Fitness costs of key point mutations that underlie acaricide target-site resistance in the two-spotted spider mite Tetranychus urticae

A Sex-Specific Trade-Off Between Pesticide Resistance and Tolerance to Heat-Induced Sterility in Tetranychus urticae

Current pest management relies extensively on pesticide application worldwide, despite the frequent rise of pesticide resistance in crop pests. This is particularly worrisome because resistance is often not costly enough to be lost in populations after pesticide application, resulting in increased dependency on pesticide application. As climate warming increases, effort should be put into understanding how heat tolerance will affect the persistence of pesticide resistance in populations. To address this, we measured heat tolerance in two populations of the spider mite crop pest Tetranychus urticae that differ in the presence or absence of a target-site mutation conferring resistance to etoxazole pesticide. We found that developmental time and fertility, but not survival, were negatively affected by increasing temperatures in the susceptible population. Furthermore, we found no difference between resistant and susceptible populations in all life-history traits when both sexes developed at control temperature, nor when females developed at high temperature. Resistant heat-stressed males, in contrast, showed lower fertility than susceptible ones, indicating a sex-specific trade-off between heat tolerance and pesticide resistance. This suggests that global warming could lead to reduced pesticide resistance in natural populations. However, resistant females, being as affected by high temperature as susceptible individuals, may buffer the toll in resistant male fertility, and the shorter developmental time at high temperatures may accelerate adaptation to temperature, the pesticide or the cost thereof. Ultimately, the complex dynamic between these two factors will determine whether resistant populations can persist under climate warming.

A glutamate-gated chloride channel as the mite-specific target-site of dicofol and other diphenylcarbinol acaricides

Dicofol has been widely used to control phytophagous mites. Although dicofol is chemically related to DDT, its mode of action has remained elusive. Here, we mapped dicofol resistance in the spider mite Tetranychus urticae to two genomic regions. Each region harbored a glutamate-gated chloride channel (GluCl) gene that contained a mutation-G314D or G326E-known to confer resistance against the unrelated acaricide abamectin. Using electrophysiology assays we showed that dicofol and other diphenylcarbinol acaricides-bromopropylate and chlorobenzilate-induce persistent currents in Xenopus oocytes expressing wild-type T. urticae GluCl3 receptors and potentiate glutamate responses. In contrast, the G326E substitution abolished the agonistic activity of all three compounds. Assays with the wild-type Drosophila GluClα revealed that this receptor was unresponsive to dicofol. Homology modeling combined with ligand-docking confirmed the specificity of electrophysiology assays. Altogether, this work elucidates the mode of action of diphenylcarbinols as mite-specific agonists of GluCl.

The complex II resistance mutation H258Y in succinate dehydrogenase subunit B causes fitness penalties associated with mitochondrial respiratory deficiency

BACKGROUND

The acaricides cyflumetofen, cyenopyrafen and pyflubumide inhibit the mitochondrial electron transport chain at complex II [succinate dehydrogenase (SDH) complex]. A target site mutation H258Y was recently discovered in a resistant strain of the spider mite pest Tetranychus urticae. H258Y causes strong cross-resistance between cyenopyrafen and pyflubumide, but not cyflumetofen. In fungal pests, fitness costs associated with substitutions at the corresponding H258 position that confer resistance to fungicidal SDH inhibitors have not been uncovered. Here, we used H258 and Y258 near-isogenic lines of T. urticae to quantify potential pleiotropic fitness effects on mite physiology.

RESULTS

The H258Y mutation was not associated with consistent significant changes of single generation life history traits and fertility life table parameters. In contrast, proportional Sanger sequencing and droplet digital polymerase chain reaction showed that the frequency of the resistant Y258 allele decreased when replicated 50:50 Y258:H258 experimentally evolving populations were maintained in an acaricide-free environment for approximately 12 generations. Using in vitro assays with mitochondrial extracts from resistant (Y258) and susceptible (H258) lines, we identified a significantly reduced SDH activity (48% lower activity) and a slightly enhanced combined complex I and III activity (18% higher activity) in the Y258 lines.

CONCLUSION

Our findings suggest that the H258Y mutation is associated with a high fitness cost in the spider mite T. urticae. Importantly, while it is the most common approach, it is clear that only comparing life history traits and life table fecundity does not allow to reliably estimate fitness costs of target site mutations in natural pest populations. © 2023 Society of Chemical Industry.

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.

Biochemical characterisation of Cytochrome P450 oxidoreductase from the cattle tick, Rhipicephalus microplus, highlights potential new acaricide target

Management of the cattle tick, Rhipicephalus microplus, presents a challenge because some populations of this cosmopolitan and economically important ectoparasite are resistant to multiple classes of acaricides. Cytochrome P450 oxidoreductase (CPR) is part of the cytochrome P450 (CYP450) monooxygenases that are involved in metabolic resistance by their ability to detoxify acaricides. Inhibiting CPR, the sole redox partner that transfers electrons to CYP450s, could overcome this type of metabolic resistance. This report represents the biochemical characterisation of a CPR from ticks. Recombinant CPR of R. microplus (RmCPR), minus its N-terminal transmembrane domain, was produced in a bacterial expression system and subjected to biochemical analyses. RmCPR displayed a characteristic dual flavin oxidoreductase spectrum. Incubation with nicotinamide adenine dinucleotide phosphate (NADPH) lead to an increase in absorbance between 500 and 600 nm with a corresponding appearance of a peak absorbance at 340-350 nm indicating functional transfer of electrons between NADPH and the bound flavin cofactors. Using the pseudoredox partner, kinetic parameters for both cytochrome c and NADPH binding were calculated as 26.6 ± 11.4 µM and 7.03 ± 1.8 µM, respectively. The turnover, K_cat, for RmCPR for cytochrome c was calculated as 0.08 s^-1 which is significantly lower than the CPR homologues of other species. IC₅₀ (Half maximal Inhibitory Concentration) values obtained for the adenosine analogues 2', 5' ADP, 2'- AMP, NADP⁺and the reductase inhibitor diphenyliodonium were: 140, 82.2, 24.5, and 75.3 µM, respectively. Biochemically, RmCPR resembles CPRs of hematophagous arthropods more so than mammalian CPRs. These findings highlight the potential of RmCPR as a target for the rational design of safer and potent acaricides against R. microplus.

