Multiple origins of pyrethroid resistance in sympatric biotypes of Bemisia tabaci (Hemiptera: Aleyrodidae)

Evaluation of the evolutionary genetics and population structure of Culex pipiens pallens in Shandong province, China based on knockdown resistance (kdr) mutations and the mtDNA-COI gene

BACKGROUND

Mosquitoes are important vectors for a range of diseases, contributing to high rates of morbidity and mortality in the human population. Culex pipiens pallens is dominant species of Culex mosquito in northern China and a major vector for both West Nile virus and Bancroftian filariasis. Insecticide application were largely applied to control the mosquito-mediated spread of these diseases, contributing to increasing rates of resistance in the mosquito population. The voltage-gated sodium channel (Vgsc) gene is the target site of pyrethroids, and mutations in this gene cause knockdown resistance (kdr). While these kdr mutations are known to be critical to pyrethroid resistance, their evolutionary origins remain poorly understood. Clarifying the origins of these mutations is potential to guide further vector control and disease prevention efforts. Accordingly, the present study was designed to study the evolutionary genetics of kdr mutations and their association with the population structure of Cx. p. pallens in Shandong province, China.

METHODS

Adult Culex females were collected from Shandong province and subjected to morphological identification under a dissection microscope. Genomic DNA were extracted from the collected mosquitoes, the Vgsc gene were amplified via PCR and sequenced to assess kdr allele frequencies, intron polymorphisms, and kdr codon evolution. In addition, population genetic diversity and related population characteristics were assessed by amplifying and sequencing the mitochondrial cytochrome C oxidase I (COI) gene.

RESULTS

Totally, 263 Cx. p. pallens specimens were used for DNA barcoding and sequencing analyses to assess kdr allele frequencies in nine Culex populations. The kdr codon L1014 in the Vgsc gene identified two non-synonymous mutations (L1014F and L1014S) in the analyzed population. These mutations were present in the eastern hilly area and west plain region of Shandong Province. However, only L1014F mutation was detected in the southern mountainous area and Dongying city of Shandong Province, where the mutation frequency was low. Compared to other cities, population in Qingdao revealed significant genetic differentiation. Spatial kdr mutation patterns are likely attributable to some combination of prolonged insecticide-mediated selection coupled with the genetic isolation of these mosquito populations.

CONCLUSIONS

These data suggest that multiple kdr alleles associated with insecticide resistance are present within the Cx. p. pallens populations of Shandong Province, China. The geographical distributions of kdr mutations in this province are likely that the result of prolonged and extensive insecticide application in agricultural contexts together with frequent mosquito population migrations. In contrast, the low-frequency kdr mutation detected in central Shandong Province populations may originate from the limited selection pressure in this area and the relative genetic isolation. Overall, the study compares the genetic patterns revealed by a functional gene with a neutral marker and demonstrates the combined impact of demographic and selection factors on population structure.

Evidence for Multiple Origins of Knockdown Resistance (kdr) in Spodoptera exigua (Hübna) (Lepidoptera: Noctuidae) From China

The beet armyworm, Spodoptera exigua (Hübna) is a serious agricultural pest that is challenging to control due to resistance to most pesticides, including pyrethroids. This resistance has previously been linked to the knockdown resistance (kdr) mutation (L1014F) of the voltage-gated sodium channel (VGSC) in S. exigua. To better understand the frequencies of the kdr mutation of SeVGSC and identify the evolutionary origins of kdr mutation in S. exigua, seven populations of S. exigua were collected in China, and partial SeVGSC genomic sequences for each individual were acquired. The bioassays showed that the survival rates of seven populations of S. exigua larvae exposed to the discriminating dose of beta-cypermethrin (0.05 mg/cm2) ranged from 91.66% to 100%, indicating that all seven populations had evolved resistance to beta-cypermethrin. The frequencies of kdr mutation (CTT to TTT) of SeVGSC of field populations ranged China were from 60% to 89.6%. The CTT to CAT substitution at this coding position resulting in the L1014H (kdr-H) mutation was found in only one individual from the QP18 population. Based on the phylogeny of SeVGSC alleles, it appeared that the kdr mutation in S. exigua populations had multiple origins, which has major consequences for pyrethroid effectiveness in the field. Thus, it is recommended to limit the use of pyrethroid and encourage rotation of insecticides with different modes of action for control of S. exigua to alleviate resistance development.

Screening for insecticide resistance in Australian field populations of Bemisia tabaci (Hemiptera: Aleyrodidae) using bioassays and DNA sequencing

BACKGROUND

Species within the Bemisia tabaci cryptic species complex can cause significant crop damage. We used high-throughput amplicon sequencing to identify the species composition and resistance allele genotypes in field populations from cotton fields in Australia. For selected populations, the resistance phenotype was determined in bioassays and compared with sequencing data.

RESULTS

A metabarcoding approach was used to analyse the species composition in 144 field populations collected between 2013 and 2021. Two mixed AUS I and MEAM1 populations were detected, whereas the remaining 142 populations consisted of MEAM1 only. High-throughput sequencing of organophosphate and pyrethroid resistance gene amplicons showed that the organophosphate resistance allele F331W was fixed (> 99%) in all MEAM1 populations, whereas the pyrethroid resistance allele L925I in the voltage-gated sodium channel gene was detected at varying frequencies [1.0%-7.0% (43 populations); 27.7% and 42.1% (two populations); 95%-97.5% (three populations)]. Neither organophosphate nor pyrethroid resistance alleles were detected in the AUS I populations. Pyrethroid bioassays of 85 MEAM1 field-derived populations detected no resistance in 51 populations, whereas 32 populations showed low frequency resistance, and 2 populations were highly resistant.

CONCLUSIONS

We demonstrate that high-throughput sequencing and bioassays are complementary approaches. The detection of target site mutations and the phenotypic provides a comprehensive analysis of the low-level resistance to pyrethroids that is present in Australian cotton farms. By contrast, a limited survey of whitefly populations from horticulture found evidence of high-level resistance against pyrethroids. Furthermore, we found that the F331W allele (linked to organophosphate resistance) is ubiquitous in Australian MEAM1. © 2022 Commonwealth of Australia. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Distribution of invasive versus native whitefly species and their pyrethroid knock-down resistance allele in a context of interspecific hybridization

The invasion success of a species in an agrosystem is greatly influenced by environmental factors such as the use of insecticides, by the intrinsic evolutionary capabilities of the species, and also by interactions with resident species. On the island of La Réunion, the successive invasions of MEAM1 and MED whitefly species over the last 20 years have not only led an increased use of insecticides, but have also challenged the resident IO species. To trace the evolution of the 3 species, and the distribution of the kdr mutation (resistance to pyrethroid) in the para-type voltage-gated sodium channel, we genotyped 41 populations (using neutral nuclear markers) and look at the prevalence of the kdr allele. MEAM1 was predominantly present in agrosystems showing quasi fixation of the resistant kdr allele whereas IO was mainly in natural environments and did not have any resistant allele. Hybridization between the two former species was detected in low frequency but has not led to introgression of resistant alleles in the resident species so far. MED showed a limited distribution in agrosystems but all individuals displayed a resistant allele. These highly contrasting patterns of distribution and resistant mutations between invasive and resident whitefly species are further discussed.

Amplicon sequencing detects mutations associated with pyrethroid resistance in Bemisia tabaci (Hemiptera: Aleyrodidae)

BACKGROUND

Bemisia tabaci (Gennadius) is a major damaging agricultural pest that exhibits high resistance to pyrethroid insecticides. L925I (TTA to ATA) and T929V (ACT to GTT) mutations in the para-type voltage-gated sodium channel (VGSC) are associated with resistance of B. tabaci to pyrethroids. Amplicon sequencing is a reliable and highly efficient method to detect the frequency of mutations linked with insecticide resistance.

