populations since the fitness cost is the less likely that resistant individuals will spread in the population. This represents an important factor to consider in designing integrated vector management programs.

Genome-wide Association Study Reveals New Loci Associated With Pyrethroid Resistance in Aedes aegypti

Genome-wide association studies (GWAS) use genetic polymorphism across the genomes of individuals with distinct characteristics to identify genotype-phenotype associations. In mosquitoes, complex traits such as vector competence and insecticide resistance could benefit from GWAS. We used the Aedes aegypti 50k SNP chip to genotype populations with different levels of pyrethroid resistance from Northern Brazil. Pyrethroids are widely used worldwide to control mosquitoes and agricultural pests, and their intensive use led to the selection of resistance phenotypes in many insects including mosquitoes. For Ae. aegypti, resistance phenotypes are mainly associated with several mutations in the voltage-gated sodium channel, known as knockdown resistance (kdr). We phenotyped those populations with the WHO insecticide bioassay using deltamethrin impregnated papers, genotyped the kdr alleles using qPCR, and determined allele frequencies across the genome using the SNP chip. We identified single-nucleotide polymorphisms (SNPs) directly associated with resistance and one epistatic SNP pair. We also observed that the novel SNPs correlated with the known kdr genotypes, although on different chromosomes or not in close physical proximity to the voltage gated sodium channel gene. In addition, pairwise comparison of resistance and susceptible mosquitoes from each population revealed differentiated genomic regions not associated with pyrethroid resistance. These new bi-allelic markers can be used to genotype other populations along with kdr alleles to understand their worldwide distribution. The functional roles of the genes near the newly discovered SNPs require new studies to determine if they act synergistically with kdr alleles or reduce the fitness cost of maintaining resistant alleles.

Reversing insecticide resistance with allelic-drive in Drosophila melanogaster

A recurring target-site mutation identified in various pests and disease vectors alters the voltage gated sodium channel (vgsc) gene (often referred to as knockdown resistance or kdr) to confer resistance to commonly used insecticides, pyrethroids and DDT. The ubiquity of kdr mutations poses a major global threat to the continued use of insecticides as a means for vector control. In this study, we generate common kdr mutations in isogenic laboratory Drosophila strains using CRISPR/Cas9 editing. We identify differential sensitivities to permethrin and DDT versus deltamethrin among these mutants as well as contrasting physiological consequences of two different kdr mutations. Importantly, we apply a CRISPR-based allelic-drive to replace a resistant kdr mutation with a susceptible wild-type counterpart in population cages. This successful proof-of-principle opens-up numerous possibilities including targeted reversion of insecticide-resistant populations to a native susceptible state or replacement of malaria transmitting mosquitoes with those bearing naturally occurring parasite resistant alleles.

Insecticide resistance (IR) poses a major global health challenge. Here, the authors generate common IR mutations in laboratory Drosophila strains and use a CRISPR-based allelic-drive to replace an IR allele with a susceptible wild-type counterpart, providing a potent new tool for vector control.

Metabolic Resistance in Permethrin-Resistant Florida Aedes aegypti (Diptera: Culicidae)