Intraspecific diversity in the mechanisms underlying abamectin resistance in a cosmopolitan pest

Pesticide resistance relies on a myriad of mechanisms, ranging from single mutations to a complex and polygenic architecture, and it involves mechanisms such as target-site insensitivity, metabolic detoxification, or a combination of these, with either additive or synergistic effects. Several resistance mechanisms against abamectin, a macrocyclic lactone widely used in crop protection, have been reported in the cosmopolitan pest Tetranychus urticae. However, it has been shown that a single mechanism cannot account for the high levels of abamectin resistance found across different mite populations. Here, we used experimental evolution combined with bulked segregant analyses to map quantitative trait loci (QTL) associated with abamectin resistance in two genetically unrelated populations of T. urticae. In these two independent QTL mapping experiments, three and four QTLs were identified, of which three were shared between experiments. Shared QTLs contained genes encoding subunits of the glutamate-gated chloride channel (GluCl) and harboured previously reported mutations, including G314D in GluCl1 and G326E in GluCl3, but also novel resistance candidate loci, including DNA helicases and chemosensory receptors. Surprisingly, the fourth QTL, present only in only one of the experiments and thus unique for one resistant parental line, revealed a non-functional variant of GluCl2, suggesting gene knock-out as resistance mechanism. Our study uncovers the complex basis of abamectin resistance, and it highlights the intraspecific diversity of genetic mechanisms underlying resistance in a cosmopolitan pest.

A mutation in chitin synthase I associated with etoxazole resistance in the citrus red mite Panonychus citri (Acari: Tetranychidae) and its uneven geographical distribution in Japan

BACKGROUND

High-levels of etoxazole resistance have not yet been frequently reported in Panonychus citri. Although a highly resistant strain was discovered in 2014, etoxazole resistance has not become a significant problem in areas of citrus production in Japan. A target site mutation in chitin synthase 1 (CHS1), I1017F, is a major etoxazole-resistance factor in Tetranychus urticae. To investigate the mechanisms of etoxazole resistance and the dispersal of resistance genes, we analyzed target-site mutations in a highly resistant strain and their geographical distribution in Japan.

RESULTS

High-level etoxazole resistance was completely recessive. The I1017F mutation was detected in CHS1 of the highly resistant strain, and its frequency was correlated with the hatchability of eggs treated with etoxazole. Sequencing and variant frequency analyses of local populations by quantitative polymerase chain reaction revealed that I1017F is restricted to the Ariake Sea area of Kyushu Island. Although a new nonsynonymous substitution, S1016L, accompanied by I1017F was found in CHS1 of the highly resistant strain, CRISPR/Cas9 engineering of flies showed that S1016L had no effect on the etoxazole resistance conferred by I1017F.

CONCLUSION

I1017F is a major target site mutation that confers high-level etoxazole resistance on P. citri. Dispersion of I1017F possibly was suppressed as a result of the completely recessive inheritance of resistance together with low gene flow between local populations. © 2022 Society of Chemical Industry.

Lack of Evidence for Fitness Costs in Soybean Aphid (Hemiptera: Aphididae) With Resistance to Pyrethroid Insecticides in the Upper Midwest Region of the United States

Twenty years after the arrival of soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), in the United States, it remains the most economically important arthropod pest of soybean in the Upper Midwest Region. After years of repeated and sustained insecticidal pressures placed on the aphid, resistance to the pyrethroid class of insecticides has been documented in multiple years over a large geographic area. In this study, the fitness of aphid isolates displaying resistant and susceptible phenotypes to λ-cyhalothrin were compared within several experiments over three soybean-growing seasons. Rates of population increase were evaluated on whole plants in the greenhouse, intrinsic rates of increase were calculated from leaf discs in the laboratory, and aphid size and asymmetry were compared through tibial measurements. No evidence of a fitness cost associated with the resistant phenotype was seen in any of our experiments. In contrast, individual resistant isolates occasionally supported significantly higher fitness values than some susceptible isolates. Additionally, a pooled analysis comparing resistant and susceptible phenotypes across years and isolates revealed that, on average, the resistant phenotype had significantly higher fitness values than the susceptible phenotype in most experiments. The lack of reproductive fitness costs associated with the pyrethroid-resistant phenotype raises concerns for longevity of pyrethroid use in soybean aphid management.