RESULTS

Similar frequencies of L925I and T929V mutations were obtained by amplicon sequencing and Sanger sequencing (L925I: 0.3548 vs 0.3619; T929V: 0.6140 vs 0.6381) with overlap of 95% confidence interval in the SX population of B. tabaci. In five populations of B. tabaci from China, the maximum and minimum frequencies of the two mutations were found in the LN (L925I: 0.1126; T929V: 0.8834) and JS (L925I: 0.8776; T929V: 0.1166) populations by amplicon sequencing. However, there was no significant difference in frequencies between the L925I and T929V mutations. The sum frequency of L925I and T929V exceeded 0.9688 in all populations. In addition, a combining mutation, L925 + T929V (L925I and T929V located in same allele), was found in five populations by amplicon sequencing even though its highest frequency was only 0.0157.

CONCLUSION

We established an efficient approach for detecting frequency of mutation by amplicon sequencing. The frequencies of L925I and T929V in VGSC associated with pyrethroid resistance were detected in this study, which could provide foundational data for resistance management of B. tabaci. © 2021 Society of Chemical Industry.

Multiple origins of a single point mutation in the cotton bollworm tetraspanin gene confers dominant resistance to Bt cotton

BACKGROUND

Transgenic crops producing insecticidal proteins derived from Bacillus thuringiensis (Bt) are used globally to kill key insect pests and provide numerous benefits, including improved pest management, increased profits, reduced insecticide use, and increased biological control. Unfortunately, such benefits are rapidly being lost by the evolution of Bt resistance by pests.

RESULTS

The main strategy to delay resistance relies on the use of non-Bt refuge plants to produce sufficient susceptible insects that mate with rare resistant insects emerging from Bt crops, essentially diluting and/or removing resistance alleles from pest populations. A key assumption for the success of this refuge strategy is that inheritance of resistance is recessive. In China, dominant resistance to Cry1Ac Bt cotton by the cotton bollworm Helicoverpa armigera is increasing and is associated with a mutation in the tetraspanin HaTSPAN1 gene, conferring more than 125-fold resistance. Here, we used amplicon sequencing to test the hypotheses that the HaTSPAN1 mutation either arose from a single event and spread or that the mutation evolved independently several times throughout northern China. From three laboratory strains and 28 field populations sampled from northern China, we identified six resistant and 50 susceptible haplotypes. Phylogenetic analysis indicates that the HaTSPAN1 mutation arose from at least four independent origins and spread to their current distributions.

CONCLUSION

The results provide valuable information about the evolutionary origins of dominant resistance to Cry1Ac Bt cotton in northern China and offer rationale for the rapid increase in field-evolved resistance in these areas, where the implementation of additional practical resistance management is needed. © 2020 Society of Chemical Industry.

Inheritance of polygenic but stable pyriproxyfen resistance in a bio-control agent Chrysoperla carnea (Neuroptera: Chrysopidae): cross-resistance and realized heritability

BACKGROUND

Chrysoperla carnea (Neuroptera: Chrysopidae) is a voracious predator frequently used in biological control programs to suppress pest populations of economic importance. However, it performs its duty in a challenging environment where various stress factors such as non-target effects of insecticides limit expected outcomes. A study providing details of genetics, cross-resistance, realized heritability, and stability of insect growth regulators (IGRs) resistance such as pyriproxyfen in this bio-control agent is essential.

RESULTS

Selection with pyriproxyfen, an IGR, resulted in 3092.10-fold and 39.60-fold resistance when judged against Susceptible and Field Pop, respectively. Very low cross-resistance to buprofezin while no cross-resistance to acetamiprid and spinosad was observed. Incompletely dominant, autosomal and polygenic resistance was also associated with high realized heritability (h² = 0.35). Furthermore, resistance to pyriproxyfen was stable in this bio-control agent.

CONCLUSION

These findings make Chrysoperla carnea an ideal fit in integrated pest management (IPM) programs where biological control approaches are employed in combination with IGRs sprays to control various insect pests especially Whitefly, Bemisia tabaci. Releasing pyriproxyfen-resistant Chrysoperla carnea in a multi-sprayed cropping environment would help to keep pest population below economic threshold level. It would also minimize risk of insecticide resistance development in pests surviving even after several insecticide applications. © 2020 Society of Chemical Industry.

Comparison of Toxicological Bioassays for Whiteflies

Simple Summary

Insecticides are commonly used to manage whiteflies in many crops including vegetables, but frequent use can cause these pests to become resistant to insecticides. Resistance can lead to control failure and severe crop damage, thus the need for insecticide efficacy testing and insecticide resistance monitoring. A study was conducted to determine whether any current methods of toxicity assays are better than others for testing whiteflies for insecticide resistance and efficacy for better information to make effective pest control decisions.

Abstract

Two Bemisia tabaci populations from Georgia and Florida, USA, were tested for their response to insecticides across different toxicological bioassay methods. Five insecticides in four Insecticide Resistance Action Committee (IRAC) groups (imidacloprid (4A), dinotefuran (4A), flupyradifurone (4D), pyriproxyfen (7C) and cyantraniliprole (28)), were evaluated against a water check. The routes of application to the plant used were either leaf drench or (systemic) root drench. The four different whitefly bioassay methodologies tested were two published IRAC methods, a clip cage method, and a new tube method. A split–split experimental design was used to assess any interactions between application route, bioassay method and insecticide treatment. Application route had no significant effect on efficacy. However, bioassay method affected overall whitefly mortality, with the dish method having reduced mortality compared to other methods, except for the clip cage method. High rates of cyantraniliprole, dinotefuran and flupyradifurone insecticides resulted in the highest incidence of adult whitefly mortality. Significant interactions relative to percent adult mortality were found between the insecticide and bioassay method for both populations assayed. The clip cage method was more sensitive in terms of dose mortality response followed by the cup and tube methods. The dish method was the least responsive to insecticide dose. Other interactions are discussed.

Recessivity of pyrethroid resistance and limited interspecies hybridization across Hyalella clades supports rapid and independent origins of resistance

Several populations of the amphipod, Hyalella azteca, have developed resistance to pyrethroid insecticides due to non-target exposure, but the dominance of the resistance trait is unknown. The current study investigated the dominance level of point mutations in natural populations of insecticide-resistant H. azteca and determined whether H. azteca from different clades with and without resistant alleles can hybridize and produce viable offspring. A parent generation (P₀) of non-resistant homozygous wild type H. azteca was crossbred with pyrethroid-resistant homozygous mutant animals and the tolerance of the filial 1 (F₁) generation to the pyrethroid insecticide, permethrin, was measured. Then the genotypes of the F₁ generation was examined to assure heterozygosity. The resistant parents had permethrin LC₅₀ values that ranged from 52 to 82 times higher than the non-resistant animals and both crossbreeding experiments produced heterozygous hybrid offspring that had LC₅₀ values similar to the non-resistant H. azteca parent. Dominance levels calculated for each of the crosses showed values close to 0, confirming that the L925I and L925V mutations were completely recessive. The lack of reproduction by hybrids of the C x D breeding confirmed that these clades are reproductively isolated and therefore introgression of adaptive alleles across these clades is unlikely. Potential evolutionary consequences of this selection include development of population bottlenecks, which may arise leading to fitness costs and reduced genetic diversity of H. azteca.

Evidence for both sequential mutations and recombination in the evolution of kdr alleles in Aedes aegypti

BACKGROUND

Aedes aegypti is a globally distributed vector of human diseases including dengue, yellow fever, chikungunya, and Zika. Pyrethroid insecticides are the primary means of controlling adult A. aegypti populations to suppress arbovirus outbreaks, but resistance to pyrethroid insecticides has become a global problem. Mutations in the voltage-sensitive sodium channel (Vssc) gene are a major mechanism of pyrethroid resistance in A. aegypti. Vssc resistance alleles in A. aegypti commonly have more than one mutation. However, our understanding of the evolutionary dynamics of how alleles with multiple mutations arose is poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the geographic distribution and association between the common Vssc mutations (V410L, S989P, V1016G/I and F1534C) in A. aegypti by analyzing the relevant Vssc fragments in 25 collections, mainly from Asia and the Americas. Our results showed all 11 Asian populations had two types of resistance alleles: 1534C and 989P+1016G. The 1534C allele was more common with frequencies ranging from 0.31 to 0.88, while the 989P+1016G frequency ranged from 0.13 to 0.50. Four distinct alleles (410L, 1534C, 410L+1534C and 410L+1016I+1534C) were detected in populations from the Americas. The most common was 410L+1016I+1534C with frequencies ranging from 0.50 to 1.00, followed by 1534C with frequencies ranging from 0.13 to 0.50. Our phylogenetic analysis of Vssc supported multiple independent origins of the F1534C mutation. Our results indicated the 410L+1534C allele may have arisen by addition of the V410L mutation to the 1534C allele, or by a crossover event. The 410L+1016I+1534C allele was the result of one or two mutational steps from a 1534C background.

