A major 6 Mb superlocus is involved in pyrethroid resistance in the common bed bug Cimex lectularius.

In the last few years, the bed bug Cimex lectularius has been an increasing problem worldwide, mainly due to the development of insecticide resistance to pyrethroids. The characterization of resistance alleles is a prerequisite to improve surveillance and resistance management. To identify genomic variants associated with pyrethroid resistance in Cimex lectularius, we compared the genetic composition of two recent and resistant populations with that of two ancient-susceptible strains using a genome-wide pool-seq design. We identified a large 6 Mb "superlocus" showing particularly high genetic differentiation and association with the resistance phenotype. This superlocus contained several clustered resistance genes and was also characterized by a high density of structural variants (inversions, duplications). The possibility that this superlocus constitutes a resistance "supergene" that evolved after the clustering of alleles adapted to insecticide and after reduction in recombination is discussed.

Superparasitism by a parasitoid wasp: The absence of sublethal effects from the neonicotinoid insecticide imidacloprid enlightens the specificity of the cholinergic pathway involved.

Imidacloprid is an insecticide that is used globally and is suspected to be at least partly responsible for the decrease in the number of pollinator insects. The effects of an LC20 of imidacloprid on the parasitic behavior of the parasitoid wasp Leptopilina boulardi were investigated. Two genetically identical L. boulardi strains were used for the experiments. The strains differed in that one was infected by LbFvirus and the other was not. LbFvirus is a virus that induces an increase in the superparasitism behavior of the wasp. Results of two previous works have shown that the organophosphorus insecticide chlorpyrifos induces an increase in the superparasitism rate of L. boulardi through its specific action on cholinergic nervous pathways. Imidacloprid targets receptors implicated in cholinergic nervous pathways and thus it was expected that imidacloprid would also increase the superparasitism rate of L. boulardi. However, the results of the present experiment demonstrate that imidacloprid does not interfere with the parasitic behavior of L. boulardi and does not increase the rate of superparasitization. It can then be concluded that the major target of imidacloprid, namely type 1 α-bungarotoxin resistant nicotinic acetylcholine receptors (nAChR1), which imidacloprid is an agonist of, and the minor target, type D α-bungarotoxin sensitive nicotinic acetylcholine receptors (nAChRD), which imidacloprid is an antagonist of, are not involved in the superparasitism behavior by L. boulardi. Therefore, the superparasitism behavior of the parasitoid wasp is controlled by cholinergic pathways that do not involve nAChR1 or nAChRD subtype receptors. These findings may enable a better understanding of the mechanisms by which the LbFvirus acts, and contribute to a better evaluation of the potential environmental impact of imidacloprid use.

Sublethal effects from endosulfan on parasitization by the parasitoid wasp Leptopilina boulardi and specificity of nervous pathways involved.

BACKGROUND: Endosulfan is a semi-permanent pollutant that can be transported long distances through the atmosphere. Although phased out in many countries, it is still used in some highly populated areas and thus greatly contributes to environmental pollution. It can impact non-target species such as Leptopilina boulardi, a hymenopteran parasitoid that is beneficial because it controls the populations of its host. Only one L. boulardi egg can successfully develop in its host, a Drosophila larva. Consequently, parasitoid females generally lay only one egg per host, except when they are infected by a virus (LbFV) that triggers superparasitization behavior (laying more than one egg per host). The effects of an LC20 of endosulfan on the parasitization behavior of two L. boulardi strains, one infected by LbFV and the other not, were evaluated. RESULTS: Endosulfan decreased the number of host larvae parasitized by both strains (decreased rate of parasitization) but had no impact on the number of eggs laid per host (i.e., the rate of superparasitization) irrespective of whether the strain was infected by LbFV or not. CONCLUSIONS: Recent research has shown that the organophosphorus insecticide chlorpyrifos induced superparasitization in parasitoid females. Both endosulfan and chlorpyrifos are neurotoxic and induce nervous system hyperstimulation. The fact that endosulfan does not trigger superparasitization, whereas chlorpyrifos does suggests that this effect is due to the specific pathway impacted by chlorpyrifos, the cholinergic nervous pathway. The consequences of these results in the context of awareness of environmental pollution by pesticides are discussed. © 2018 Society of Chemical Industry.