A H258Y mutation in subunit B of the succinate dehydrogenase complex of the spider mite Tetranychus urticae confers resistance to cyenopyrafen and pyflubumide, but likely reinforces cyflumetofen binding and toxicity

Succinate dehydrogenase (SDH) inhibitors such as cyflumetofen, cyenopyrafen and pyflubumide, are selective acaricides that control plant-feeding spider mite pests. Resistance development to SDH inhibitors has been investigated in a limited number of populations of the spider mite Tetranychus urticae and is associated with cytochrome P450 based detoxification and target-site mutations such as I260 T/V in subunit B and S56L in subunit C of SDH. Here, we report the discovery of a H258Y substitution in subunit B of SDH in a highly pyflubumide resistant population of T. urticae. As this highly conserved residue corresponds to one of the ubiquinone binding residues in fungi and bacteria, we hypothesized that H258Y could have a strong impact on SDH inhibitors toxicity. Marker assisted introgression and toxicity bioassays revealed that H258Y caused high cross resistance between cyenopyrafen and pyflubumide, but increased cyflumetofen toxicity. Resistance associated with H258Y was determined as dominant for cyenopyrafen, but recessive for pyflubumide. In vitro SDH assays with extracted H258 mitochondria showed that cyenopyrafen and the active metabolites of pyflubumide and cyflumetofen, interacted strongly with complex II. However, a clear shift in IC50s was observed for cyenopyrafen and the metabolite of pyflubumide when Y258 mitochondria were investigated. In contrast, the mutation slightly increased affinity of the cyflumetofen metabolite, likely explaining its increased toxicity for the mite lines carrying the substitution. Homology modeling and ligand docking further revealed that, although the three acaricides share a common binding motif in the Q-site of SDH, H258Y eliminated an important hydrogen bond required for cyenopyrafen and pyflubumide binding. In addition, the hydrogen bond between cyenopyrafen and Y117 in subunit D was also lost upon mutation. In contrast, cyflumetofen affinity was enhanced due to an additional hydrogen bond to W215 and hydrophobic interactions with the introduced Y258 in subunit B. Altogether, our findings not only highlight the importance of the highly conserved histidine residue in the binding of SDH inhibitors, but also reveal that a resistance mutation can provide both positive and negative cross-resistance within the same acaricide mode of action group.

Cross resistance, inheritance and fitness advantage of cyetpyrafen resistance in two-spotted spider mite, Tetranychus urticae

Cyetpyrafen belonging to mitochondrial electron transport inhibitors of complex II (METI II) has been widely applied to manage pest mites in China. To investigate the adaption of Tetranychus urticae in the evolution of cyetpyrafen resistance, a study of cross resistance, mode of inheritance and fitness comparison of resistance using indoor cyetpyrafen resistant strain (resistance ratio, RR > 2, 000-fold) was executed. Cyet-R showed serious cross resistance to cyenopyrafen (>2500-fold) and cyflumetofen (~190-fold). The number of resistant genes was evaluated via chi-square (χ²) test and the concentration-response curve regarding goodness-of-fit between observed and the expected mortality. The LC₅₀s of F_1RS (Cyet-R^♀ × Tu-YN^♂) and F_1SR (Tu-YN^♀ × Cyet-R^♂) were 3126.30 mg/L and 2743.97 mg/L, respectively, without significance, suggesting autosomal inheritance. The degree of dominance (D) values of F_1RS and F_1SR ranged between 0 and 1, revealing an incompletely dominant inheritance in the tested population of Tetranychus urticae. Plots of concentration-response data for the orthogonal backcross and reverse backcross progenies showed a significant deviation from the expected lines, pointing out a polygenic inheritance. Besides, lifetable analysis showed a fitness advantage of Cyet-R with a significantly decreased adult preadult period and significantly increased total fecundity. This study suggested that cyetpyrafen resistance against T. urticae was inherited as autosomal, incompletely dominant and multigenetic and characterized with serious cross resistance and fitness advantage. Therefore, rational application and preventive strategy should be considered to sustain the efficacy of cyetpyrafen against T. urticae.

Fitness studies of insecticide resistant strains: lessons learned and future directions

The evolution of insecticide resistance is generally thought to be associated with a fitness cost in the absence of insecticide exposure. However, it is not clear how these fitness costs manifest or how universal this phenomenon is. To investigate this, we conducted a literature review of publications that studied fitness costs of insecticide resistance, selected papers that met our criteria for scientific rigor, and analyzed each class of insecticides separately as well as in aggregate. The more than 170 publications on fitness costs of insecticide resistance show that in 60% of the experiments there is a cost to having resistance, particularly for measurements of reversion of resistance and reproduction. There were differences between classes of insecticides, with fitness costs seen less commonly for organochlorines. There was considerable variation in the experiments performed. We suggest that future papers will have maximum value to the community if they quantitatively determine resistance levels, identify the resistance mechanisms present (and the associated mutations), have replicated experiments, use related strains (optimally congenic with the resistance mutation introgressed into different genetic backgrounds) and measure fitness by multiple metrics. Studies on the fitness costs of insecticide resistance will continue to enlighten our understanding of the evolutionary process and provide valuable information for resistance management. © 2021 Society of Chemical Industry.