CONCLUSIONS/SIGNIFICANCE

Our data corroborated previous geographic distributions of resistance mutations and provided evidence for both recombination and sequential accumulation of mutations contributing to the molecular evolution of resistance alleles in A. aegypti.

Comparative Proteomics Analysis Between the Short-Term Stress and Long-Term Adaptation of the Blattella germanica (Blattodea: Blattellidae) in Response to Beta-Cypermethrin

A proteomic method combining two-dimensional polyacrylamide gel electrophoresis and tandem mass spectrometry was used to compare the hemolymph expression profiles of a beta-cypermethrin-resistant Blattella germanica L. strain (R) and a susceptible strain (S) after 24 h of beta-cypermethrin induction. The results showed that there were 42 differentially expressed proteins after induction of the R strain: 4 proteins were upregulated and 38 proteins were downregulated. One hundred one hemolymph proteins were differentially expressed after induction of the S strain: 53 proteins were upregulated and 48 proteins were downregulated. The identified proteins were mainly classified into the following categories: energy metabolism proteins such as arginine kinase and triose phosphate isomerase, detoxification-related proteins such as glutathione S-transferases (GSTs), signal molecule-regulated proteins such as nitric oxide synthase (NOS), and other proteins such as kinetic-related proteins and gene expression-related proteins. Several proteins show significant differences in response to short-term stress and long-term adaptation, and differential expression of these proteins reflects an overall change in cellular structure and metabolism associated with resistance to pyrethroid insecticides. In summary, our research has improved the understanding of the molecular mechanisms of beta-cypermethrin resistance in German cockroaches, which will facilitate the development of rational methods to improve the management of this pest.

Unintentional exposure to terrestrial pesticides drives widespread and predictable evolution of resistance in freshwater crustaceans

Pesticide runoff from terrestrial environments into waterways is often lethal to freshwater organisms, but exposure may also drive evolution of pesticide resistance. We analyzed the degree of resistance and molecular genetic changes underlying resistance in Hyalella azteca, a species complex of freshwater crustaceans inadvertently exposed to pesticide pollution via runoff. We surveyed 16 waterways encompassing most major watersheds throughout California and found that land use patterns are predictive of both pyrethroid presence in aquatic sediments and pyrethroid resistance in H. azteca. Nonsynonymous amino acid substitutions in the voltage-gated sodium channel including the M918L, L925I, or L925V confer resistance in H. azteca. The most frequently identified mutation, L925I, appears to be preferred within the species complex. The L925V substitution has been associated with pyrethroid resistance in another insect, but is novel in H. azteca. We documented a variety of pyrethroid resistance mutations across several species groups within this complex, indicating that pyrethroid resistance has independently arisen in H. azteca at least six separate times. Further, the high frequency of resistance alleles indicates that pesticide-mediated selection on H. azteca populations in waterways equals or exceeds that of targeted terrestrial pests. Widespread resistance throughout California suggests current practices to mitigate off-site movement of pyrethroids are inadequate to protect aquatic life from negative ecological impacts and implies the likelihood of similar findings globally.

Susceptibility of MED-Q1 and MED-Q3 Biotypes of Bemisia tabaci (Hemiptera: Aleyrodidae) Populations to Essential and Seed Oils

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a major pest of many agricultural and ornamental crops in tropical and subtropical regions causing damages that result in important economic losses. Insecticides are commonly used in greenhouses or fields to control B. tabaci populations leading to rapid evolution of resistance that render treatments inefficient. Therefore, and for environmental and human health concerns, other approaches must be developed for this pest management. In the present study, we compare, using the leaf dip method, the toxicity of three essential oils (Cymbopogon citratus, Ocimum americanum, and Hyptis spicigera) and three seed oils (Lannea microcarpa, Lannea acida, and Carapa procera) with three chemical insecticides (acetamiprid, deltamethrin, and chlorpyrifos-ethyl) on adults. Two B. tabaci biotypes (MED-Q1 and MED-Q3) belonging to the Mediterranean species and collected in Burkina Faso were used. Essential oils were analyzed by gas chromatography-mass spectrometry and gas chromatography-flame ionization detector. We showed that these two biotypes have different levels of resistance to the three insecticides, MED-Q3 being more sensitive than MED-Q1. Moreover, they differ in the frequency of resistance alleles to insecticides, especially for organophosphates, as these alleles are almost fixed in MED-Q1. On the other hand, the two biotypes prove to be more susceptible to the plant extracts than to insecticides except for chlorpyrifos-ethyl, with essential oils that showed the highest insecticidal activities. Monoterpenes content were the most abundant and showed the highest insecticidal activities. Our results indicated that essential oils, but also seed oils, have the potential to constitute an alternative strategy of pest management.

Characterization of the voltage-gated sodium channel of the Asian citrus psyllid, Diaphorina citri

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), is an important insect pest of citrus. It is the vector of 'Candidatus' Liberibacter asiaticus, a phloem-limited bacterium that infects citrus, resulting in the disease Huanglongbing (HLB). Disease management relies heavily on suppression of D. citri populations with insecticides, including pyrethroids. In recent annual surveys to monitor insecticide resistance, reduced susceptibility to fenpropathrin was identified in several field populations of D. citri. The primary target of pyrethroids is the voltage-gated sodium channel (VGSC). The VGSC is prone to target-site insensitivity because of mutations that either reduce pyrethroid binding and/or alter gating kinetics. These mutations, known as knockdown resistance or kdr, have been reported in a wide diversity of arthropod species. Alternative splicing, in combination with kdr mutations, has been also associated with reduced pyrethroid efficacy. Here we report the molecular characterization of the VGSC in D. citri along with a survey of alternative splicing across developmental stages of this species. Previous studies demonstrated that D. citri has an exquisite enzymatic arsenal to detoxify insecticides resulting in reduced efficacy. The results from the current investigation demonstrate that target-site insensitivity is also a potential basis for insecticide resistance to pyrethroids in D. citri. The VGSC sequence and its molecular characterization should facilitate early elucidation of the underlying cause of an established case of resistance to pyrethroids. This is the first characterization of a VGSC from a hemipteran to this level of detail, with the majority of the previous studies on dipterans and lepidopterans.

Insecticide resistance status in the whitefly, Bemisia tabaci genetic groups Asia-I, Asia-II-1 and Asia-II-7 on the Indian subcontinent

The present study is a summary of the current level of the insecticide resistance to selected organophosphates, pyrethroids, and neonicotinoids in seven Indian field populations of Bemisia tabaci genetic groups Asia-I, Asia-II-1, and Asia-II-7. Susceptibility of these populations was varied with Asia-II-7 being the most susceptible, while Asia-I and Asia-II-1 populations were showing significant resistance to these insecticides. The variability of the LC₅₀ values was 7x for imidacloprid and thiamethoxam, 5x for monocrotophos and 3x for cypermethrin among the Asia-I, while, they were 7x for cypermethrin, 6x for deltamethrin and 5x for imidacloprid within the Asia-II-1 populations. When compared with the most susceptible, PUSA population (Asia-II-7), a substantial increase in resistant ratios was observed in both the populations of Asia-I and Asia-II-1. Comparative analysis during 2010-13 revealed a decline in susceptibility in Asia-I and Asia-II-1 populations of B. tabaci to the tested organophosphate, pyrethroid, and neonicotinoid insecticides. Evidence of potential control failure was detected using probit analysis estimates for cypermethrin, deltamethrin, monocrotophos and imidacloprid. Our results update resistance status of B. tabaci in India. The implications of insecticide resistance management of B. tabaci on Indian subcontinent are discussed.

