Eating to survive … or not?: Feeding compensation and toxicity in a songbird exposed to thiamethoxam treated millet seeds

Abstract

Thiamethoxam (TMX) is a neonicotinoid insecticide widely used for seed treatment in agriculture. Birds can be exposed to TMX by ingesting unburied seeds after sowing. This study evaluated the effects of a 21-day dietary exposure to TMX-treated millet seeds in the grayish baywing (Agelaioides badius), a farmland songbird. Birds (N = 8 per group) were exposed ad libitum to seeds treated with 0.27 (Low), 0.33 (Medium) and 4.3 (High) g TMX/kg seed; concentrations representing a concentration below the minimum recommended rate, the minimum approved rate, and a worst-case exposure scenario; respectively. A dose-dependent increase in seed consumption was observed in the latter stages of the experiment, suggesting compensatory feeding that amplifies toxicant intake. Feeding behavior was altered in medium and high treatments, while activity patterns were altered across all exposed birds. TMX reduced hematocrit in medium and high treatments, while the birds exposed to the lowest concentration presented tissue-specific modulations in enzymatic biomarkers, including elevated cholinesterase and glutathione S-transferase activities, indicating oxidative stress and cholinergic alterations. The high-dose group exhibited 50% mortality, finding of concern given TMX's classification as of low acute toxicity. High-dose survivor birds also displayed significant loss of body weight (−20.5%) and impaired anti-predator responsiveness. While the high treatment represents an unlike exposure scenario, acute concentrations can be ingested by birds gorging on spilled seeds. These findings emphasize the importance of minimizing the availability of treated seeds for birds and using the lowest effective approved seed treatment rates to reduce the risk of TMX to granivorous birds.