The spread of resistance to imidacloprid is restricted by thermotolerance in natural populations of Drosophila melanogaster

Pollution offsets the rapid evolution of increased heat tolerance in a natural population

Despite the increasing evidence for rapid thermal evolution in natural populations, evolutionary rescue under global warming may be constrained by the presence of other stressors. Highly relevant in our polluted planet, is the largely ignored evolutionary trade-off between heat tolerance and tolerance to pollutants. By using two subpopulations (separated 40 years in time) from a resurrected natural population of the water flea Daphnia magna that experienced a threefold increase in heat wave frequency during this period, we tested whether rapid evolution of heat tolerance resulted in reduced tolerance to the widespread metal zinc and whether this would affect heat tolerance upon exposure to the pollutant. Our results revealed rapid evolution of increased heat tolerance in the recent subpopulation. Notably, the sensitivity to the metal tended to be stronger (reduction in net energy budget) or was only present (reductions in heat tolerance and in sugar content) in the recent subpopulation. As a result, the rapidly evolved higher heat tolerance of the recent subpopulation was fully offset when exposed to zinc. Our results highlight that the many reports of evolutionary rescue to global change stressors may give a too optimistic view as our warming planet is polluted by metals and other pollutants.

Predicting the Combined Effects of Multiple Stressors and Stress Adaptation in Gammarus pulex

Global change confronts organisms with multiple stressors causing nonadditive effects. Persistent stress, however, leads to adaptation and related trade-offs. The question arises: How can the resulting effects of these contradictory processes be predicted? Here we show that Gammarus pulex from agricultural streams were more tolerant to clothianidin (mean EC50 148 μg/L) than populations from reference streams (mean EC50 67 μg/L). We assume that this increased tolerance results from a combination of physiological acclimation, epigenetic effects, and genetic evolution, termed as adaptation. Further, joint exposure to pesticide mixture and temperature stress led to synergistic interactions of all three stressors. However, these combined effects were significantly stronger in adapted populations as shown by the model deviation ratio (MDR) of 4, compared to reference populations (MDR = 2.7). The pesticide adaptation reduced the General-Stress capacity of adapted individuals, and the related trade-off process increased vulnerability to combined stress. Overall, synergistic interactions were stronger with increasing total stress and could be well predicted by the stress addition model (SAM). In contrast, traditional models such as concentration addition (CA) and effect addition (EA) substantially underestimated the combined effects. We conclude that several, even very disparate stress factors, including population adaptations to stress, can act synergistically. The strong synergistic potential underscores the critical importance of correctly predicting multiple stresses for risk assessment.

Dissipation and residues of imidacloprid in amaranth under greenhouse and open field cultivations

Despite the extensive exposure to imidacloprid residues in food plants, there has been little research on imidacloprid residues in amaranth. The dissipation trend and residue behavior of imidacloprid were evaluated to provide guidelines for imidacloprid application on amaranth under open field and greenhouse. The dissipation rate of imidacloprid in amaranth conformed to the first-order kinetic equation, and the half-lives of imidacloprid in amaranth ranged from 0.29 days in open field to 1.29 days in the greenhouse. After 7 and 14 days from the application of imidacloprid (pesticide dosage, 45 or 67.5 g a.i./ha), the amaranth under the open field and greenhouse growth could be consumed safely with average residues of 0.19 and 0.38 mg/kg, respectively. This result demonstrated that the cultivation has the dominant influence on imidacloprid residue, and the residue of imidacloprid in amaranth planting on open field was much lower than that in the greenhouse, indicating a significant difference in the pesticide residues between the two cultivations with a p-value less than 0.05.

Revealing the cascade of pesticide effects from gene to community

Global pesticide exposure in agriculture leads to biodiversity loss, even at ultra-low concentrations below the legal limits. The mechanisms by which the effects of toxicants act at such low concentrations are still unclear, particularly in relation to their propagation across the different biological levels. In this study, we demonstrate, for the first time, a cascade of effects from the gene to the community level. At the gene level, agricultural pesticide exposure resulted in reduced genetic diversity of field-collected Gammarus pulex, a dominant freshwater crustacean in Europe. Additionally, we identified alleles associated with adaptations to pesticide contamination. At the individual level, this genetic adaptation to pesticides was linked to a lower fecundity, indicating related fitness costs. At the community level, the combined effect of pesticides and competitors caused a decline in the overall number and abundance of pesticides susceptible macroinvertebrate competing with gammarids. The resulting reduction in interspecific competition provided an advantage for pesticide-adapted G. pulex to dominate macroinvertebrate communities in contaminated areas, despite their reduced fitness due to adaptation. These processes demonstrate the complex cascade of effects, and also illustrate the resilience and adaptability of biological systems across organisational levels to meet the challenges of a changing environment.