Landscape genetic structure and evolutionary genetics of insecticide resistance gene mutations in Anopheles sinensis

Background

Anopheles sinensis is one of the most abundant vectors of malaria and other diseases in Asia. Vector control through the use of insecticides is the front line control method of vector-borne diseases. Pyrethroids are the most commonly used insecticides due to their low toxicity to vertebrates and low repellency. However, the extensive use of insecticides has imposed strong selection pressure on mosquito populations for resistance. High levels of resistance to pyrethroid insecticides and various mutations and haplotypes in the para sodium channel gene that confers knockdown resistance (kdr) have been detected in An. sinensis. Despite the importance of kdr mutations in pyrethroid resistance, the evolutionary origin of the kdr mutations is unknown. This study aims to examine the evolutionary genetics of kdr mutations in relation to spatial population genetic structure of An. sinensis.

Methods

Adults or larvae of Anopheles sinensis were collected from various geographic locations in China. DNA was extracted from individual mosquitoes. PCR amplification and DNA sequencing of the para-type sodium channel gene were conducted to analyse kdr allele frequency distribution, kdr codon upstream and downstream intron polymorphism, population genetic diversity and kdr codon evolution. The mitochondrial cytochrome c oxidase COI and COII genes were amplified and sequenced to examine population variations, genetic differentiation, spatial population structure, population expansion and gene flow patterns.

Results

Three non-synonymous mutations (L1014F, L1014C, and L1014S) were detected at the kdr codon L1014 of para-type sodium channel gene. A patchy distribution of kdr mutation allele frequencies from southern to central China was found. Near fixation of kdr mutation was detected in populations from central China, but no kdr mutations were found in populations from southwestern China. More than eight independent mutation events were detected in the three kdr alleles, and at least one of them evolved multiple times subsequent to their first divergence. Based on sequence analysis of the mitochondrial COI and COII genes, significant and large genetic differentiation was detected between populations from southwestern China and central China. The patchy distribution of kdr mutation frequencies is likely a consequence of geographic isolation in the mosquito populations and the long-term insecticide selection.

Conclusion

Our results indicate multiple origins of the kdr insecticide-resistant alleles in An. sinensis from southern and central China. Local selection related to intense and prolonged use of insecticide for agricultural purposes, as well as frequent migrations among populations are likely the explanations for the patchy distribution of kdr mutations in China. On the contrary, the lack of kdr mutations in Yunnan and Sichuan is likely a consequence of genetic isolation and absence of strong selection pressure. The present study compares the genetic patterns revealed by a functional gene with a neutral marker and demonstrates the combined impact of demographic and selection factors on population structure.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1513-6) contains supplementary material, which is available to authorized users.

Development of a lateral flow test to detect metabolic resistance in Bemisia tabaci mediated by CYP6CM1, a cytochrome P450 with broad spectrum catalytic efficiency

Cotton whitefly, Bemisia tabaci (Genn.) (Homoptera: Aleyrodidae) is a major sucking pest in many agricultural and horticultural cropping systems globally. The frequent use of insecticides of different mode of action classes resulted in populations resisting treatments used to keep numbers under economic damage thresholds. Recently it was shown that resistance to neonicotinoids such as imidacloprid is linked to the over-expression of CYP6CM1, a cytochrome P450 monooxygenase detoxifying imidacloprid and other neonicotinoid insecticides when recombinantly expressed in insect cells. However over-expression of CYP6CM1 is also known to confer cross-resistance to pymetrozine, an insecticide not belonging to the chemical class of neonicotinoids. In addition we were able to demonstrate by LC-MS/MS analysis the metabolisation of pyriproxyfen by recombinantly expressed CYP6CM1. Based on our results CYP6CM1 is one of the most versatile detoxification enzymes yet identified in a pest of agricultural importance, as it detoxifies a diverse range of chemical classes used to control whiteflies. Therefore we developed a field-diagnostic antibody-based lateral flow assay which detects CYP6CM1 protein at levels providing resistance to neonicotinoids and other insecticides. The ELISA based test kit can be used as a diagnostic tool to support resistance management strategies based on the alternation of different modes of action of insecticides.

Detection of genetically isolated entities within the Mediterranean species of Bemisia tabaci: new insights into the systematics of this worldwide pest

BACKGROUND

The taxonomy of the species complex Bemisia tabaci, a serious agricultural pest worldwide, is not well resolved yet, even though species delimitation is critical for designing effective control strategies. Based on a threshold of 3.5% mitochondrial (mtCOI) sequence divergence, recent studies have identified 28 putative species. Among them, mitochondrial variability associated with particular symbiotic compositions (=cytotypes) can be observed, as in MED, which raises the question of whether it is a single or a complex of biological species.

RESULTS

Using microsatellites, an investigation was made of the genetic relatedness of Q1 and ASL cytotypes that belong to MED. Samples of the two cytotypes were collected in West Africa where they live in sympatry on the same hosts. Genotyping revealed a high level of differentiation, without evidence of gene flow. Moreover, they differed highly in frequencies of resistance alleles to insecticides, which were much higher in Q1 than in ASL.

CONCLUSION

Q1 and ASL are sufficiently reproductively isolated for the introgression of neutral alleles to be prevented, suggesting that they are actually different species. This indicates that nuclear genetic differentiation must be investigated within groups with less than 3.5% mtCOI divergence in order to elucidate the taxonomy of B. tabaci at a finer level. Overall, these data provide important information for pest management.

Newly rare or newly common: evolutionary feedbacks through changes in population density and relative species abundance, and their management implications

Environmental management typically seeks to increase or maintain the population sizes of desirable species and to decrease population sizes of undesirable pests, pathogens, or invaders. With changes in population size come long-recognized changes in ecological processes that act in a density-dependent fashion. While the ecological effects of density dependence have been well studied, the evolutionary effects of changes in population size, via changes in ecological interactions with community members, are underappreciated. Here, we provide examples of changing selective pressures on, or evolution in, species as a result of changes in either density of conspecifics or changes in the frequency of heterospecific versus conspecific interactions. We also discuss the management implications of such evolutionary responses in species that have experienced rapid increases or decreases in density caused by human actions.

Genetic structure of Bemisia tabaci Med populations from home-range countries, inferred by nuclear and cytoplasmic markers: impact on the distribution of the insecticide resistance genes

BACKGROUND

Insecticide resistance management in Bemisia tabaci is one of the main issues facing agricultural production today. An extensive survey was undertaken in five Mediterranean countries to examine the resistance status of Med B. tabaci species in its range of geographic origin and the relationship between population genetic structure and the distribution of resistance genes. The investigation combined molecular diagnostic tests, sequence and microsatellite polymorphism studies and monitoring of endosymbionts.

RESULTS

High frequencies of pyrethroid (L925I and T929V, VGSC gene) and organophosphate (F331W, ace1 gene) resistance mutations were found in France, Spain and Greece, but not in Morocco or Tunisia. Sequence analyses of the COI gene delineated two closely related mitochondrial groups (Q1 and Q2), which were found either sympatrically (Spain) or separately (France). Only Q1 was observed in Greece, Morocco and Tunisia. Bayesian analyses based on microsatellite loci revealed three geographically delineated genetic groups (France, Spain, Morocco/Greece/Tunisia) and high levels of genetic differentiation even between neighbouring samples. Evidence was also found for hybridisation and asymmetrical gene flow between Q1 and Q2.

CONCLUSIONS

Med B. tabaci is more diverse and structured than reported so far. On a large geographic scale, resistance is affected by population genetic structure, whereas on a local scale, agricultural practices appear to play a major role.