Elicitation of superparasitization behavior from the parasitoid wasp Leptopilina boulardi by the organophosphorus insecticide chlorpyrifos.

Chlorpyrifos is an organophosphorus insecticide that largely contributes to environmental pollution. Parasitoids, as any other non-target species, can be exposed to insecticides through environmental pollution. Parasitoids are key species because they regulate natural populations of other insects. The hymenopterous parasitoid Leptopilina boulardi, whose larvae develop inside Drosophila larvae, is a solitary parasitoid; thus, only one larva can successfully develop per host. Therefore, females generally lay only one egg per host because any increase in the number of eggs laid will decrease its fitness. The effects of an LC20 of chlorpyrifos on the parasitization behavior of two strains (NS and S) of L. boulardi were evaluated. The NS and S strains were genetically identical but differed in that the S strain was infected by a virus, LbFV, which modifies the parasitization behavior of the parasitoid. In control conditions, parasitoid females from the NS strain rarely superparasitized (laid more than one egg per host) their host whereas females from the S strain frequently superparasitized their host. When parasitoids were exposed to an LC20 of chlorpyrifos, the rates of host larvae superparasitized by females and the mean numbers of eggs laid per host larva increased for both NS and S strains. Therefore, both the insecticide and the virus induced an increase in the superparasitization of the host. The effect of the insecticide on the superparasitization behavior of the parasitoid is discussed according to its mode of action.

The sublethal effects of endosulfan on the circadian rhythms and locomotor activity of two sympatric parasitoid species.

The organochlorine insecticide endosulfan is dispersed worldwide and significantly contributes to environmental pollution. It is an antagonist of the neurotransmitter gamma-aminobutyric acid (GABA), which is also indirectly involved in photoperiodic time measurement. In this study, we show that endosulfan at a dose as low as LC 0.1 modified the rhythm of locomotor activity of two sympatric parasitoid species, Leptopilina boulardi and Leptopilina heterotoma. The insecticide strongly increased the nocturnal activity of both species and synchronized their diurnal activity; these activities were not synchronized under control conditions. Parasitoids are important species in ecosystems because they control the populations of other insects. In this paper, we discuss the possible consequences of these sublethal effects and highlight the importance of such effects in evaluating the consequences of environmental pollution due to insecticides.

The sublethal effects of deltamethrin on Trichogramma behaviors during the exploitation of host patches.

Trichogramma and parasitoids as a whole are key species because they regulate natural populations of other insects. As any non-target species, this parasitoid can be exposed to insecticides by environmental pollution. This study identified the effects of an LD 20 of deltamethrin (a pyrethroid) on the behavior of Trichogramma brassicae females infesting a patch of host eggs. The study found that females that survived exposure to the insecticide infested fewer host eggs; spent more time on unsuitable, previously infested host eggs; and infested more previously infested host eggs than controls. The insecticide also induced an increase in antennal and ovipositor rejection of previously infested host eggs. These results are discussed in the light of the mode of action of pyrethroid insecticides. The findings of the study highlight sublethal effects that reduce the fitness of parasitoids and that could consequently modify the equilibrium of natural ecosystems.

Effects of deltamethrin on the specific discrimination of sex pheromones in two sympatric Trichogramma species.