Simple Summary

Pyrethroid-oriented vector control programs have increased worldwide to control adult Aedes aegypti mosquitoes and quell Aedes-borne disease epidemics. Due to years of pyrethroid use, resistance to pyrethroids in Ae. aegypti has become a global issue. In Florida, permethrin is the most common pyrethroid adulticide active ingredient used to control mosquito populations. Thus far, all wild Florida Ae. aegypti populations tested against permethrin have been found to be resistant. Metabolic resistance is a major mechanism of resistance in insects in which enzyme-mediated reactions cause the degradation or sequestration of insecticides. We performed assays to investigate the presence of metabolic resistance in 20 Florida Ae. aegypti populations and found that 11 populations (55%) exhibited metabolic resistance due to the action of at least one of three classes of metabolizing enzymes: oxidases, esterases, and glutathione transferases. Additionally, we identified two metabolic enzyme inhibitors: S.S.S-tributyl phosphorotrithioate (DEF; inhibits esterase activity) and diethyl maleate (DM; inhibits glutathione transferase activity), in addition to the commonly used piperonyl butoxide (PBO; inhibits oxidase activity), which were able to increase the efficacy of permethrin against resistant Ae. aegypti populations. Pre-exposure to DEF, PBO, and DM resulted in increased mortality after permethrin exposure in eight (73%), seven (64%), and six (55%) of the Ae. aegypti populations, respectively. Increasing the effectiveness of pyrethroids is important for mosquito control, as it is the primary method used for adult control during mosquito-borne disease outbreaks. Considering that DEF and DM performed similarly to PBO, they may be good candidates for inclusion in formulated pyrethroid products to increase their efficacy against resistant mosquitoes.

Abstract

Aedes aegypti is the principal mosquito vector for many arthropod-borne viruses (arboviruses) including dengue, chikungunya, and Zika. In the United States, excessive permethrin use has led to a high frequency of resistance in mosquitoes. Insecticide resistance is a significant obstacle in the struggle against vector-borne diseases. To help overcome metabolic resistance, synergists that inhibit specific metabolic enzymes can be added to formulated pyrethroid products. Using modified CDC bottle bioassays, we assessed the effect of three inhibitors (piperonyl butoxide (PBO), which inhibits oxidase activity; S.S.S-tributyl phosphorotrithioate (DEF), which inhibits esterase activity; and diethyl maleate (DM), which inhibits glutathione transferase activity) + permethrin. We performed these against 20 Florida Ae. aegypti populations, all of which were resistant to permethrin. Our data indicated that 11 out of 20 populations (55%) exhibited metabolic resistance. Results revealed 73% of these populations had significant increases in mortality attributed to DEF + permethrin, 64% to PBO + permethrin, and 55% to DM + permethrin compared to permethrin alone. Currently, PBO is the only metabolic enzyme inhibitor added to formulated pyrethroid products used for adult mosquito control. Our results suggest that the DEF and DM inhibitors could also be useful additives in permethrin products, especially against metabolically resistant Ae. aegypti mosquitoes. Moreover, metabolic assays should be conducted to better inform mosquito control programs for designing and implementing integrated vector management strategies.

Aedes aegypti continuously exposed to Bacillus thuringiensis svar. israelensis does not exhibit changes in life traits but displays increased susceptibility for Zika virus

Background

Aedes aegypti can transmit arboviruses worldwide, and Bacillus thuringiensis svar. israelensis (Bti)-based larvicides represent an effective tool for controlling this species. The safety of Bti and lack of resistance have been widely reported; however, little is known regarding the impact of the extensive use of these larvicides on the life traits of mosquitoes. Therefore, this study investigated biological parameters, including susceptibility to arbovirus, of an Ae. aegypti strain (RecBti) subjected to 29 generations of exposure to Bti compared with the RecL reference strain.

Methods

The biological parameters of individuals reared under controlled conditions were compared. Also, the viral susceptibility of females not exposed to Bti during their larval stage was analysed by oral infection and followed until 14 or 21 days post-infection (dpi).

Results

RecBti individuals did not display alterations in the traits that were assessed (fecundity, fertility, pupal weight, developmental time, emergence rate, sex ratio and haematophagic capacity) compared to RecL individuals. Females from both strains were susceptible to dengue serotype 2 (DENV-2) and Zika virus (ZIKV). However, RecBti females showed significantly higher rates of ZIKV infection compared with RecL females at 7 (90% versus 68%, Chi-square: χ² = 7.27, df = 1, P = 0.006) and 14 dpi (100% versus 87%, Chi-square: χ² = 7.69, df = 1, P = 0.005) and for dissemination at 7 dpi (83.3% versus 36%, Fisher’s exact test: P < 0.0001, OR = 0.11, 95% CI 0.03–0.32). Quantification of DENV-2 and ZIKV viral particles produced statistically similar results for females from both strains.