Identification of a receptor for the sex pheromone of the vine mealybug, Planococcus ficus

The vine mealybug, Planococcus ficus, is a significant pest of vineyards in all major grape growing regions of the world. This pest causes significant aesthetic damage to berry clusters through its feeding behavior and secretion of "honeydew", which leads to significant decreases in crop marketability. More importantly, the vine mealybug is a vector of several grapevine viruses which are the causal agent of grapevine leafroll disease, one of the most destructive and economically devastating diseases of the grape industry worldwide. As there is no cure for grapevine leafroll disease, the only control measures available to reduce its spread are to remove infected vines whilst simultaneously controlling mealybug populations. Using transcriptomic libraries prepared from male and female mealybugs and a draft genome, we identified and evaluated expression levels of members of the odorant receptor gene family. Interestingly, of the 50 odorant receptors identified from these P. ficus genetic resources, only 23 were found to be expressed in females, suggesting this flightless life stage has a decreased reliance on the olfactory system. In contrast, 46 odorant receptors were found to be expressed in the alate male life stage. Heterologous expression of eight of these receptors, along with the obligate co-receptor, Orco, in HEK293 cells allowed for the identification of two receptors that respond to lavandulyl senecioate, the sole constituent of the sex pheromone used by this species. Interestingly, one of these receptors, PficOR8, also responded to the sex pheromone used by the Japanese mealybug, Planococcus kraunhiae. The data presented here represent the first report of odorant receptor gene family expression levels, as well as the identification of the first sex pheromone receptor, in soft-scale insects. The identification of a receptor for the vine mealybug sex pheromone will allow for the development of novel, species-specific pest control tools and monitoring devices.

Application of CRISPR/Cas9-based genome editing in ecotoxicology

Chemicals are widely used and released into the environment, and their degradation, accumulation, migration, and transformation processes in the environment can pose a threat to the ecosystem. The advancement in analytical methods with high-throughput screening of biomolecules has revolutionized the way toxicologists used to explore the effects of chemicals on organisms. CRISPR/Cas is a newly developed tool, widely used in the exploration of basic science and biologically engineered products given its high efficiency and low cost. For example, it can edit target genes efficiently, and save loss of the crop yield caused by environmental pollution as well as gain a better understanding of the toxicity mechanisms from various chemicals. This review briefly introduces the development history of CRISPR/Cas and summarizes the current application of CRISPR/Cas in ecotoxicology, including its application on improving crop yield and drug resistance towards agricultural pollution, antibiotic pollution and other threats. The benefits by applying the CRISPR/Cas9 system in conventional toxicity mechanism studies are fully demonstrated here together with its foreseeable expansions in other area of ecotoxicology. Finally, the prospects and disadvantages of CRISPR/Cas system in the field of ecotoxicology are also discussed.

Rapid adaptation in a fast-changing world: Emerging insights from insect genomics

Many researchers have questioned the ability of biota to adapt to rapid anthropogenic environmental shifts. Here, we synthesize emerging genomic evidence for rapid insect evolution in response to human pressure. These new data reveal diverse genomic mechanisms (single locus, polygenic, structural shifts; introgression) underpinning rapid adaptive responses to a variety of anthropogenic selective pressures. While the effects of some human impacts (e.g. pollution; pesticides) have been previously documented, here we highlight startling new evidence for rapid evolutionary responses to additional anthropogenic processes such as deforestation. These recent findings indicate that diverse insect assemblages can indeed respond dynamically to major anthropogenic evolutionary challenges. Our synthesis also emphasizes the critical roles of genomic architecture, standing variation and gene flow in maintaining future adaptive potential. Broadly, it is clear that genomic approaches are essential for predicting, monitoring and responding to ongoing anthropogenic biodiversity shifts in a fast-changing world.