Pyrethroid Resistance and Fitness Cost Conferred by the super-kdr Mutation M918L in Rhopalosiphum padi (Hemiptera: Aphididae)

Pyrethroid insecticides have been widely utilized for insect pest control. Target-site resistance is one of the major mechanisms explaining pest resistance to pyrethroids. This study quantified pyrethroid resistance and fitness cost conferred by the voltage-gated sodium channel (VGSC) M918L mutation in Rhopalosiphum padi. Six s-kdr-SS and six s-kdr-RS parthenogenetic lineages were established from the same field population and were reared in the laboratory without exposure to pesticides for more than one year. Enzyme activity analysis demonstrated that metabolic resistance had no impact on these lineages. Bioassays showed that the M918L mutation strongly affected pyrethroid efficiency, conferring moderate resistance to bifenthrin (type I) (39.0-fold) and high resistance to lambda-cyhalothrin (type II) (194.7-fold). Compared with the life table of s-kdr-SS lineages, s-kdr-RS lineages exhibited a relative fitness cost with significant decreases in longevity and fecundity. Meanwhile, competitive fitness was measured by blending various ratios of s-kdr-SS and s-kdr-SS aphids. The results indicated that M918L-mediated resistance showed a significant fitness cost in the presence of wild aphids without insecticide pressure. The fitness cost strongly correlated with the initial resistance allele frequency. This work characterized the novel s-kdr M918L mutation in R. padi, defined its function in resistance to different types of pyrethroids, and documented that the M918L-mediated resistance has a significant fitness cost.

All resistance alleles are not equal: the high fitness cost of super-kdr in the absence of insecticide

BACKGROUND

Mutations in the voltage-sensitive sodium channel are an important mechanism of resistance to pyrethroid insecticides. In Musca domestica, common resistance alleles are kdr, super-kdr and kdr-his. The levels of resistance that these alleles confer is known, but the fitness of these alleles relative to each other and to susceptible alleles is unknown. We used crosses from congenic strains of M. domestica to establish populations with known allele frequencies and then examined the changes in allele and genotype frequencies over 25 generations under laboratory conditions.

RESULTS

There was a significant fitness cost for the super-kdr allele, which decreased from the starting frequency of 0.25 to 0.05 after 25 generations. The fitness of the kdr, kdr-his and susceptible alleles were similar. The greatest change in genotype frequency was seen for the super-kdr/super-kdr genotype, which was no longer detected after 25 generations.

CONCLUSION

The fitness cost associated with the super-kdr allele is consistent with previous reports and appears to be a factor in helping to restrain high levels of resistance in field populations (the super-kdr allele confers higher levels of resistance than kdr or kdr-his). It is known that the relative costs of different alleles are environmentally dependent, but our results also demonstrate that the relative fitness of given alleles depends on which alleles are present in a given population, as previous pairwise comparisons of allele fitness do not exactly match (except for super-kdr) the results obtained using this four allele study. © 2020 Society of Chemical Industry.

The fitness advantages of bistrifluron resistance related to chitin synthase A in Spodoptera litura (Fab.) (Noctuidae: Lepidoptera)

BACKGROUND

Spodoptera litura is one of the major agricultural pests in China, and it has developed serious resistance to many traditional chemical insecticides. In the present study, the bistrifluron-resistant (Bis-SEL) strain accompanied by a higher oviposition, 113.8-fold RR compared to the bistrifluron-susceptible (Bis-UNSEL) strain, was obtained by bidirectional screening. A comparison of their gonad coefficiency and genes related to oviposition or resistance was used to elucidate the resurgence mechanism.

RESULTS

The ovarian index, oviposition, and potential egg production in the Bis-SEL strain of female adults were significantly higher than those in the Bis-UNSEL strain, and the length of ovariole in the Bis-SEL strain was also significantly elongated. The protein contents of vitellogenin (Vg) and vitellogenin receptor (VgR) in the Bis-UNSEL strain were lower than those in the Bis-SEL strain, consistent with their gene expressions levels, and there was a significantly positive linear correlation between Vg and VgR protein contents, further confirming that resistant strains have high reproductive fitness. Moreover, the chitin synthase A in the Bis-SEL strain was clearly up-regulated, and a mutation (H866Y) near the QRRRW in the catalytic domain caused a rise in the hydrogen bond between UDP-GlcNAc and chitin synthase, and its chitin content was higher than that in the Bis-UNSEL strain. Nevertheless, the sensitivity of the Bis-SEL strain to bistrifluron was significantly recovered when it was knocked down though RNA interference.

CONCLUSION

The fitness advantages of bistrifluron resistance may be related to the up-regulation and mution of chitin synthase A. © 2021 Society of Chemical Industry.

Fitness Costs Associated with Pyrethroid Resistance in Halotydeus destructor (Tucker) (Acari: Penthaleidae) Elucidated Through Semi-field Trials

Pyrethroid resistance in the redlegged earth mite, Halotydeus destructor (Tucker), is primarily attributed to a kdr (knockdown resistance) mutation in the parasodium channel gene. To assess fitness costs associated with this resistance, adult resistant and susceptible populations were mixed in different proportions in microcosm tubs and placed in a shade-house simulating field conditions. Three separate experiments were undertaken whereby parental mites were collected from the field and offspring were followed for two to three generations. The association between fitness costs and kdr-mediated resistance was investigated by examining differences in mite numbers and changes in resistant allele frequencies across generations. In two (of the three) experiments, the population fitness measure of mites was significantly lower in microcosms containing a higher proportion of resistant individuals compared with treatments containing susceptible mites. No differences in mite fitness were observed between treatments in the third experiment; in this instance, the starting proportion of individuals homozygous for the resistant mutation was much lower (~40%) than in the other experiments (>90%). In all three experiments, a decrease in the resistant allele frequency across mite generations was observed. These findings indicate a potential deleterious pleiotropic effect of the kdr mutation on the fitness of H. destructor and have implications for resistance management strategies aimed at this important agricultural pest. Further experiments investigating fitness costs directly in the field are warranted.