Nuclear and cytoplasmic differentiation among Mediterranean populations of Bemisia tabaci: testing the biological relevance of cytotypes

BACKGROUND

The taxonomy of the species complex Bemisia tabaci is still an unresolved issue. Recently, phylogenetic analysis based on mtCOI identified 31 cryptic species. However, mitochondrial diversity is observed within these species, associated with distinct symbiotic bacterial communities forming associations, which here are called cytotypes. The authors investigated the biological significance of two cytotypes (Q1 and Q2) belonging to the Mediterranean species, which have only been found in allopatry in the Western Mediterranean to date. Sampling was done over a few years in Western Europe, and sympatric situations were found that allowed their reproductive compatibility to be tested in the field with the use of microsatellites.

RESULTS

The field survey indicated that, in spite of its recent introduction, Q2 is well established in France and Spain, where it coexists with Q1. Microsatellite data showed that, in allopatry, Q1 and Q2 are highly differentiated, while there is little or no genetic differentiation when they coexist in sympatry, suggesting a high rate of hybridisation. Crossing experiments in the lab confirmed their interfertility.

CONCLUSION

Q1 and Q2 hybridise, which confirms that they belong to the same species, in spite of the high degree of genetic differentiation at both the cytoplasmic and nuclear levels, and also suggests that their symbiotic bacteria do not prevent hybridisation.

Differential expression of hemolymph proteins between susceptible and insecticide-resistant Blattella germanica (Blattodea: Blattellidae)

A proteomic approach combining two-dimensional polyacrylamide gel electrophoresis and tandem mass spectrometry was used to compare hemolymph expression profiles of a beta-cypermethrin-resistant Blattella germanica L. strain and a beta-cypermethrin-susceptible strain. Twenty-eight hemolymph proteins were differentially expressed in the resistant cockroach strain; 19 proteins were upregulated and 9 proteins were downregulated compared with the susceptible strain. Protein identification indicated that expression of putative cuticular protein, nitric oxide synthase, triosephosphate isomerase, alpha-amylase, ABC transporter, and Per a 3 allergen was elevated, and expression of arginine kinase and glycosidase was reduced. The differential expression of these proteins reflects the overall change in cellular structure and metabolism related to the resistance of pyrethroid insecticides.

Insecticide resistance in Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) and Anopheles gambiae Giles (Diptera: Culicidae) could compromise the sustainability of malaria vector control strategies in West Africa

Insecticides from the organophosphate (OP) and pyrethroid (PY) chemical families, have respectively, been in use for 50 and 30 years in West Africa, mainly against agricultural pests, but also against vectors of human disease. The selection pressure, with practically the same molecules year after year (mainly on cotton), has caused insecticide resistance in pest populations such as Bemisia tabaci, vector of harmful phytoviruses on vegetables. The evolution toward insecticide resistance in malaria vectors such as Anopheles gambiae sensus lato (s.l.) is probably related to the current use of these insecticides in agriculture. Thus, successful pest and vector control in West Africa requires an investigation of insect susceptibility, in relation to the identification of species and sub species, such as molecular forms or biotypes. Identification of knock down resistance (kdr) and acetylcholinesterase gene (Ace1) mutations modifying insecticide targets in individual insects and measure of enzymes activity typically involved in insecticide metabolism (oxidase, esterase and glutathion-S-transferase) are indispensable in understanding the mechanisms of resistance. Insecticide resistance is a good example in which genotype-phenotype links have been made successfully. Insecticides used in agriculture continue to select new resistant populations of B. tabaci that could be from different biotype vectors of plant viruses. As well, the evolution of insecticide resistance in An. gambiae threatens the management of malaria vectors in West Africa. It raises the question of priority in the use of insecticides in health and/or agriculture, and more generally, the question of sustainability of crop protection and vector control strategies in the region. Here, we review the susceptibility tests, biochemical and molecular assays data for B. tabaci, a major pest in cotton and vegetable crops, and An. gambiae, main vector of malaria. The data reviewed was collected in Benin and Burkina Faso between 2008 and 2010 under the Corus 6015 research program. This review aims to show: (i) the insecticide resistance in B. tabaci as well as in An. gambiae; and (ii) due to this, the impact of selection of resistant populations on malaria vector control strategies. Some measures that could be beneficial for crop protection and vector control strategies in West Africa are proposed.

Multiple origins of pyrethroid insecticide resistance across the species complex of a nontarget aquatic crustacean, Hyalella azteca

Use of pesticides can have substantial nonlethal impacts on nontarget species, including driving evolutionary change, often with unknown consequences for species, ecosystems, and society. Hyalella azteca, a species complex of North American freshwater amphipods, is widely used for toxicity testing of water and sediment and has frequently shown toxicity due to pyrethroid pesticides. We demonstrate that 10 populations, 3 from laboratory cultures and 7 from California water bodies, differed by at least 550-fold in sensitivity to pyrethroids. The populations sorted into four phylogenetic groups consistent with species-level divergence. By sequencing the primary pyrethroid target site, the voltage-gated sodium channel, we show that point mutations and their spread in natural populations were responsible for differences in pyrethroid sensitivity. At least one population had both mutant and WT alleles, suggesting ongoing evolution of resistance. Although nonresistant H. azteca were susceptible to the typical neurotoxic effects of pyrethroids, gene expression analysis suggests the mode of action in resistant H. azteca was not neurotoxicity but was oxidative stress sustained only at considerably higher pyrethroid concentrations. The finding that a nontarget aquatic species has acquired resistance to pesticides used only on terrestrial pests is troubling evidence of the impact of chronic pesticide transport from land-based applications into aquatic systems. Our findings have far-reaching implications for continued uncritical use of H. azteca as a principal species for monitoring and environmental policy decisions.

DNA-based screening for an intracellular cadherin mutation conferring non-recessive Cry1Ac resistance in field populations of Helicoverpa armigera

A number of cadherin mutants conferring resistance to Bt toxin Cry1Ac have been reported in three major lepidopteran pests, including Helicoverpa armigera. Unlike most of the cadherin mutants conferring recessive resistance, an allele (r15) with a 55aa deletion in the intracellular domain of cadherin (HaCad) was previously identified to cause non-recessive resistance to Cry1Ac in H. armigera. In the present study, a DNA-based PCR method was developed to screen the r15 allele from field populations of H. armigera collected from the main cotton planting areas of China in 2011 and 2012. Three heterozygous r15 alleles were detected from 562 moths collected from northern China (with intensive Bt cotton planting), and r15 allele frequency was estimated to be 0.0027. However, no r15 allele was detected from 314 moths collected from Xinjiang (with limited Bt cotton use). Although all the r15 alleles have the same deletion in the cDNA sequence, at least four different indels causing loss of exon 32 have been detected in the genomic DNA sequences flanking exon 32 of HaCad. Multiple origins of the r15 alleles illustrate parallel genotypic adaption of H. armigera to the selection pressure of Bt cotton.

Assessment of potential sublethal effects of various insecticides on key biological traits of the tobacco whitefly, Bemisia tabaci

The tobacco whitefly Bemisia tabaci is one of the most devastating pests worldwide. Current management of B. tabaci relies upon the frequent applications of insecticides. In addition to direct mortality by typical acute toxicity (lethal effect), insecticides may also impair various key biological traits of the exposed insects through physiological and behavioral sublethal effects. Identifying and characterizing such effects could be crucial for understanding the global effects of insecticides on the pest and therefore for optimizing its management in the crops. We assessed the effects of sublethal and low-lethal concentrations of four widely used insecticides on the fecundity, honeydew excretion and feeding behavior of B. tabaci adults. The probing activity of the whiteflies feeding on treated cotton seedlings was recorded by an Electrical Penetration Graph (EPG). The results showed that imidacloprid and bifenthrin caused a reduction in phloem feeding even at sublethal concentrations. In addition, the honeydew excretions and fecundity levels of adults feeding on leaf discs treated with these concentrations were significantly lower than the untreated ones. While, sublethal concentrations of chlorpyrifos and carbosulfan did not affect feeding behavior, honeydew excretion and fecundity of the whitefly. We demonstrated an antifeedant effect of the imidacloprid and bifenthrin on B. tabaci, whereas behavioral changes in adults feeding on leaves treated with chlorpyrifos and carbosulfan were more likely caused by the direct effects of the insecticides on the insects' nervous system itself. Our results show that aside from the lethal effect, the sublethal concentration of imidacloprid and bifenthrin impairs the phloem feeding, i.e. the most important feeding trait in a plant protection perspective. Indeed, this antifeedant property would give these insecticides potential to control insect pests indirectly. Therefore, the behavioral effects of sublethal concentrations of imidacloprid and bifenthrin may play an important role in the control of whitefly pests by increasing the toxicity persistence in treated crops.

