Linking micropollutant gradients and multigenerational microplastic exposure to insecticide tolerance in field populations of Anopheles gambiae s.l

Abstract

Urban freshwater systems increasingly serve as breeding habitats for Anopheles gambiae; however, the effects of prolonged larval exposure to complex waterborne pollutants remain insufficiently characterised. Therefore, larvae were collected from two mosquito breeding sites with pronounced spatial differences in baseline concentrations of heavy metals (Pb, Cd, Hg, As), pesticides (organochlorines/organophosphates), inorganic nutrients (NO₃−, NO₂−, NH₄+, PO₄3−), and microplastics (MPs). Principal Component Analysis demonstrated a pronounced difference in the overall pollutant profiles of the two breeding sites, with substantially greater contamination at the downstream site. The influence of repeated exposure to MPs and the insecticide deltamethrin (either on its own or with MPs) on field populations of larvae was quantified in laboratory experiments. Larval survival and development were recorded over six generations, and insecticide tolerance of adult mosquitoes derived from these larvae quantified by WHO resistance bioassay tests. Initially, MP exposure reduced larval survival and delayed development; however, after several generations, larvae exhibited increased survival, accelerated development, and enhanced adult tolerance to deltamethrin. These responses were more pronounced in mosquitoes originating from the more polluted site, suggesting potential pre-existing environmental conditioning, although differences in sibling-species composition between sites cannot be excluded as a contributing factor. Exposure to combined MPs and insecticide resulted in higher survival and shorter development times compared to insecticide exposure alone. Collectively, these findings provide evidence that repeated MP exposure, in conjunction with environmental pollution, can progressively increase phenotypic insecticide tolerance in An. gambiae. Polluted urban breeding habitats may therefore contribute to reduced insecticide susceptibility, emphasizing the need to incorporate environmental pollution considerations into vector-control and insecticide-susceptibility management strategies.