Evaluating neonicotinoid insecticide uptake by plants used as buffers and cover crops

Runoff and drainage from fields planted with neonicotinoid-coated seeds often contain insecticides that adversely affect aquatic life and other non-target organisms. Management practices such as in-field cover cropping and edge-of-field buffer strips may reduce insecticide mobility, making it important to understand the ability of different plants used in these interventions to absorb neonicotinoids. In this greenhouse study we evaluated uptake of thiamethoxam, a commonly used neonicotinoid, in six plant species - crimson clover, fescue, oxeye sunflower, Maximillian sunflower, common milkweed, and butterfly milkweed - along with a native forb mixture and a native grass plus native forb mixture. All plants were irrigated with water containing 100 or 500 μg/L of thiamethoxam for 60 days, then plant tissues and soils were analyzed for thiamethoxam and its metabolite clothianidin. Crimson clover accumulated up to 50% of the applied thiamethoxam, which was significantly more than other plants and indicates this species may be a hyper-accumulator that can sequester thiamethoxam. In contrast, milkweed plants had relatively low neonicotinoid uptake (<0.5%), meaning that those species may not pose excessive risk to beneficial insects that feed on them. In all plants, accumulated masses of thiamethoxam and clothianidin were greater in above-ground tissues (leaves and stems) than in below-ground roots, with more accrual in leaves than stems. Plants treated with the higher thiamethoxam concentration retained proportionally more of the insecticides. Because thiamethoxam primarily accumulates in above-ground tissues, management strategies that include biomass removal may reduce the input of such insecticides into the environment.