Untangling a Gordian knot: the role of a GluCl3 I321T mutation in abamectin resistance in Tetranychus urticae

BACKGROUND

The cys-loop ligand-gated ion channels, including the glutamate-gated chloride channel (GluCl) and GABA-gated chloride channel (Rdl) are important targets for drugs and pesticides. The macrocyclic lactone abamectin primarily targets GluCl and is commonly used to control the spider mite Tetranychus urticae, an economically important crop pest. However, abamectin resistance has been reported for multiple T. urticae populations worldwide, and in several cases was associated with the mutations G314D in GluCl1 and G326E in GluCl3. Recently, an additional I321T mutation in GluCl3 was identified in several abamectin resistant T. urticae field populations. Here, we aim to functionally validate this mutation and determine its phenotypic strength.

RESULTS

The GluCl3 I321T mutation was introgressed into a T. urticae susceptible background by marker-assisted backcrossing, revealing contrasting results in phenotypic strength, ranging from almost none to 50-fold. Next, we used CRISPR-Cas9 to introduce I321T, G314D and G326E in the orthologous Drosophila GluCl. Genome modified flies expressing GluCl I321T were threefold less susceptible to abamectin, while CRISPRed GluCl G314D and G326E flies were lethal. Last, functional analysis in Xenopus oocytes revealed that the I321T mutation might reduce GluCl3 sensitivity to abamectin, but also suggested that all three T. urticae Rdls are affected by abamectin.

CONCLUSION

Three different techniques were used to characterize the role of I321T in GluCl3 in abamectin resistance and, combining all results, our analysis suggests that the I321T mutation has a complex role in abamectin resistance. Given the reported subtle effect, additional synergistic factors in resistance warrant more investigation. © 2020 Society of Chemical Industry.

Transcriptome profiling and functional analysis suggest that the constitutive overexpression of four cytochrome P450s confers resistance to abamectin in Tetranychus urticae from China

BACKGROUND

The two-spotted spider mite Tetranychus urticae is a polyphagous and cosmopolitan pest that has developed high resistance to abamectin, making it difficult to control. Although 'target resistance' related to glutamate-gated chloride channel mutations was found in T. urticae field populations in China, other resistance mechanisms appear to be involved. Here, we conducted genome-wide transcriptome profiling using RNA-sequencing of two abamectin-resistant populations (NB-ZJ and SY-BJ) and one susceptible strain (Lab-SS) to identify differentially expressed genes that might contribute to the resistance of T. urticae to abamectin in China.

RESULTS

Our experiments showed that abamectin resistance was synergized by piperonyl butoxide (PBO) and triphenyl phosphate (TPP), with synergistic ratios (SR) of 2.95-fold and 2.21-fold for PBO and 3.55-fold and 2.84-fold for TPP in NB-ZJ and SY-BJ populations, respectively. Transcriptome data and quantitative real-time PCR (qRT-PCR) revealed that seven detoxification enzyme genes were overexpressed in the two resistant populations. Furthermore, functional analysis by RNA interference (RNAi) indicated that the mortality caused by abamectin was significantly increased by the separate silencing of the P450 genes CYP389C10, CYP392D8, CYP392A11, and CYP392A12.

CONCLUSION

qRT-PCR expression and RNAi data suggest that the overexpression of P450 genes CYP389C10, CYP392D8, CYP392A11, and CYP392A12 may be involved in the abamectin-resistance of field populations of T. urticae in China. This knowledge could facilitate the elucidation of resistance mechanisms and the development of resistance management of T. urticae field populations. © 2020 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.

Identification and characterization of striking multiple-insecticide resistance in a Tetranychus urticae field population from Greece

BACKGROUND

Tetranychus urticae is a notorious crop pest with a worldwide distribution that has developed resistance to a wide range of acaricides. Here, we investigated the resistance levels of a T. urticae population collected from an ornamental greenhouse in Peloponnese, Greece, and analyzed its resistance mechanisms at the molecular level.

RESULTS

Toxicological assays showed resistance against compounds with different modes of action, with resistance ratios of: 89-fold for abamectin; > 1000-fold for clofentezine; > 5000-fold for etoxazole; 27-fold for fenpyroximate and pyridaben; 20- and 36-fold for spirodiclofen and spirotetramat, respectively; and 116- and > 500-fold for cyenopyrafen and cyflumetofen, respectively. Bioassays with synergists indicated the involvement of detoxification enzymes in resistance to abamectin, but not to cyflumetofen and spirodiclofen. RNA sequencing (RNA-seq) analysis showed significant over-expression of several genes encoding detoxification enzymes such as cytochrome P450 monooxygenases and UDP-glycosyltransferases, which have been previously associated with acaricide resistance. Known target-site resistance mutations were identified in acetyl-choline esterase, chitin synthase 1 and NDUFS7/psst, but putative novel resistance mutations were also discovered in targets such as glutamate-gated chloride channel subunit 3. Interestingly, target-site resistance mutations against pyrethroids or bifenazate were not identified, possibly indicating a recent reduced selection pressure in Greece, as well as a possible opportunity to rotate these chemistries.

CONCLUSION

We identified and characterized a striking case of multiple acaricide resistance in a field population of T. urticae. Exceptionally strong resistance phenotypes, with accumulation of multiple resistance mutations and over-expression of P450s and other detoxification genes in the same field population are reported.