Multiple origins of kdr-type resistance in the house fly, Musca domestica

Insecticide resistance is a model phenotype that can be used to investigate evolutionary processes underlying the spread of alleles across a global landscape, while offering valuable insights into solving the problems that resistant pests present to human health and agriculture. Pyrethroids are one of the most widely used classes of insecticides world-wide and they exert their toxic effects through interactions with the voltage-sensitive sodium channel (Vssc). Specific mutations in Vssc (kdr, kdr-his and super-kdr) are known to cause resistance to pyrethroid insecticides in house flies. In order to determine the number of evolutionary origins of kdr, kdr-his and super-kdr, we sequenced a region of Vssc from house flies collected in the USA, Turkey and China. Our phylogenetic analysis of Vssc unequivocally supports the hypothesis of multiple independent origins of kdr, super-kdr and kdr-his on an unprecedented geographic scale. The implications of these evolutionary processes on pest management are discussed.

Short-term and transgenerational effects of the neonicotinoid nitenpyram on susceptibility to insecticides in two whitefly species

The cosmopolitan silverleaf whitefly, Bemisia tabaci which had coexisted with Trialeurodes vaporariorum in Northern China for many years, has become the dominant species in the last years. Recent reports show that it is gradually displacing the other greenhouse whitefly species. Neonicotinoid, which includes nitenpyram, is a major group of insecticides used against whiteflies in various crops. When exposed to low doses of insecticides, insects may develop resistance by adapting physiologically. The short- and long-term effects of nitenpyram on insecticide sensitivity in B. tabaci biotype B and T. vaporariorum adult populations have been compared in the present study. After being exposed to LC(25) of nitenpyram for 24 h, the B. tabaci biotype B adults showed no significant change in susceptibility to nitenpyram or to five other insecticides: imidacloprid, acetamiprid, abamectin, chlorpyrifos and beta-cypermethrin. By contrast, exposure to the LC(25) of nitenpyram for 24 h led to a significant increase in the susceptibility of T. vaporariorum to nitenpyram and imidacloprid, by 1.8- and 2-fold, respectively. When exposed for seven generations to the LC(25) of nitenpyram, B. tabaci developed 6-fold resistance to nitenpyram, and 3.1- and 5-fold cross-resistance to imidacloprid and acetamiprid, respectively, whereas T. vaporariorum developed lower resistance (3.7-fold) to the nitenpyram and very low cross-resistance to imidacloprid (2.5-fold). The higher adaptable nature of B. tabaci (demonstrated here in the case of nitenpyram) when exposed to low doses of insecticides may provide a selective advantage when competing with T. vaporariorum in crops.

Multiple origins of the sodium channel kdr mutations in codling moth populations

Resistance to insecticides is one interesting example of a rapid current evolutionary change. DNA variability in the voltage-gated sodium channel gene (trans-membrane segments 5 and 6 in domain II) was investigated in order to estimate resistance evolution to pyrethroid in codling moth populations at the World level. DNA variation among 38 sequences revealed a unique kdr mutation (L1014F) involved in pyrethroid resistance in this gene region, which likely resulted from several convergent substitutions. The analysis of codling moth samples from 52 apple orchards in 19 countries using a simple PCR-RFLP confirmed that this kdr mutation is almost worldwide distributed. The proportions of kdr mutation were negatively correlated with the annual temperatures in the sampled regions. Homozygous kdr genotypes in the French apple orchards showed lower P450 cytochrome oxidase activities than other genotypes. The most plausible interpretation of the geographic distribution of kdr in codling moth populations is that it has both multiple independent origins and a spreading limited by low temperature and negative interaction with the presence of alternative resistance mechanisms to pyrethroid in the populations.

Identification of mutations associated with pyrethroid resistance in the voltage-gated sodium channel of the tomato leaf miner (Tuta absoluta)

The tomato leaf miner, Tuta absoluta (Lepidoptera) is a significant pest of tomatoes that has undergone a rapid expansion in its range during the past six years and is now present across Europe, North Africa and parts of Asia. One of the main means of controlling this pest is through the use of chemical insecticides. In the current study insecticide bioassays were used to determine the susceptibility of five T. absoluta strains established from field collections from Europe and Brazil to pyrethroids. High levels of resistance to λ cyhalothrin and tau fluvalinate were observed in all five strains tested. To investigate whether pyrethroid resistance was mediated by mutation of the para-type sodium channel in T. absoluta the IIS4-IIS6 region of the para gene, which contains many of the mutation sites previously shown to confer knock down (kdr)-type resistance to pyrethroids across a range of different arthropod species, was cloned and sequenced. This revealed that three kdr/super-kdr-type mutations (M918T, T929I and L1014F), were present at high frequencies within all five resistant strains at known resistance 'hot-spots'. This is the first description of these mutations together in any insect population. High-throughput DNA-based diagnostic assays were developed and used to assess the prevalence of these mutations in 27 field strains from 12 countries. Overall mutant allele frequencies were high (L1014F 0.98, M918T 0.35, T929I 0.60) and remarkably no individual was observed that did not carry kdr in combination with either M918T or T929I. The presence of these mutations at high frequency in T. absoluta populations across much of its range suggests pyrethroids are likely to be ineffective for control and supports the idea that the rapid expansion of this species over the last six years may be in part mediated by the resistance of this pest to chemical insecticides.

Mutations in the sodium channel associated with pyrethroid resistance in the greenhouse whitefly, Trialeurodes vaporariorum

BACKGROUND

Trialeurodes vaporariorum Westwood is an important pest of protected crops in temperate regions of the world. Resistance to pyrethroid insecticides is long established in this species, but the molecular basis of the mechanism(s) responsible has not previously been disclosed.

RESULTS

Mortality rates of three European strains of T. vaporariorum to the pyrethroid bifenthrin were calculated, and each possessed significant resistance (up to 662-fold) when compared with a susceptible reference strain. Direct sequencing revealed three amino acid substitutions in the para-type voltage-gated sodium channel (the pyrethroid and DDT target site) of bifenthrin-resistant T. vaporariorum at positions previously implicated with pyrethroid or DDT resistance (M918L, L925I and T929I) in other related species.

CONCLUSION

This study indicates that resistance to bifenthrin in T. vaporariorum is associated with target-site insensitivity, and that the specific mutations in the sodium channel causing resistance may differ between localities.

Pyrethroid insecticide resistance in British populations of the cabbage whitefly, Aleyrodes proletella

BACKGROUND

The cabbage whitefly, Aleyrodes proletella L., is emerging as a significant pest of field brassica crops in certain regions of the United Kingdom. In order to investigate the contribution of pesticide resistance to this phenomenon, A. proletella populations were sampled from five different areas in England in 2008 and 2009. Adult residual leaf-dip bioassays were carried out using pyrethroid and neonicotinoid insecticides.

RESULTS

Significant resistance to pyrethroids was found in multiple samples collected from two areas. No evidence of cross-resistance to neonicotinoids was found in a subset of the pyrethroid-resistant populations. While the patterns of resistance to different pyrethroids were broadly correlated, the magnitude of resistance factors differed substantially. Survival of strains at a putative diagnostic concentration of lambda-cyhalothrin was found to provide a guide to their LC50. Significant differences in LC50 were found when different brassica crops were used in the bioassay, although the resistance patterns between strains were maintained.

CONCLUSION

Reduced susceptibility to multiple pyrethroid insecticides exists in populations of A. proletella in the United Kingdom, corresponding to recent major outbreaks. The mechanism(s) of resistance are yet to be determined, but molecular structural differences in pyrethroids probably influence the magnitude of cross-resistance within this group of insecticides.