Genetic variation of the interaction type between two stressors in a single population: From antagonism to synergism when combining a heat spike and a pesticide

Despite the surging interest in the interactions between toxicants and non-chemical stressors, and in evolutionary ecotoxicology, we have poor knowledge whether these patterns differ among genotypes within a population. Warming and toxicants are two widespread stressors in aquatic systems that are known to modify each other's effects. We studied to what extent effects of sequential exposure to a heat spike and the pesticide esfenvalerate differed among genotypes in the water flea Daphnia magna. Esfenvalerate had similar negative effects on survival and body size across genotypes, and for most genotypes it increased time to maturation, yet the effects on the reproductive performance were only detected in some genotypes and were inconsistent in direction. Across genotypes, the heat spike increased the heat tolerance, yet the negative effects of the heat spike on survival, reproductive performance and body size, and the positive effects on grazing rate and the shortened time to maturation were only seen in some genotypes. Notably, the interaction type between both stressors differed among genotypes. In contrast to our expectation, the impact of esfenvalerate was only magnified by the heat spike in some genotypes and only for a subset of the traits. For survival and time to maturation, the interaction type for the same stressor combination covered all three categories: additions, synergisms and antagonisms. This illustrates that categorizing the interaction type between stressors at the level of populations may hide considerable intrapopulation variation among genotypes. Opposite to our expectation, the more pesticide-tolerant genotypes showed a stronger synergism between both stressors. Genotype-dependent interaction patterns between toxicants and non-chemical stressors may explain inconsistencies among studies and challenges ecological risk assessment based on single genotypes. The observed genetic differences in the responses to the (combined) stressors may fuel the evolution of the stressor interaction pattern, a largely ignored topic in evolutionary ecotoxicology.

Warmer temperatures reduce chemical tolerance in the redlegged earth mite (Halotydeus destructor), an invasive winter-active pest

BACKGROUND

Quantifying how chemical tolerance of pest arthropods varies with temperature is important for understanding the outcomes of chemical control, for measuring and monitoring resistance, and for predicting how pesticide resistance will evolve under future climate change. We studied the redlegged earth mite, Halotydeus destructor (Tucker), a winter-active invasive agricultural pest in Australia. Using a replicated block experiment, we tested the effect of different thermal conditions on the expression of chemical tolerance to a pyrethroid and two organophosphates. Our chemical bioassays were conducted on two redlegged earth mite populations: one possessed organophosphate resistance, whilst the other was susceptible to pesticides. Mites were first acclimated at cool (4 °C) and warm (14 °C) conditions and then exposed to pesticides in both cool (11 °C) and warm (18 °C) test conditions.

RESULTS

Warm test conditions generally reduced chemical tolerance to all pesticides relative to cool test conditions. Median lethal dose (LD₅₀ ) values of mites tested under cool conditions were 1.12-3.57-fold greater than of mites tested under warm conditions. Acclimation had a variable and small impact on chemical responses. Thermal factors (ratio between test temperatures) were similar between populations for each active ingredient. Despite reduced chemical tolerances under warm test conditions for individual mite populations, resistance factors (ratio between resistant and susceptible mite populations) were relatively consistent.

CONCLUSION

Our data provides context for prior theoretical work demonstrating climatically constrained pesticide resistances in Australian redlegged earth mites. Estimates of temperature dependent toxicity measured in this study may be useful in parameterizing models of redlegged earth mite control under an increasingly warm and more variable climate. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Emerging Contaminant Imidacloprid in Mediterranean Soils: The Risk of Accumulation Is Greater than the Risk of Leaching

Imidacloprid (IMI) is an extensively used neonicotinoid insecticide whose occurrence in the environment is a worldwide problem. Its sorption/transport properties are recognized as one of the key knowledge gaps hindering policymaking regarding its international routine monitoring in soils. Therefore, we studied IMI transport behaviour in Croatian Mediterranean soils using column experiments. Breakthrough curves were analysed using the two-site adsorption model and compared against dimethoate (DIM). Transport parameters were correlated to soil physicochemical properties. The results indicate that IMI shows a high degree of preference for soil organic matter over any other soil constituent. For IMI, the clay did not exhibit any sorption activity, while hematite did act as an active sorbent. Contrarily, hematite increased the leachability of DIM by blocking the active sorption sites on clay platelets. Both hematite and clay sorption acted as type-2 (i.e., rate-limiting) sites. In all soils, IMI exhibited lower short-term leachability than DIM. Combined with a body of data concerning other aspects of IMI environmental behaviour, the results indicate that the risk of accumulation of IMI in the soil is greater than the risk of contamination by leaching. Thus, continuous monitoring of IMI in soils should be incorporated into future soil health protection programs.

