Seed-applied insecticides in corn reduce herbivore performance on adjacent non-crop plants

Insecticide seed treatments are widely used in field crop systems to protect seeds and seedlings from early pest damage, through the coating of seeds with insecticides, fungicides, or nematicides. While effective against target pests, these treatments may also have unintended effects on non-target organisms by accumulating on neighboring plants. We evaluated the indirect impact of corn seed treatments on herbivores feeding on a co-occurring non-crop species, wild mustard (Rhamphospermum arvense). In each pot, one corn seed and one wild mustard seed were co-planted. Corn seeds were assigned to one of four treatments: 1) untreated control, 2) fungicides only, 3) neonicotinoid + fungicides, 4) anthranilic diamide + fungicides. After five weeks, we harvested leaves from wild mustard plants and used them to feed larvae of two generalist herbivores, fall armyworm, Spodoptera frugiperda and Diamondback moth, Plutella xylostela, reared individually in Petri dishes on agar to maintain humidity. Larvae were allowed to feed on plant material, and relative growth rate (RGR) was calculated after three days. ANOVA followed by Tukey’s test revealed that both herbivore species had significantly lower relative growth rates (RGR) when fed on wild mustard grown next to corn treated with anthranilic diamide + fungicide, compared to those fed on wild mustard grown next to untreated control, fungicide-only, or neonicotinoid + fungicide-treated corn. These results suggest systemic insecticide transfer from treated crops can negatively affect herbivores on neighboring non-target plants, with potential implications for agroecosystem dynamics.