Co-Expression of a Homologous Cytochrome P450 Reductase Is Required for In Vivo Validation of the Tetranychus urticae CYP392A16-Based Abamectin Resistance in Drosophila

Simple Summary

The two-spotted spider mite, Tetranychus urticae, is one of the most damaging agricultural pests worldwide, feeding on over 1100 plant species and causing extensive damage to several crops. Chemical acaricides remain the most widely used strategy to control this pest. However, T. urticae has developed significant resistance to numerous acaricide compounds, due to certain features of mite biology and extensive acaricide applications that lead to the selection of resistant pests and subsequently the emergence of resistant populations. Several molecular/genetic mechanisms may contribute to these highly resistant phenotypes. Such mechanisms frequently involve expression of P450 detoxification enzymes, which act together with a partner protein named cytochrome P450 reductase (CPR). In this study, we investigated the potential of a mite P450 enzyme, CYP392A16, to confer resistance to the acaricide abamectin in vivo, when expressed in tissues of the model fruit fly Drosophila melanogaster. We confirmed that expression of this enzyme contributes to abamectin resistance in the fruit fly model, but only when a homologous mite CPR is co-expressed. Our findings indicate that the Drosophila model system can be engineered to facilitate validation of the candidate mite P450s, in order to elucidate resistance mechanisms and their underlying interactions.

Abstract

Overexpression of the cytochrome P450 monooxygenase CYP392A16 has been previously associated with abamectin resistance using transcriptional analysis in the two-spotted spider mite Tetranychus urticae, an important pest species worldwide; however, this association has not been functionally validated in vivo despite the demonstrated ability of CYP392A16 to metabolize abamectin in vitro. We expressed CYP392A16 in vivo via a Gal4 transcription activator protein/Upstream Activating Sequence (GAL4/UAS) system in Drosophila melanogaster flies, driving expression with detoxification tissue-specific drivers. We demonstrated that CYP392A16 expression confers statistically significant abamectin resistance in toxicity bioassays in Drosophila only when its homologous redox partner, cytochrome P450 reductase (TuCPR), is co-expressed in transgenic flies. Our study shows that the Drosophila model can be further improved, to facilitate the functional analysis of insecticide resistance mechanisms acting alone or in combination.

Geographical distribution and molecular insights into abamectin and milbemectin cross-resistance in European field populations of Tetranychus urticae

BACKGROUND

Milbemectin and abamectin are frequently used to control the spider mite Tetranychus urticae. The development of abamectin resistance in this major pest has become an increasing problem worldwide, potentially compromising the use of milbemectin. In this study, a large collection of European field populations was screened for milbemectin and abamectin resistance, and both target-site and metabolic (cross-)resistance mechanisms were investigated.

RESULTS

High to very high levels of abamectin resistance were found in one third of all populations, while milbemectin resistance levels were low for most populations. The occurrence of well-known target-site resistance mutations in glutamate-gated chloride channels (G314D in GluCl1 and G326E in GluCl3) was documented in the most resistant populations. However, a new mutation, I321T in GluCl3, was also uncovered in three resistant populations, while a V327G and L329F mutation was found in GluCl3 of one resistant population. A differential gene-expression analysis revealed the overexpression of detoxification genes, more specifically cytochrome P450 monooxygenase (P450) and UDP-glycosyltransferase (UGT) genes. Multiple UGTs were functionally expressed, and their capability to glycosylate abamectin and milbemectin, was tested and confirmed.

CONCLUSIONS

We found a clear correlation between abamectin and milbemectin resistance in European T. urticae populations, but as milbemectin resistance levels were low, the observed cross-resistance is probably not of operational importance. The presence of target-site resistance mutations in GluCl genes was confirmed in most but not all resistant populations. Gene-expression analysis and functional characterization of P450s and UGTs suggests that also metabolic abamectin resistance mechanisms are common in European T. urticae populations. © 2020 Society of Chemical Industry.

Evolutionary trade-offs of insecticide resistance - The fitness costs associated with target-site mutations in the nAChR of Drosophila melanogaster

The evolution of resistance to drugs and pesticides poses a major threat to human health and food security. Neonicotinoids are highly effective insecticides used to control agricultural pests. They target the insect nicotinic acetylcholine receptor and mutations of the receptor that confer resistance have been slow to develop, with only one field‐evolved mutation being reported to date. This is an arginine‐to‐threonine substitution at position 81 of the nAChR_β1 subunit in neonicotinoid‐resistant aphids. To validate the role of R81T in neonicotinoid resistance and to test whether it may confer any significant fitness costs to insects, CRISPR/Cas9 was used to introduce an analogous mutation in the genome of Drosophila melanogaster. Flies carrying R81T showed an increased tolerance (resistance) to neonicotinoid insecticides, accompanied by a significant reduction in fitness. In comparison, flies carrying a deletion of the whole nAChR_α6 subunit, the target site of spinosyns, showed an increased tolerance to this class of insecticides but presented almost no fitness deficits.