ddRAD Sequencing Identifies Pesticide Resistance-Related Loci and Reveals New Insights into Genetic Structure of Bactericera cockerelli as a Plant Pathogen Vector

(1) Background: Many hemipteran insects transmit plant pathogens that cause devastating crop diseases, while pest management frequently relies primarily on insecticide applications. These intense insecticide applications lead to the development of insecticide resistance, as was the case for potato psyllid, Bactericera cockerelli (Hemiptera: Triozidae), a vector of Candidatus Liberibacter solanacearum, which causes zebra chip disease in potato. (2) Methods: Here, we use double-digest restriction site-associated DNA (ddRAD) to genotype eight psyllid populations (one susceptible and seven resistant to neonicotinoid insecticides). (3) Results: Association tests identified over 400 loci that were strongly segregated between susceptible and resistant populations. Several loci were located within genes involved in insecticide resistance, gene regulation, fertility, and development. Moreover, we explored the genetic structure of these eight populations and discovered that routinely utilized haplotyping was not an accurate predictor of population structure. Pairwise comparisons of the fixation index (FST) of populations of the same haplotype were not different from pairwise FST of populations that belonged to different haplotypes. (4) Conclusions: Our findings suggest that neonicotinoid insecticide resistance has a genetic basis, most likely as a result of similar selection pressure. Furthermore, our results imply that using a single maternally inherited gene marker to designate genetic lineages for potato psyllids should be re-evaluated.

Multiple Stress Reduces the Advantage of Pesticide Adaptation

Under global change scenarios, multistress conditions may occur regularly and require adaptation. However, the adaptation to one stressor might be associated with the increased sensitivity to another stressor. Here, we investigated the ecological consequences of such trade-off under multiple stress. We compared the pesticide tolerance of the crustacean Gammarus pulex from agricultural streams with populations from reference streams. Under optimum temperature, G. pulex from agricultural streams were considerably more tolerant to pesticides as compared to the reference populations. Here, we assume that the increased tolerance in agricultural populations is the combination of acclimation, epigenetic effect, and genetic evolution. After experimental pre-exposure to very low concentration (LC₅₀/1000), reference populations showed increased pesticide tolerance. In contrast, pre-exposure did not further increase the tolerance of agricultural populations. Moreover, these populations were more sensitive to elevated temperature alone due to the hypothesized fitness cost of genetic adaptation to pesticides. However, both reference and agricultural populations showed a similar tolerance to the combined stress of pesticides and warming due to stronger synergistic effects in adapted populations. As a result, pesticide adaptation loses its advantage. The combined effect was predicted well using the stress addition model, developed for predicting the synergistic interaction of independent stressors. We conclude that under multistress conditions, adaptation to pesticides reduces the general stress capacity of individuals and trade-off processes increase the sensitivity to additional stressors. This causes strong synergistic effects of additional stressors on pesticide-adapted individuals.

Optimizing sampling design and sequencing strategy for the genomic analysis of quantitative traits in natural populations

Mapping the genes underlying ecologically relevant traits in natural populations is fundamental to develop a molecular understanding of species adaptation. Current sequencing technologies enable the characterization of a species' genetic diversity across the landscape or even over its whole range. The relevant capture of the genetic diversity across the landscape is critical for a successful genetic mapping of traits and there are no clear guidelines on how to achieve an optimal sampling and which sequencing strategy to implement. Here we determine, through simulation, the sampling scheme that maximizes the power to map the genetic basis of a complex trait in an outbreeding species across an idealized landscape and draw genomic predictions for the trait, comparing individual and pool sequencing strategies. Our results show that quantitative trait locus detection power and prediction accuracy are higher when more populations over the landscape are sampled and this is more cost-effectively done with pool sequencing than with individual sequencing. Additionally, we recommend sampling populations from areas of high genetic diversity. As progress in sequencing enables the integration of trait-based functional ecology into landscape genomics studies, these findings will guide study designs allowing direct measures of genetic effects in natural populations across the environment.

