Selective grazing behaviour of chironomids on microalgae under pesticide pressure

The herbicide diuron and the insecticide imidacloprid are amongst the most frequently detected pesticides in French rivers, and each is known to affect many aquatic organisms. However, the question of whether and how both pesticides together might induce multi-stress conditions, which could induce indirect effects such as the modification of biological interactions within freshwater microbial communities has not received much attention. This study was undertaken to determine the effect of diuron and imidacloprid alone and in combination on the feeding behaviour of chironomid larvae. An initial experiment measured the impact of the different contamination conditions at environmental concentrations (5 μg L^-1 for each pesticide) on the grazing rate of chironomids on three microalgae species, independently. Two diatom species, Gomphonema gracile (two different morphotypes: normal and teratogen) and Planothidium lanceolatum, and one green alga Desmodesmus sp. were offered as food, during 24 h. Chironomids grazing rates varied according to the pesticide and algae species. Indeed, diuron impacted algae more strongly and probably affected their palatability, leading chironomids to increase grazing pressure on less nutritionally interesting algae. Imidacloprid, by targeting insect larvae, increased or inhibited their grazing capacity depending on the food source. In a second experiment (cafeteria design), the food selectivity of chironomids on previous algae was determined under similar contamination conditions during 4 h: under diuron, larvae switched equally between the microalgae and were as mobile as in the control without pesticide. However, imidacloprid and the pesticide mixture condition altered chironomid movements and grazing behaviour. By investigating the impact of an herbicide and an insecticide, alone and in combination, on the responses of food (algae growth rate) and biological (mortality) and behavioural (mobility, food selection) responses of chironomid larvae, this study provided new insights on the direct and indirect effects of pesticide contamination on a simplified trophic web.