Conclusions

Prolonged exposure of Ae. aegypti larvae to Bti did not alter most of the evaluated biological parameters, except that RecBti females exhibited a higher vector susceptibility for ZIKV. This finding is related to a background of Bti exposure for several generations but not to a previous exposure of the tested females during the larval stage. This study highlights mosquito responses that could be associated with the chronic exposure to Bti in addition to the primary larvicidal effect elicited by this control agent.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04880-6.

Bacterial Toxins Active against Mosquitoes: Mode of Action and Resistance

Larvicides based on the bacteria Bacillus thuringiensis svar. israelensis (Bti) and Lysinibacillus sphaericus are effective and environmentally safe compounds for the control of dipteran insects of medical importance. They produce crystals that display specific and potent insecticidal activity against larvae. Bti crystals are composed of multiple protoxins: three from the three-domain Cry type family, which bind to different cell receptors in the midgut, and one cytolytic (Cyt1Aa) protoxin that can insert itself into the cell membrane and act as surrogate receptor of the Cry toxins. Together, those toxins display a complex mode of action that shows a low risk of resistance selection. L. sphaericus crystals contain one major binary toxin that display an outstanding persistence in field conditions, which is superior to Bti. However, the action of the Bin toxin based on its interaction with a single receptor is vulnerable for resistance selection in insects. In this review we present the most recent data on the mode of action and synergism of these toxins, resistance issues, and examples of their use worldwide. Data reported in recent years improved our understanding of the mechanism of action of these toxins, showed that their combined use can enhance their activity and counteract resistance, and reinforced their relevance for mosquito control programs in the future years.

The impact of sublethal permethrin exposure on susceptible and resistant genotypes of the urban disease vector Aedes aegypti

BACKGROUND

In urban environments, some of the most common control tools used against the mosquito disease vector Aedes aegypti are pyrethroid insecticides applied as aerosols, fogs or residual sprays. Their efficacy is compromised by patchy deployment, aging residues, and the evolution and invasion of pyrethroid-resistant mosquitoes. A large proportion of mosquitoes in a given environment will therefore receive sublethal doses of insecticide. The potential impact of this sublethal exposure on the behaviour and biology of Ae. aegypti carrying commonly reported resistance alleles is poorly documented.

RESULTS

In susceptible insects, sublethal exposure to permethrin resulted in reductions in egg viability (13.9%), blood avidity (16.7%) and male mating success (28.3%). It caused a 70% decrease in the lifespan of exposed susceptible females and a 66% decrease in the insecticide-resistant females from the parental strain. Exposure to the same dose of insecticide in the presence of the isolated kdr genotype resulted in a smaller impact on female longevity (a 58% decrease) but a 26% increase in eggs per female and a 37% increase in male mating success. Sublethal permethrin exposure reduced host-location success by 20-30% in all strains.

CONCLUSION

The detrimental effects of exposure on susceptible insects were expected, but resistant insects demonstrated a less predictable range of responses, including negative effects on longevity and host-location but increases in fecundity and mating competitiveness. Overall, sublethal insecticide exposure is expected to increase the competitiveness of resistant phenotypes, acting as a selection pressure for the evolution of permethrin resistance. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Fitness costs of individual and combined pyrethroid resistance mechanisms, kdr and CYP-mediated detoxification, in Aedes aegypti

BACKGROUND

Aedes aegypti is an important vector of many human diseases and a serious threat to human health due to its wide geographic distribution and preference for human hosts. A. aegypti also has evolved widespread resistance to pyrethroids due to the extensive use of this insecticide class over the past decades. Mutations that cause insecticide resistance result in fitness costs in the absence of insecticides. The fitness costs of pyrethroid resistance mutations in A. aegypti are still poorly understood despite their implications for arbovirus transmission.