Development of a low-density DNA microarray for diagnosis of target-site mutations of pyrethroid and organophosphate resistance mutations in the whitefly Bemisia tabaci

BACKGROUND

Rapid and accurate detection of mutations related to insecticide resistance is essential for development of resistance management strategies to support sustainable agriculture. The M918V, L925I and T929V mutations of the voltage-gated sodium channel gene (vgsc) and the F392W mutation of the acetylcholinesterase I gene (ace1) are reportedly associated with resistance to pyrethroids and organophosphates, respectively, in Bemisia tabaci. In order to detect known base substitutions in the ace1 and vgsc genes, a low-density microarray with an allele-specific probe was developed.

RESULTS

Specific regions of the ace1 and vgsc gene mutations were amplified by multiplex asymmetrical PCR using Cy3-labelled primers, and then the PCR products were hybridised on the microarray. After analysing the probe signal data, the microarray containing 12 allele-specific probes produced a unique pattern of probe signals for field DNA samples of B. tabaci. To determine the optimal cut-off value of each probe, receiver operating characteristic (ROC) curve analysis was conducted using SPSS. Among 60 individual samples, microarray data for 57 samples were consistent with direct sequencing data.

CONCLUSION

Although many molecular detection methods have been employed to monitor insecticide resistance, the present microarray provides rapid and accurate identification of target mutations in B. tabaci for resistance management.

Pyrethroid resistance in Sitophilus zeamais is associated with a mutation (T929I) in the voltage-gated sodium channel

The maize weevil, Sitophilus zeamais, is the most important pest affecting stored grain in Brazil and its control relies heavily on the use of insecticides. The intensive use of compounds such as the pyrethroids has led to the emergence of resistance, and previous studies have suggested that resistance to both pyrethroids and 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) may result from reduced sensitivity of the insecticide target, the voltage-gated sodium channel. To identify the molecular mechanisms underlying pyrethroid resistance in S. zeamais, the domain II region of the voltage-gated sodium channel (para-orthologue) gene was amplified by PCR and sequenced from susceptible and resistant laboratory S. zeamais strains that were selected with a discriminating dose of DDT. A single point mutation, T929I, was found in the para gene of the resistant S. zeamais populations and its presence in individual weevils was strongly associated with survival after DDT exposure. This is the first identification of a target-site resistance mutation in S. zeamais and unusually it is a super-kdr type mutation occurring in the absence of the more common kdr (L1014F) substitution. A high-throughput assay based on TaqMan single nucleotide polymorphism genotyping was developed for sensitive detection of the mutation and used to screen field-collected strains of S. zeamais. This showed that the mutation is present at low frequency in field populations and is a useful tool for informing control strategies.

Evidence for pre-zygotic reproductive barrier between the B and Q biotypes of Bemisia tabaci (Hemiptera: Aleyrodidae)

The degree of reproductive isolation between the B and Q biotypes of the whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is currently not clear. Laboratory experiments have shown that the two biotypes are capable of producing viable F1 hybrids but that these females are sterile as their F2 generation failed to develop, indicating, most likely, a post-zygotic reproductive barrier. Here, we confirm, by molecular and ecological tools, that the B and Q biotypes of Israel are genetically isolated and provide two independent lines of evidence that support the existence of a pre-zygotic reproductive barrier between them. Firstly, monitoring of mating behaviors in homogeneous and heterogeneous couples indicated no copulation events in heterogeneous couples compared to approximately 50% in homogeneous B and Q couples. Secondly, we could not detect the presence of sperm in the spermathecae of females from heterogeneous couples, compared to 50% detection in intra-B biotype crosses and 15% detection in intra-Q biotype crosses. The existence of pre-zygotic reproductive barriers in Israeli B and Q colonies may indicate a reinforcement process in which mating discrimination is strengthened between sympatric taxa that were formerly allopatric, to avoid maladaptive hybridization. As the two biotypes continued to perform all courtship stages prior to copulation, we also conducted mixed cultures experiments in order to test the reproductive consequences of inter-biotype courtship attempts. In mixed cultures, a significant reduction in female fecundity was observed for the Q biotype but not for the B biotype, suggesting an asymmetric reproductive interference effect in favour of the B biotype. The long-term outcome of this effect is yet to be determined since additional environmental forces may reduce the probability of demographic displacement of one biotype by the other in overlapping niches.

Change in the biotype composition of Bemisia tabaci in Shandong Province of China from 2005 to 2008

Certain biotypes of the Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) complex cause extensive damage and are important pests and virus vectors in agricultural crops throughout the world. Among the most invasive and well studied are the B and Q biotypes. Recent reports in Shandong Province, China, have indicated that the Q biotype was introduced there in approximately 2005, whereas the B biotype has been established there for approximately 10 yr. Even so, the present distribution of the two biotypes in Shandong has not been examined. The results of this study showed that the B and Q biotypes are both present in Shandong Province based on bar-coding using a approximately 450-base fragment of the mitochondrial cytochrome oxidase I (mtCOI) gene. In addition, a B biotype-specific polymerase chain reaction primer pair that amplifies a approximately 300 bp mtCOI fragment was designed and used to examine the biotype composition of B. tabaci in selected crops from six provincial locations, using the general mtCOI primers as an internal positive control for DNA quality. The results of this study indicated that the Q biotype was the predominant B. tabaci colonizing all of the crops in the study sites examined. This suggests that the Q biotype has displaced the B biotype in Shandong Province of China, which until now was the predominant biotype. This is the first report of the displacement of the B by the Q biotype in field grown crops in China, and in a locale where neither the B nor the Q biotype is native. We hypothesize that this phenomenon may have been exacerbated by the widespread use of neonicotinoid insecticides for whitefly control, given the sustained efficacy thus far of neonicotinoids against the B biotype, and their failure at times to effectively control the Q biotype.

Current status of insecticide resistance in Q biotype Bemisia tabaci populations from Crete

BACKGROUND

A major problem of crop protection in Crete, Greece, is the control of Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) with chemical insecticides owing to the rapid development of resistance. The aim of this study was to investigate the establishment of resistance and the underlying mechanisms to major insecticide classes with classical bioassays and known biochemical resistance markers.

RESULTS

During a 2005-2007 survey, 53 Q biotype populations were collected. Application history records showed extensive use of neonicotinoids, organophosphates, carbamates and pyrethroids. High resistance levels were identified in the majority of populations (>80%) for imidacloprid (RF: 38-1958x) and alpha-cypermethrin (RF: 30-600x). Low resistance levels (RF < 12) were observed for pirimiphos-methyl. A strong correlation between resistance to imidacloprid and the number of applications with neonicotinoids was observed. Significant correlations were observed between COE and P450-dependent monoxygenase activity with resistance to alpha-cypermethrin and imidacloprid respectively. A propoxur-based AChE diagnostic test indicated that iAChE was widespread in most populations. Resistance levels for alpha-cypermethrin were increased when compared with a previous survey (2002-2003). Differentiation of LC(50) values between localities was observed for imidacloprid only.

CONCLUSION

Bemisia tabaci resistance evolved differently in each of the three insecticides studied. Imidacloprid resistance seems less established and less persistent than alpha-cypermethrin resistance. The low resistance levels for pirimiphos-methyl suggest absence of cross-resistance with other organophosphates or carbamates used.

Age-specific expression of resistance to a neonicotinoid insecticide in the whitefly Bemisia tabaci

Neonicotinoid insecticides retain a crucial role within many chemical and integrated control strategies for the tobacco whitefly, Bemisia tabaci Gennadius, in spite of the establishment of potent and widespread resistance in many areas. Metabolic resistance mechanisms mediated by overexpression of P450-dependent monooxygenases have been implicated in neonicotinoid resistance in the two most prevalent B. tabaci biotypes. Further characterisation of resistance to the neonicotinoid imidacloprid in populations of both these B- and Q-types is reported.Expression of resistance to imidacloprid was age specific in B- and Q-type strains of B. tabaci. The highest observed resistance ratio at LC(50) expressed in prepupal nymphs was 13, compared with at least 580 in their adult counterparts. For all strains, resistance expressed in immatures was not sufficiently potent to compromise recommended imidacloprid application rates.Targeting neonicotinoids towards immature life stages of B. tabaci may circumvent the protection conferred by current mechanisms of resistance, simultaneously reducing the selection pressures imposed. However, such tactics may enhance the expression of existing resistance mechanisms in immatures, or promote the establishment of novel ones expressed in all life stages.

