Deciphering pollinator exposure hazards: Systemic translocation and accumulation of seed-coating pesticides in maize and their synergistic exposure risk to bees.

Seed coating agents, especially neonicotinoid-fungicide combinations, is widely used for crop pest and disease control, their distribution behavior and potential ecological risks are largely overlooked. This study aimed to reveal the translocation pattern of 7 seed-coating pesticides in maize tissues and leaf guttation, and the exposure risk to bees. Hydrophilic pesticides including thiamethoxam, imidacloprid, clothianidin, and metalaxyl-M (log K = -0.13 to 1.75) demonstrated efficient root-to-leaf translocation and subsequent release through leaf guttation, establishing a concentration gradient of leaves > roots > stems. The peak concentrations of these compounds in guttation were 2.06-50.56 mg/L. Conversely, lipophilic pesticides (tebuconazole, ipconazole and fludioxonil, log K = 3.70-4.21) remained mainly in roots with negligible upward translocation, exhibiting the highest guttation concentrations of 0.004 - 0.156 mg/L. Toxicity tests showed strong synergy in thiamethoxam-metalaxyl-M and imidacloprid-tebuconazole combinations, with cotoxicity coefficient values of 220.36 and 337.49, respectively. The risk assessment revealed that exposure risk of guttation and pollen to bees were unacceptable both in laboratory cultivation and actual field when neonicotinoids and fungicides were mixed coated (RQ = 1.1-31496). Our findings highlight potential ecological risk of seed-coating pesticides during maize growth, and provide essential data for rational application and development.