METHODOLOGY/PRINCIPLE FINDINGS

We evaluated fitness based both on allele-competition and by measuring specific fitness components (i.e. life table and mating competition) to determine the costs of the different resistance mechanisms individually and in combination. We used four congenic A. aegypti strains: Rockefeller (ROCK) is susceptible to insecticides; KDR:ROCK (KR) contains only voltage-sensitive sodium channel (Vssc) mutations S989P+V1016G (kdr); CYP:ROCK (CR) contains only CYP-mediated resistance; and CYP+KDR:ROCK (CKR) contains both CYP-mediated resistance and kdr. The kdr allele frequency decreased over nine generations in the allele-competition study regardless of the presence of CYP-mediated resistance. Specific fitness costs were variable by strain and component measured. CR and CKR had a lower net reproductive rate (R0) than ROCK or KR, and KR was not different than ROCK. There was no correlation between the level of permethrin resistance conferred by the different mechanisms and their fitness cost ratio. We also found that CKR males had a reduced mating success relative to ROCK males when attempting to mate with ROCK females.

CONCLUSIONS/SIGNIFICANCE

Both kdr and CYP-mediated resistance have a fitness cost affecting different physiological aspects of the mosquito. CYP-mediated resistance negatively affected adult longevity and mating competition, whereas the specific fitness costs of kdr remains elusive. Understanding fitness costs helps us determine whether and how quickly resistance will be lost after pesticide application has ceased.

The presence of knockdown resistance mutations reduces male mating competitiveness in the major arbovirus vector, Aedes aegypti

Background

The development of insecticide resistance in mosquitoes can have pleiotropic effects on key behaviours such as mating competition and host-location. Documenting these effects is crucial for understanding the dynamics and costs of insecticide resistance and may give researchers an evidence base for promoting vector control programs that aim to restore or conserve insecticide susceptibility.

Methods and findings

We evaluated changes in behaviour in a backcrossed strain of Aedes aegypti, homozygous for two knockdown resistance (kdr) mutations (V1016G and S989P) isolated in an otherwise fully susceptible genetic background. We compared biting activity, host location behaviours, wing beat frequency (WBF) and mating competition between the backcrossed strain, and the fully susceptible and resistant parental strains from which it was derived. The presence of the homozygous kdr mutations did not have significant effects on blood avidity, the time to locate a host, or WBF in females. There was, however, a significant reduction in mean WBF in males and a significant reduction in estimated male mating success (17.3%), associated with the isolated kdr genotype.

Conclusions

Our results demonstrate a cost of insecticide resistance associated with an isolated kdr genotype and manifest as a reduction in male mating success. While there was no recorded difference in WBF between the females of our strains, the significant reduction in male WBF recorded in our backcrossed strain might contribute to mate-recognition and mating disruption. These consequences of resistance evolution, especially when combined with other pleiotropic fitness costs that have been previously described, may encourage reversion to susceptibility in the absence of insecticide selection pressures. This offers justification for the implementation of insecticide resistance management strategies based on the rotation or alternation of different insecticide classes in space and time.

The mosquito Aedes aegypti is the main vector of dengue, chikungunya, and Zika. Its control relies heavily on the use of insecticides but the rapid evolution of resistance to these chemicals compromises their efficacy. The conservation or restoration of insecticide susceptibility in Ae. aegypti populations is therefore of great importance. Insecticide susceptibility can be encouraged if the evolution of resistance is accompanied by fitness costs that favour susceptible mosquitoes in the absence of insecticides. This paper documents the first report of a reduction in mating success directly associated with an isolated mutation that confers insecticide resistance in Ae. aegypti. This change in behaviour appears related to alterations in male wing-beat frequency. Our results provide evidence of behavioural changes related to insecticide resistance in Ae. aegypti, suggesting a competitive advantage of susceptible individuals in the absence of insecticides in the field.