The Power of Drosophila melanogaster for Modeling Neonicotinoid Effects on Pollinators and Identifying Novel Mechanisms

Neonicotinoids are the most widely used insecticides in the world and are implicated in the widespread population declines of insects including pollinators. Neonicotinoids target nicotinic acetylcholine receptors which are expressed throughout the insect central nervous system, causing a wide range of sub-lethal effects on non-target insects. Here, we review the potential of the fruit fly Drosophila melanogaster to model the sub-lethal effects of neonicotinoids on pollinators, by utilizing its well-established assays that allow rapid identification and mechanistic characterization of these effects. We compare studies on the effects of neonicotinoids on lethality, reproduction, locomotion, immunity, learning, circadian rhythms and sleep in D. melanogaster and a range of pollinators. We also highlight how the genetic tools available in D. melanogaster, such as GAL4/UAS targeted transgene expression system combined with RNAi lines to any gene in the genome including the different nicotinic acetylcholine receptor subunit genes, are set to elucidate the mechanisms that underlie the sub-lethal effects of these common pesticides. We argue that studying pollinators and D. melanogaster in tandem allows rapid elucidation of mechanisms of action, which translate well from D. melanogaster to pollinators. We focus on the recent identification of novel and important sublethal effects of neonicotinoids on circadian rhythms and sleep. The comparison of effects between D. melanogaster and pollinators and the use of genetic tools to identify mechanisms make a powerful partnership for the future discovery and testing of more specific insecticides.

Role of nicotinic acetylcholine receptor subunits in the mode of action of neonicotinoid, sulfoximine and spinosyn insecticides in Drosophila melanogaster

Insecticides remain valuable tools for the control of insect pests that significantly impact human health and agriculture. A deeper understanding of insecticide targets is important in maintaining this control over pests. Our study systematically investigates the nicotinic acetylcholine receptor (nAChR) gene family, in order to identify the receptor subunits critical to the insect response to insecticides from three distinct chemical classes (neonicotinoids, spinosyns and sulfoximines). Applying the CRISPR/Cas9 gene editing technology in D. melanogaster, we were able to generate and maintain homozygous mutants for eight nAChR subunit genes. A ninth gene (Dβ1) was investigated using somatic CRISPR in neural cells to overcome the low viability of the homozygous germline knockout mutant. These findings highlight the specificity of the spinosyn class insecticide, spinosad, to receptors containing the Dα6 subunit. By way of contrast, neonicotinoids are likely to target multiple receptor subtypes, beyond those receptor subunit combinations previously identified. Significant differences in the impacts of specific nAChR subunit deletions on the resistance level of flies to neonicotinoids imidacloprid and nitenpyram indicate that the receptor subtypes they target do not completely overlap. While an R81T mutation in β1 subunits has revealed residues co-ordinating binding of sulfoximines and neonicotinoids differ, the resistance profiles of a deletion of Dβ1 examined here provide new insights into the mode of action of sulfoxaflor (sulfoximine) and identify Dβ1 as a key component of nAChRs targeted by both these insecticide classes. A comparison of resistance phenotypes found in this study to resistance reported in insect pests reveals a strong conservation of subunit targets across many different insect species and that mutations have been identified in most of the receptor subunits that our findings would predict to have the potential to confer resistance.

In Silico Studies of Lamiaceae Diterpenes with Bioinsecticide Potential against Aphis gossypii and Drosophila melanogaster

Background: The growing demand for agricultural products has led to the misuse/overuse of insecticides; resulting in the use of higher concentrations and the need for ever more toxic products. Ecologically, bioinsecticides are considered better and safer than synthetic insecticides; they must be toxic to the target organism, yet with low or no toxicity to non-target organisms. Many plant extracts have seen their high insecticide potential confirmed under laboratory conditions, and in the search for plant compounds with bioinsecticidal activity, the Lamiaceae family has yielded satisfactory results. Objective: The aim of our study was to develop computer-assisted predictions for compounds with known insecticidal activity against Aphis gossypii and Drosophila melanogaster. Results and conclusion: Structure analysis revealed ent-kaurane, kaurene, and clerodane diterpenes as the most active, showing excellent results. We also found that the interactions formed by these compounds were more stable, or presented similar stability to the commercialized insecticides tested. Overall, we concluded that the compounds bistenuifolin L (1836) and bistenuifolin K (1931), were potentially active against A. gossypii enzymes; and salvisplendin C (1086) and salvixalapadiene (1195), are potentially active against D. melanogaster. We observed and highlight that the diterpenes bistenuifolin L (1836), bistenuifolin K (1931), salvisplendin C (1086), and salvixalapadiene (1195), present a high probability of activity and low toxicity against the species studied.