Organophosphates' resistance in the B-biotype of Bemisia tabaci (Hemiptera: Aleyrodidae) is associated with a point mutation in an ace1-type acetylcholinesterase and overexpression of carboxylesterase

Organophosphate (OP) insecticides are inhibitors of the enzyme acetylcholinesterase (AChE), which terminates nerve impulses by catalyzing the hydrolysis of the neurotransmitter acetylcholine. Previous biochemical studies in Bemisia tabaci (Hemiptera: Aleyrodidae) proposed the existence of two molecular mechanisms for OPs' resistance: carboxylesterase- (COE) mediated hydrolysis or sequestration and decreased sensitivity of AChE. Here, two acetylcholinesterase genes, ace1 and ace2, have been fully cloned and sequenced from an OP-resistant strain and an OP-susceptible strain of B. tabaci. Comparison of nucleic acid and deduced amino acid sequences revealed only silent nucleotide polymorphisms in ace2, and one mutation, Phe392Trp (Phe331 in Torpedo californica), in ace1 of the resistant strain. The Phe392Trp mutation is located in the acyl pocket of the active site gorge and was recently shown to confer OP insensitivity in Culex tritaeniorhynchus. In addition, we also report on the isolation of two carboxylesterase genes (coe1 and coe2) from B. tabaci, the first carboxylesterases to be reported from this species. We show that one of the genes, coe1, is overexpressed ( approximately 4-fold) in the OP-resistant strain, and determine, by quantitative PCR, that the elevated expression is not related to gene amplification but probably to modified transcriptional control. Lastly, we bring new biochemical evidence that support the involvement of both AChE insensitivity and COE metabolism in resistance to OP insecticides in the resistant strain.

Over-expression of cytochrome P450 CYP6CM1 is associated with high resistance to imidacloprid in the B and Q biotypes of Bemisia tabaci (Hemiptera: Aleyrodidae)

The two most damaging biotypes of Bemisia tabaci, B and Q, have both evolved strong resistance to the neonicotinoid insecticide imidacloprid. The major mechanism in all samples investigated so far appeared to be enhanced detoxification by cytochrome P450s monooxygenases (P450s). In this study, a polymerase chain reaction (PCR) technology using degenerate primers based on conserved P450 helix I and heme-binding regions was employed to identify P450 cDNA sequences in B. tabaci that might be involved in imidacloprid resistance. Eleven distinct P450 cDNA sequences were isolated and classified as members of the CYP4 or CYP6 families. The mRNA expression levels of all 11 genes were compared by real-time quantitative RT-PCR across nine B and Q field-derived strains of B. tabaci showing strong resistance, moderate resistance or susceptibility to imidacloprid. We found that constitutive over-expression (up to approximately 17-fold) of a single P450 gene, CYP6CM1, was tightly related to imidacloprid resistance in both the B and Q biotypes. Next, we identified three single-nucleotide polymorphic (SNP) markers in the intron region of CYP6CM1 that discriminate between the resistant and susceptible Q-biotype CYP6CM1 alleles (r-Q and s-Q, respectively), and used a heterogeneous strain to test for association between r-Q and resistance. While survivors of a low imidacloprid dose carried both the r-Q and s-Q alleles, approximately 95% of the survivors of a high imidacloprid dose carried only the r-Q allele. Together with previous evidence, the results reported here identify enhanced activity of P450s as the major mechanism of imidacloprid resistance in B. tabaci, and the CYP6CM1 gene as a leading target for DNA-based screening for resistance to imidacloprid and possibly other neonicotinoids in field populations.

Multiple origins of knockdown resistance mutations in the Afrotropical mosquito vector Anopheles gambiae

How often insecticide resistance mutations arise in natural insect populations is a fundamental question for understanding the evolution of resistance and also for modeling its spread. Moreover, the development of resistance is regarded as a favored model to study the molecular evolution of adaptive traits. In the malaria vector Anopheles gambiae two point mutations (L1014F and L1014S) in the voltage-gated sodium channel gene, that confer knockdown resistance (kdr) to DDT and pyrethroid insecticides, have been described. In order to determine whether resistance alleles result from single or multiple mutation events, genotyping of the kdr locus and partial sequencing of the upstream intron-1 was performed on a total of 288 A. gambiae S-form collected from 28 localities in 15 countries. Knockdown resistance alleles were found to be widespread in West Africa with co-occurrence of both 1014S and 1014F in West-Central localities. Differences in intron-1 haplotype composition suggest that kdr alleles may have arisen from at least four independent mutation events. Neutrality tests provided evidence for a selective sweep acting on this genomic region, particularly in West Africa. The frequency and distribution of these kdr haplotypes varied geographically, being influenced by an interplay between different mutational occurrences, gene flow and local selection. This has important practical implications for the management and sustainability of malaria vector control programs.

Reversal of resistance to pyriproxyfen in the Q biotype of Bemisia tabaci (Hemiptera: Aleyrodidae)

Pyriproxyfen, a juvenile hormone (JH) mimic, is a biorational insecticide that disrupts insect development. It is one of the principal insecticides being used to control Bemisia tabaci (Gennadius) on cotton, and has many environmentally positive attributes that make it compatible with integrated pest management (IPM) programs. In Israel, a high level of resistance to pyriproxyfen has been observed in several isolated regions. Here, tests were conducted to establish whether temporal refuges from exposure to pyriproxyfen could be useful for restoring the effectiveness of the compound. Resistance was found to decrease by a factor of 8 when exposure to pyriproxyfen was ceased for 13 generations. Reversal of resistance was accompanied with increased biotic fitness of the revertant colony. By incorporating experimental estimates of nymph survival, sex ratio, fecundity, egg hatching rate and developmental time, the seasonal cost per generation for resistant insects was estimated to be 25%. A genetic simulation model, optimized by empirical data from bioassays, predicted fitness cost per generation of 19% for resistant homozygous (RR) females and hemizygous (R) males, and produced rates of reversal similar to the experimental results. The model also predicted that, even after 5 years ( approximately 55 generations) without pyriproxyfen treatments, the frequency of the resistance allele (R) will still remain high (0.02). It is therefore concluded, on the basis of experimental and modeling results, that the effectiveness of temporal refuges for reversing development of resistance to pyriproxyfen in B. tabaci may be limited.

At least two origins of fungicide resistance in grapevine downy mildew populations

Quinone outside inhibiting (QoI) fungicides represent one of the most widely used groups of fungicides used to control agriculturally important fungal pathogens. They inhibit the cytochrome bc1 complex of mitochondrial respiration. Soon after their introduction onto the market in 1996, QoI fungicide-resistant isolates were detected in field plant pathogen populations of a large range of species. However, there is still little understanding of the processes driving the development of QoI fungicide resistance in plant pathogens. In particular, it is unknown whether fungicide resistance occurs independently in isolated populations or if it appears once and then spreads globally by migration. Here, we provide the first case study of the evolutionary processes that lead to the emergence of QoI fungicide resistance in the plant pathogen Plasmopara viticola. Sequence analysis of the complete cytochrome b gene showed that all resistant isolates carried a mutation resulting in the replacement of glycine by alanine at codon 143 (G143A). Phylogenetic analysis of a large mitochondrial DNA fragment including the cytochrome b gene (2,281 bp) across a wide range of European P. viticola isolates allowed the detection of four major haplotypes belonging to two distinct clades, each of which contains a different QoI fungicide resistance allele. This is the first demonstration that a selected substitution conferring resistance to a fungicide has occurred several times in a plant-pathogen system. Finally, a high population structure was found when the frequency of QoI fungicide resistance haplotypes was assessed in 17 French vineyards, indicating that pathogen populations might be under strong directional selection for local adaptation to fungicide pressure.