Do I eat or do I walk? Determining the exposure route of Imidacloprid to green peach aphids in cultivated tobacco

Neonicotinoid insecticides are a widely used family of active ingredients that mimic the action of nicotine, they are an effective protection against green peach aphid (Myzus persicae) and, when applied systemically, move through the xylem. However, aphids feed mostly from the plants’ phloem, so we hypothesized that contact with the leaf was the main exposure route to the pesticide by the green peach aphid. To prove this hypothesis we allowed tobacco leaves to absorb water with four concentrations of imidacloprid for 24h. We took two discs from each leaf, cover one with stretched Parafilm and put 10 adult apterae aphids on each disc. We counted the number of live and dead adult and nymphs after 24h and live and dead nymphs after 48h. Our results show that aphids survive and reproduce better on untreated leaves and that the three pesticide concentrations used produced comparable results in both mortality and reproduction. Adult aphids are able to recognize the presence of pesticide regardless of lack of contact with the leaf, reducing their fecundity, but the presence of the barrier improves their survival slightly, probably due to behavioral effects.  Nymphs are readily killed by all concentrations of the pesticide in at least 24h of exposure. These results suggest that the aphids are most likely to be exposed to the pesticide by feeding and that the antifeedant effect of imidacloprid is very important to delay aphid colonization by reducing fecundity of aphids.