Effect of Selection for Pyrethroid Resistance on Abiotic Stress Tolerance in Aedes aegypti from Merida, Yucatan, Mexico

Simple Summary

Aedes aegypti is the principal vector of major human pathogens, including dengue, Zika, chikungunya, and yellow fever viruses. Vector control relies mostly on the use of pyrethroid insecticides that kill mosquitoes by disabling the nervous system through binding to the voltage-gated sodium channel (vgsc). Resistance mechanisms have evolved most commonly as mutations in the vgsc gene or in genes associated with detoxification. These mutations are thought to associate with fitness costs, such that the frequency of resistant genotypes should decrease in the absence of insecticide use, and this assumption is critical to managing resistance through insecticide rotation strategies. While most studies to date have investigated life history parameters such as fecundity, we sought to investigate whether environmental stress resistance traits might also vary with insecticide resistance. We found, contrary to our expectations, that a strain selected for enhanced insecticide resistance had higher thermotolerance than its sister insecticide susceptible counterpart. Overall, our results indicate that abiotic resistance traits can correlate with insecticide resistance in surprising and variable ways, potentially complicating the management of insecticide resistance in the field.

Abstract

The study of fitness costs of insecticide resistance mutations in Aedes aegypti has generally been focused on life history parameters such as fecundity, mortality, and energy reserves. In this study we sought to investigate whether trade-offs might also exist between insecticide resistance and other abiotic stress resistance parameters. We evaluated the effects of the selection for permethrin resistance specifically on larval salinity and thermal tolerance. A population of A. aegypti originally from Southern Mexico was split into two strains, one selected for permethrin resistance and the other not. Larvae were reared at different salinities, and the fourth instar larvae were subjected to acute thermal stress; then, survival to both stresses was compared between strains. Contrary to our predictions, we found that insecticide resistance correlated with significantly enhanced larval thermotolerance. We found no clear difference in salinity tolerance between strains. This result suggests that insecticide resistance does not necessarily carry trade-offs in all traits affecting fitness and that successful insecticide resistance management strategies must account for genetic associations between insecticide resistance and abiotic stress resistance, as well as traditional life history parameters.

Neonicotinoid Insecticides: Molecular Targets, Resistance, and Toxicity

Neonicotinoids have been used to protect crops and animals from insect pests since the 1990s, but there are concerns regarding their adverse effects on nontarget organisms, notably on bees. Enhanced resistance to neonicotinoids in pests is becoming well documented. We address the current understanding of neonicotinoid target site interactions, selectivity, and metabolism not only in pests but also in beneficial insects such as bees. The findings are relevant to the management of both neonicotinoids and the new generation of pesticides targeting insect nicotinic acetylcholine receptors.

Fitness effects for Ace insecticide resistance mutations are determined by ambient temperature

BACKGROUND

Insect pest control programs often use periods of insecticide treatment with intermittent breaks, to prevent fixing of mutations conferring insecticide resistance. Such mutations are typically costly in an insecticide-free environment, and their frequency is determined by the balance between insecticide treatment and cost of resistance. Ace, a key gene in neuronal signaling, is a prominent target of many insecticides and across several species, three amino acid replacements (I161V, G265A, and F330Y) provide resistance against several insecticides. Because temperature disturbs neuronal signaling homeostasis, we reasoned that the cost of insecticide resistance could be modulated by ambient temperature.

RESULTS

Experimental evolution of a natural Drosophila simulans population at hot and cold temperature regimes uncovered a surprisingly strong effect of ambient temperature. In the cold temperature regime, the resistance mutations were strongly counter selected (s = - 0.055), but in a hot environment, the fitness costs of resistance mutations were reduced by almost 50% (s = - 0.031). We attribute this unexpected observation to the advantage of the reduced enzymatic activity of resistance mutations in hot environments.

CONCLUSION

We show that fitness costs of insecticide resistance genes are temperature-dependent and suggest that the duration of insecticide-free periods need to be adjusted for different climatic regions to reflect these costs. We suggest that such environment-dependent fitness effects may be more common than previously assumed and pose a major challenge for modeling climate change.