The large amounts of insecticides used for crop protection lead to widespread environmental pollution. Determination of the potential impacts induced by this contamination on key species involved in the equilibrium of ecosystems is therefore a necessity. In this study, we tested the effects of a pyrethroid insecticide, deltamethrin, on the capacity of males from two sympatric Trichogramma species to discriminate the sex pheromones emitted by females of their own species (Trichogramma are parasitoids of Lepidopterous). The impact of an acute exposure as could occur at field edges was evaluated using a dose inducing 20% mortality (LD 20). The impact of a low exposure corresponding to diffuse environmental pollution was evaluated by applying an LD 0.1 (a dose inducing no apparent mortality). For T. semblidis, deltamethrin decreased the specific recognition of sexual pheromones at the higher dose (LD 20) but had no effect on this recognition at the lower dose (LD 0.1). However, deltamethrin decreased the saturation of pheromone receptors at both doses. For T. evanescens, deltamethrin increased the recognition of sexual pheromones at both doses, though not during the same period of observation (at the beginning for the LD 20, at the end for the LD 0.1), but it did not decrease the saturation of the pheromone receptors. These differing results were analyzed considering the behavior of the insects, their level of sensitivity to the insecticide and its mode of action. They provide new insights regarding possible consequences of environmental pollution by insecticides on functional biodiversity.

Modifications of trichogramma behaviors during the exploitation of host patches induced by the insecticide chlorpyrifos.

Parasitoid species are key species because they regulate numerous insect species, including pests. An efficient infestation of hosts is critical to the development of parasitoid populations. In this article, we investigate the effects of the widely used insecticide chlorpyrifos on the exploitation of a patch of host by a parasitoid, Trichogramma brassicae. We show that chlorpyrifos increased the efficiency of parasitoid females in the infestation of the first host egg by decreasing its super-parasitization. Except for the first egg, all infested eggs were infested only once by both control and treated females; therefore, the insecticide did not impede the detection of a host that had already been infested. We did find that the insecticide affected the mode of rejection of infested eggs. At the beginning of the exploitation of the patch, females exposed to the insecticide made more antennal rejections than controls but eventually made more ovipositor rejections. These results suggest that the insecticide initially stimulated the antennal perception of the infested host but finally led to the saturation of this perception. Parasitoids compensated for this loss of antennal perception via ovipositor perception of infested eggs. This switch of behavior corresponds to a decrease in efficiency, as it is much more time consuming; therefore, females exposed to the insecticide had to stay longer on the patch for an equal rate of exploitation relative to controls. The infestation of host eggs is a crucial behavior for parasitoids, enabling their reproduction and the development of their species. By decreasing the antennal recognition of infested eggs, chlorpyrifos continues to be detrimental even when parasitoids survive exposure.

Decrease in fecundity induced by interspecific mating between two Trichogramma parasitoid species.

Trichogramma are used for the biological control of numerous pests. For Trichogramma, as for other insects, the specificity of matings is ensured by several barriers that prevent copulation attempts between insects from different species. We have recently shown that insecticides may totally suppress species recognition that occurs from pheromonal communications between two Trichogramma species, a sublethal effect that will increase mating attempts between two different species. In this work, we have assessed the fitness cost of such interspecific matings and demonstrate that they are very costly for females. After an interspecific mating, females can generate only males because fertilized eggs degenerate (Trichogramma are haplo-diploid species; males are issued from unfertilized eggs and females from fertilized eggs). The resulting offspring are reduced in number by more than half, corresponding to the missing progeny from fertilized eggs. After an interspecific mating, the fecundity of females cannot be restored even if females subsequently mate intraspecifically. These results highlight the strong fitness cost of any event that would decrease the specificity of matings in Trichogramma. Because Trichogramma are key species regulating insect populations, these effects must be considered in the context of sustainable development.

The sublethal effects of pesticides on beneficial arthropods.

Traditionally, measurement of the acute toxicity of pesticides to beneficial arthropods has relied largely on the determination of an acute median lethal dose or concentration. However, the estimated lethal dose during acute toxicity tests may only be a partial measure of the deleterious effects. In addition to direct mortality induced by pesticides, their sublethal effects on arthropod physiology and behavior must be considered for a complete analysis of their impact. An increasing number of studies and methods related to the identification and characterization of these effects have been published in the past 15 years. Review of sublethal effects reported in published literature, taking into account recent data, has revealed new insights into the sublethal effects of pesticides including effects on learning performance, behavior, and neurophysiology. We characterize the different types of sublethal effects on beneficial arthropods, focusing mainly on honey bees and natural enemies, and we describe the methods used in these studies. Finally, we discuss the potential for developing experimental approaches that take into account these sublethal effects in integrated pest management and the possibility of integrating their evaluation in pesticide registration procedures.