Identification and functional characterization of a novel acetyl-CoA carboxylase mutation associated with ketoenol resistance in Bemisia tabaci

Insecticides of the tetronic/tetramic acid family (cyclic ketoenols) are widely used to control sucking pests such as whiteflies, aphids and mites. They act as inhibitors of acetyl-CoA carboxylase (ACC), a key enzyme for lipid biosynthesis across taxa. While it is well documented that plant ACCs targeted by herbicides have developed resistance associated with mutations at the carboxyltransferase (CT) domain, resistance to ketoenols in invertebrate pests has been previously associated either with metabolic resistance or with non-validated candidate mutations in different ACC domains. A recent study revealed high levels of spiromesifen and spirotetramat resistance in Spanish field populations of the whitefly Bemisia tabaci that was not thought to be associated with metabolic resistance. We confirm the presence of high resistance levels (up to >640-fold) against ketoenol insecticides in both Spanish and Australian B. tabaci strains of the MED and MEAM1 species, respectively. RNAseq analysis revealed the presence of an ACC variant bearing a mutation that results in an amino acid substitution, A2083V, in a highly conserved region of the CT domain. F1 progeny resulting from reciprocal crosses between susceptible and resistant lines are almost fully resistant, suggesting an autosomal dominant mode of inheritance. In order to functionally investigate the contribution of this mutation and other candidate mutations previously reported in resistance phenotypes, we used CRISPR/Cas9 to generate genome modified Drosophila lines. Toxicity bioassays using multiple transgenic fly lines confirmed that A2083V causes high levels of resistance to commercial ketoenols. We therefore developed a pyrosequencing-based diagnostic assay to map the spread of the resistance alleles in field-collected samples from Spain. Our screening confirmed the presence of target-site resistance in numerous field-populations collected in Sevilla, Murcia and Almeria. This emphasizes the importance of implementing appropriate resistance management strategies to prevent or slow the spread of resistance through global whitefly populations.

Significance and interpretation of molecular diagnostics for insecticide resistance management of agricultural pests

Insecticide resistant pests become increasingly difficult to control in current day agriculture. Because of environmental and health concerns, the insecticide portfolio to combat agricultural pests is gradually decreasing. It is therefore crucial to make rational decisions on insecticide use to assure effective resistance management. However, resistance monitoring programs that inform on pest susceptibility and resistance are not yet common practice in agriculture. Molecular markers of resistance that are turned into convenient diagnostic tools are urgently needed and will only increase in importance. This review investigates which factors determine the strength, diagnostic value, and success of a diagnostic marker, and in which cases recent technical advances might provide new opportunities for decision making in an operational meaningful way.

Identification and characterization of new mutations in mitochondrial cytochrome b that confer resistance to bifenazate and acequinocyl in the spider mite Tetranychus urticae

BACKGROUND

In spider mites, mutations in the mitochondrial cytochrome b Q_o pocket have been reported to confer resistance to the Q_o inhibitors bifenazate and acequinocyl. In this study, we surveyed populations of the two-spotted spider mite Tetranychus urticae for mutations in cytochrome b, linked newly discovered mutations with resistance and assessed potential pleiotropic fitness costs.

RESULTS

We identified two novel mutations in the Q_o site: G132A (equivalent to G143A in fungi resistant to strobilurins) and G126S + A133T (previously reported to cause bifenazate and acequinocyl resistance in Panonychus citri). Two T. urticae strains carrying G132A were highly resistant to bifenazate but not acequinocyl, whereas a strain with G126S + A133T displayed high levels of acequinocyl resistance, but only moderate levels of bifenazate resistance. Bifenazate and acequinocyl resistance were inherited maternally, providing strong evidence for the involvement of these mutations in the resistance phenotype. Near isogenic lines carrying G132A revealed several fitness penalties in T. urticae; a lower net reproductive rate (R₀ ), intrinsic rate of increase (rm) and finite rate of increase (LM); a higher doubling time (DT); and a more male-biased sex ratio.

CONCLUSIONS

Several lines of evidence were provided to support the causal role of newly discovered cytochrome b mutations in bifenazate and acequinocyl resistance. Because of the fitness costs associated with the G132A mutation, resistant T. urticae populations might be less competitive in a bifenazate-free environment, offering opportunities for resistance management. © 2019 Society of Chemical Industry.

Physiological resistance alters behavioral response of Tetranychus urticae to acaricides

Multiple acaricide resistance in Tetranychus urticae continues to threaten crop production globally, justifying the need to adequately study resistance for sustainable pest management. Most studies on acaricide resistance have focused on the acute contact toxicity of acaricides with little or no information on the behavioral responses elicited after acaricide exposure. Furthermore, the impact of physiological resistance on these behavioral responses remains unknown in most pest species, including T. urticae. We tested the effect of acaricide resistance on contact toxicity, irritancy and repellency of mitochondrial electron transport inhibitor of complex I (MET-I) and mite growth inhibitor (MGI) acaricides on multiple T. urticae strains. We also tested whether acaricides with similar physiological target site/mode of action also elicit similar behavioral effects on T. urticae strains. MET-I acaricides (fenazaquin, fenpyroximate, and pyrabiden) and MGIs (clofentezine, hexythiazox and etoxazole) elicited a dose-dependent irritant and repellent effect on T. urticae. Selection of strains for physiological resistance to these acaricides affected the behavioral response of T. urticae, especially in MET-I resistant strains, that showed reduced irritancy and repellency to MET-I acaricides. Behavioral response also affected the oviposition of T. urticae, where strains generally showed preferential oviposition away from the acaricides. The outcome of this study highlights negative consequences of acaricide resistance that can potentially affect T. urticae management.