Low doses of the neonicotinoid insecticide imidacloprid induce ROS triggering neurological and metabolic impairments in Drosophila

Declining insect population sizes are provoking grave concern around the world as insects play essential roles in food production and ecosystems. Environmental contamination by intense insecticide usage is consistently proposed as a significant contributor, among other threats. Many studies have demonstrated impacts of low doses of insecticides on insect behavior, but have not elucidated links to insecticidal activity at the molecular and cellular levels. Here, the histological, physiological, and behavioral impacts of imidacloprid are investigated in Drosophila melanogaster, an experimental organism exposed to insecticides in the field. We show that oxidative stress is a key factor in the mode of action of this insecticide at low doses. Imidacloprid produces an enduring flux of Ca2+ into neurons and a rapid increase in levels of reactive oxygen species (ROS) in the larval brain. It affects mitochondrial function, energy levels, the lipid environment, and transcriptomic profiles. Use of RNAi to induce ROS production in the brain recapitulates insecticide-induced phenotypes in the metabolic tissues, indicating that a signal from neurons is responsible. Chronic low level exposures in adults lead to mitochondrial dysfunction, severe damage to glial cells, and impaired vision. The potent antioxidant, N-acetylcysteine amide (NACA), reduces the severity of a number of the imidacloprid-induced phenotypes, indicating a causal role for oxidative stress. Given that other insecticides are known to generate oxidative stress, this research has wider implications. The systemic impairment of several key biological functions, including vision, reported here would reduce the resilience of insects facing other environmental challenges.

Contrasting population structure and demographic history of cereal aphids in different environmental and agricultural landscapes

Genetic diversity of populations has important ecological and evolutionary consequences, whose understanding is fundamental to improve the sustainability of agricultural production. Studies of how differences in agricultural management and environment influence the population structure of insect pests are central to predict outbreaks and optimize control programs. Here, we have studied the population genetic diversity and evolution of Sitobion avenae and Sitobion miscanthi, which are among the most relevant aphid pests of cereals across Europe and Asia, respectively. We have used genotyping by sequencing (GBS) to identify genome-wide single nucleotide polymorphisms (SNPs) to infer the geographic structure and migration patterns. In the present study, we show that the population structure in present-day populations is different from that described in previous studies, which suggest that they have evolved recently possibly as a response to human-induced changes in agriculture. This study shows that S. avenae in England is predominantly parthenogenetic and there has been a demographic and spatial expansion of a single genetic cluster, which could correspond with the insecticide resistance superclone identified in previous studies. Conversely, in China, S. miscanthi populations are mostly cyclical parthenogenetic, with one sexual stage in autumn to produce overwintering eggs, and there are six genetically differentiated subpopulations and high genetic differentiation between geographic locations, which suggests that further taxonomical research is needed. Unlike S. avenae in England, there is no evidence for insecticide resistance and there is no predominance of a single lineage in S. miscanthi in China.

Using CRISPR/Cas9 genome modification to understand the genetic basis of insecticide resistance: Drosophila and beyond

Chemical insecticides are a major tool for the control of many of the world's most damaging arthropod pests. However, their intensive application is often associated with the emergence of resistance, sometimes with serious implications for sustainable pest control. To mitigate failure of insecticide-based control tools, the mechanisms by which insects have evolved resistance must be elucidated. This includes both identification and functional characterization of putative resistance genes and/or mutations. Research on this topic has been greatly facilitated by using powerful genetic model insects like Drosophila melanogaster, and more recently by advances in genome modification technology, notably CRISPR/Cas9. Here, we present the advances that have been made through the application of genome modification technology in insecticide resistance research. The majority of the work conducted in the field to date has made use of genetic tools and resources available in D. melanogaster. This has greatly enhanced our understanding of resistance mechanisms, especially those mediated by insensitivity of the pesticide target-site. We discuss this progress for a series of different insecticide targets, but also report a number of unsuccessful or inconclusive attempts that highlight some inherent limitations of using Drosophila to characterize resistance mechanisms identified in arthropod pests. We also discuss an experimental framework that may circumvent current limitations while retaining the genetic versatility and robustness that Drosophila has to offer. Finally, we describe examples of direct CRISPR/Cas9 use in non-model pest species, an approach that will likely find much wider application in the near future.

Climate change and the genetics of insecticide resistance

Changes in global temperature and humidity as a result of climate change are producing rapid evolutionary changes in many animal species, including agricultural pests and disease vectors, leading to changes in allele frequencies of genes involved in thermotolerance and desiccation resistance. As some of these genes have pleiotropic effects on insecticide resistance, climate change is likely to affect insecticide resistance in the field. In this review, we discuss how the interactions between adaptation to climate change and resistance to insecticides can affect insecticide resistance in the field using examples in phytophagous and hematophagous pest insects, focusing on the effects of increased temperature and increased aridity. We then use detailed genetic and mechanistic studies in the model insect, Drosophila melanogaster, to explain the mechanisms underlying this phenomenon. We suggest that tradeoffs or facilitation between adaptation to climate change and resistance to insecticides can alter insecticide resistance allele frequencies in the field. The dynamics of these interactions will need to be considered when managing agricultural pests and disease vectors in a changing climate. © 2019 Society of Chemical Industry.