Phenotypic plasticity of sternopleural bristle number in temperate and tropical populations of Drosophila melanogaster.

We investigated the phenotypic plasticity of sternopleural bristle (SB) number as a function of growth temperature in isofemale lines from temperate (France) and tropical (Congo) populations of Drosophila melanogaster. We found concave reaction norms with a maximum in the middle of the thermal range, except in four African lines which exhibited a regularly decreasing response curve. Genetic variability (intraclass correlation) and evolvability (genetic CV, coefficient of variation) were independent properties and did not change with temperature. Residual, within-line variability was, however, strongly influenced by growth temperature, showing a U-shaped response curve and a minimum CV of 9% at 21.5 degrees C. As expected from a previously known latitudinal cline, maximum values (MV) were higher in temperate than in tropical flies. The temperature of maximum value (TMV) was observed at a higher temperature in the tropical population, in agreement with similar adaptive trends already observed for other quantitative traits. Significant negative correlations within each population were observed between a plasticity curvature parameter and MV or TMV. No difference in curvature was, however, observed between populations, in spite of their very different MVs.

Stimulating effects of the insecticide chlorpyrifos on host searching and infestation efficacy of a parasitoid wasp.

Hymenopterous parasitoids play an important role in the control of insect populations. During oviposition, Hymenopterous parasitoids use cues such as odours from their environment to locate their specific host. Leptopilina heterotoma (parasitoid of Drosophila larvae) locate their host by probing the substrate with the ovipositor. This behaviour can be induced by the odour of the host substrate alone. We analysed the sub-lethal effects of chlorpyrifos at LD20 on the probing activity in response to a fruit odour (banana). The insecticide increased the percentage of females spontaneously probing in response to the odour. Parasitoid females were then conditioned to associate banana odour with the oviposition in host larvae. This conditioning enables parasitoids to memorize the odour and to increase their probing response to this odour. During the olfactory conditioning, females exposed to the insecticide found and oviposited in host larvae more quickly than control females. One hour after the olfactory conditioning, females exposed to the insecticide presented a higher increase of their probing response to the odour than controls. Twenty-four hours after conditioning, the stimulation produced by chlorpyrifos was no longer perceptible, but the level of response of conditioned females was still higher than that of non-conditioned females, showing that odour memory was not impaired by the insecticide treatment. These sub-lethal effects, that stimulate host searching by parasitoids without impairing odour memorization, could increase their parasitic efficiency.

PHENOTYPIC PLASTICITY AND REACTION NORMS IN TEMPERATE AND TROPICAL POPULATIONS OF DROSOPHILA MELANOGASTER: OVARIAN SIZE AND DEVELOPMENTAL TEMPERATURE.

The plasticity of ovariole number relative to developmental temperature was studied in three populations of Drosophila melanogaster at both ends of the cline: a temperate French population and two equatorial Congolese. Ovary size was much greater in the French flies, in agreement with an already known latitudinal cline. Among isofemale lines, significant differences in genetic variability were observed between populations with a maximum variability at intermediate temperatures. Parameters of phenotypic variability (CV and FA) were not statistically different among lines or populations, but a significant increase at low temperature was demonstrated for both. The shapes of the response curves (i.e., the norm of reaction) were analyzed by adjusting the data to a quadratic equation. The parameters of the equation were highly variable among lines. On the other hand, the temperature for maximum value of ovarioles (TMV) was much less variable and exhibited only a slightly significant difference between temperate and tropical flies (22.2°C vs. 22.7°C). During its geographic extension toward colder places, D. melanogaster underwent a large, presumably adaptative, increase in ovariole number but very little change in the norm of reaction of that trait.