Pyrethroid resistance in the pest mite, Halotydeus destructor: Dominance patterns and a new method for resistance screening

An L1024F substitution in the para gene, which encodes a subunit of the voltage-gated sodium channel, has been implicated in pyrethroid resistance in a mite pest, Halotydeus destructor, which attacks rape and other grain crops. A high-resolution melt (HRM) genotyping assay was developed for testing the relative pyrethroid susceptibility of different para genotypes and for high-throughput field screening of resistant alleles. The L1024F mutation was found to be incompletely recessive in phenotypic laboratory bioassays with the pyrethroid pesticide, bifenthrin. While the resistance ratio of heterozygotes (RS) to susceptible homozygotes (SS) was <6 in 24 h bioassays, the resistant homozygotes (RR) (with a resistance ratio > 200,000) survived the recommended field rate of bifenthrin (100 mgL^-1). HRM genotyping of mites from field populations across Australia indicated the presence of resistant alleles in Western Australia and South Australia, but not in Victoria and New South Wales. The assay developed will be useful for routine screening of pyrethroid resistance, and the dominance relationships established here point to useful resistance management strategies involving the maintenance of reservoirs of susceptible mites to dilute resistant homozygotes in a population.

Resistance incidence and presence of resistance mutations in populations of Tetranychus urticae from vegetable crops in Turkey

Tetranychus urticae Koch is one of the most common and harmful pests in vegetable production areas. Similar to other countries, control of T. urticae is mainly based on acaricides in Turkey. However, T. urticae rapidly develops resistance and failures in chemical control have occurred frequently. The toxicity of various acaricides was investigated in ten T. urticae populations collected from vegetable crops in Turkey. In addition, populations were screened for the presence of currently known target-site resistance mutations. It was shown that resistance to bifenthrin was the most widespread, but also half of the populations were resistant to abamectin and hexythiazox. Resistance mutations in the voltage-gated sodium channel (VGSC) and chitin synthase 1 were found in various populations. Moreover, for the first time, F1538I and L1024V VGSC mutations were reported for Turkish populations. Mutations that confer resistance to abamectin, bifenazate and METI-I acaricides such as pyridaben were not detected. These results will contribute to the design of an effective resistance management program in Turkey.

Point mutations in the voltage-gated sodium channel gene associated with pyrethroid resistance in Iranian populations of the European red mite Panonychus ulmi

The European red mite Panonychus ulmi (Koch) is a major pest of apple trees worldwide and causes significant damage to apple orchards in Iran. Pyrethroid insecticides/acaricides, such as fenpropathrin and fenvalerate, are widely used to control P. ulmi, but their long-term use may lead to low efficacy. Earlier studies investigating pyrethroid resistance in closely related mites such as Tetranychus urticae revealed that pyrethroid resistance was associated with point mutations in the voltage-gated sodium channel gene (vgsc). The aim of this study was to investigate the biochemical and molecular mechanisms of fenpropathrin and fenvalerate resistance in Iranian populations of P. ulmi. Pyrethroid toxicity bioassays were carried out on different P. ulmi field populations. Marand (resistance ratio, RR = 149), Maraqeh (RR = 90) and Mianeh2 (RR = 71) populations exhibited high levels of resistance to fenpropathrin, compared to a susceptible field population (Shahin Dej). Resistance was also observed for fenvalerate with resistance ratio's ranging from 2- to 20-fold. Synergism experiments and enzyme activity assays predicted a minor role for classical detoxification enzymes. In contrast, two amino acid substitutions in the VGSC, L1024V and F1538I, that were previously shown to confer pyrethroid resistance, were detected in all three resistant P. ulmi populations and point towards target-site insensitivity as the most likely resistance mechanism. Furthermore, sequencing after cloning of vgsc fragments from single haploid males revealed the presence of multiple copies of vgsc in a highly resistant strain. The link between resistance mutations and vgsc copy number variation should be the subject of future study, as this might be used to develop molecular markers for monitoring pyrethroid resistance of P. ulmi in the field.

Long-Term Population Studies Uncover the Genome Structure and Genetic Basis of Xenobiotic and Host Plant Adaptation in the Herbivore Tetranychus urticae

Pesticide resistance arises rapidly in arthropod herbivores, as can host plant adaptation, and both are significant problems in agriculture. These traits have been challenging to study as both are often polygenic and many arthropods are genetically intractable. Here, we examined the genetic architecture of pesticide resistance and host plant adaptation in the two-spotted spider mite, Tetranychus urticae, a global agricultural pest. We show that the short generation time and high fecundity of T. urticae can be readily exploited in experimental evolution designs for high-resolution mapping of quantitative traits. As revealed by selection with spirodiclofen, an acetyl-CoA carboxylase inhibitor, in populations from a cross between a spirodiclofen-resistant and a spirodiclofen-susceptible strain, and which also differed in performance on tomato, we found that a limited number of loci could explain quantitative resistance to this compound. These were resolved to narrow genomic intervals, suggesting specific candidate genes, including acetyl-CoA carboxylase itself, clustered and copy variable cytochrome P450 genes, and NADPH cytochrome P450 reductase, which encodes a redox partner for cytochrome P450s. For performance on tomato, candidate genomic regions for response to selection were distinct from those responding to the synthetic compound and were consistent with a more polygenic architecture. In accomplishing this work, we exploited the continuous nature of allele frequency changes across experimental populations to resolve the existing fragmented T. urticae draft genome to pseudochromosomes. This improved assembly was indispensable for our analyses, as it will be for future research with this model herbivore that is exceptionally amenable to genetic studies.