Adaptation to the abiotic environment in insects: the influence of variability on ecophysiology and evolutionary genomics

Advances in tools to gather environmental, phenotypic, and molecular data have accelerated our ability to detect abiotic drivers of variation across the genome-to-phenome spectrum in model and non-model insects. However, differences in the spatial and temporal resolution of these data sets may create gaps in our understanding of linkages between environment, genotype, and phenotype that yield missed or misleading results about adaptive variation. In this review we highlight sources of variability that might impact studies of phenotypic and 'omic environmental adaptation, challenges to collecting data at relevant scales, and possible solutions that link intensive fine-scale reductionist studies of mechanisms to large-scale biogeographic patterns.

Sublethal larval exposure to imidacloprid impacts adult behaviour in Drosophila melanogaster

Pesticides are now chronically found in numerous ecosystems incurring widespread toxic effects on multiple organisms. For insects, the larvae are very exposed to pesticide pollution and the acute effect of insecticides on larvae has been characterized in a range of species. However, the carry-on effects in adults of sublethal exposure occurring in larvae are not well characterized. Here, we use a collection of strains of Drosophila melanogaster differing in their larval resistance to a commonly used insecticide, imidacloprid, and we test the effect of larval exposure on behavioural traits at the adult stage. Focusing on locomotor activity and on courtship and mating behaviour, we observed a significant carry-on effect of imidacloprid exposure. The heritability of activity traits measured in flies exposed to imidacloprid was higher than measured in controls and in these, courtship traits were genetically less correlated from mating success. Altogether, we did not observe a significant effect of the larval insecticide resistance status on adult behavioural traits, suggesting that selection for resistance in larvae does not involve repeatable behavioural changes in adults. This lack of correlation between larval resistance and adult behaviour also suggests that resistance at the larval stage does not necessarily result in increased behavioural resilience at a later life stage. These findings imply that selection for resistance in larvae as well as for behavioural resilience to sublethal exposure in adult will combine and impose a greater evolutionary constraint. Our conclusions further substantiate the need to encompass multiple trait measures and life stages in toxicological assays to properly assess the environmental impact of pesticides.

Cis- and trans-acting variants contribute to survivorship in a naïve Drosophila melanogaster population exposed to ryanoid insecticides

Around the world insecticides are being deregistered and banned, as their environmental costs are deemed too great or their efficacy against pest insects is reduced through the evolution of insecticide resistance. With the introduction of replacement insecticides comes the responsibility to assess the way new insecticides perturb various levels of biological systems, from insect physiology to ecosystems. We used a systems genetics approach to identify genetic variants affecting survivorship of Drosophila melanogaster exposed to chlorantraniliprole. The study population was completely naïve to this insecticide chemistry and yet we find associations with variants in neuromuscular genes and coregulated detoxification genes. We predict that these variants will increase in populations of this “sentinel species” as these insecticides are applied in the environment.

Insecticide resistance is a paradigm of microevolution, and insecticides are responsible for the strongest cases of recent selection in the genome of Drosophila melanogaster. Here we use a naïve population and a novel insecticide class to examine the ab initio genetic architecture of a potential selective response. Genome-wide association studies (GWAS) of chlorantraniliprole susceptibility reveal variation in a gene of major effect, Stretchin Myosin light chain kinase (Strn-Mlck), which we validate with linkage mapping and transgenic manipulation of gene expression. We propose that allelic variation in Strn-Mlck alters sensitivity to the calcium depletion attributable to chlorantraniliprole’s mode of action. GWAS also reveal a network of genes involved in neuromuscular biology. In contrast, phenotype to transcriptome associations identify differences in constitutive levels of multiple transcripts regulated by cnc, the homolog of mammalian Nrf2. This suggests that genetic variation acts in trans to regulate multiple metabolic enzymes in this pathway. The most outstanding association is with the transcription level of Cyp12d1 which is also affected in cis by copy number variation. Transgenic overexpression of Cyp12d1 reduces susceptibility to both chlorantraniliprole and the closely related insecticide cyantraniliprole. This systems genetics study reveals multiple allelic variants segregating at intermediate frequency in a population that is completely naïve to this new insecticide chemistry and it foreshadows a selective response among natural populations to these chemicals.