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 insecticides that mimic the action of nicotine that, when applied to the soil, move systemically through the xylem and are effective against green peach aphid (GPA, Myzus persicae). However, aphids mostly feed on plants’ phloem, so we hypothesized that contact with the leaf was the main pesticide exposure route. To test this hypothesis, we allowed tobacco leaves to absorb water with four concentrations (0g, 0.12g, 0.24g and 0.48g AI/L) of imidacloprid for 24h. We took two discs from each leaf, covered one with stretched Parafilm, and put 10 adult apterae on each disc. We counted the number of live and dead adults and nymphs after 24h and live and dead nymphs after 48h. The presence of pesticide in the leaves, regardless of concentration and contact, did not affect adult mortality after 24 h, but had a significant effect on the number of nymphs produced. It also had an effect in the mortality of those nymphs for the next 24. Overall, these results suggest that GPA are most likely to be exposed to the pesticide by feeding than by contact, although we cannot rule out exposure through contact with the stylet.