Local selection in the presence of high levels of gene flow: Evidence of heterogeneous insecticide selection pressure across Ugandan Culex quinquefasciatus populations

Background: Culex quinquefasciatus collected in Uganda, where no vector control interventions directly targeting this species have been conducted, was used as a model to determine if it is possible to detect heterogeneities in selection pressure driven by insecticide application targeting other insect species. Methodology/Principal findings: Population genetic structure was assessed through microsatellite analysis, and the impact of insecticide pressure by genotyping two target-site mutations, Vgsc-1014F of the voltage-gated sodium channel target of pyrethroid and DDT insecticides, and Ace1-119S of the acetylcholinesterase gene, target of carbamate and organophosphate insecticides. No significant differences in genetic diversity were observed among populations by microsatellite markers with HE ranging from 0.597 to 0.612 and low, but significant, genetic differentiation among populations (FST = 0.019, P = 0.001). By contrast, the insecticide-resistance markers display heterogeneous allelic distributions with significant differences detected between Central Ugandan (urban) populations relative to Eastern and Southwestern (rural) populations. In the central region, a frequency of 62% for Vgsc-1014F, and 32% for the Ace1-119S resistant allele were observed. Conversely, in both Eastern and Southwestern regions the Vgsc-1014F alleles were close to fixation, whilst Ace1-119S allele frequency was 12% (although frequencies may be underestimated due to copy number variation at both loci). Conclusions/Significance: Taken together, the microsatellite and both insecticide resistance target-site markers provide evidence that in the face of intense gene flow among populations, disjunction in resistance frequencies arise due to intense local selection pressures despite an absence of insecticidal control interventions targeting Culex

Mode of action of plant-derived natural insecticides

Most of the chemical insecticides are neurotoxic, acting on targets in the central nervous system such as the membrane ion channels (DDT, pyrethroids), the enzyme acetylcholinesterase (organophosphate, carbamate), and the receptors of neurotransmitters (avermectins, neonicotinoids). The recently introduced diamide group of insecticides target the novel ryanodine receptor in the nervous system. Since pests continue to evolve resistance to compounds currently in use, new compounds with new modes of action are needed. Natural products could be a promising source for novel pest control agents. The origin of many of the important insecticide classes is traceable to a natural source as in the case of pyrethroids, avermectins, spinosads, and neonicotinoids. Although insect control agents acting on targets other than the nervous system such as insect growth regulators (e.g., azadirachtin, JH analogues, ecdysone antagonists) have been developed, due to their lack of contact toxicity, they are not quite successful, but find a place in the integrated pest management. Recent progress in understanding the biology of insect olfaction and taste offers new strategies for developing selective pest control agents. Decalesides, recently discovered natural insecticides, represent a new class of plant-derived insecticides targeting the tarsal gustatory receptors. In this chapter, we focus on the toxicity and mode of action of natural insecticides