Sublethal and Lethal Effects of Low-Dose Prothioconazole Alone and in Combination with Low-Dose Lambda-Cyhalothrin on Carabid Beetles in a Field-Realistic Scenario

Environmental risk assessment (ERA) for pesticide approval in the context of predatory insects remains inadequate as it often overlooks the influence of agricultural practices. An increasing number of studies have shown that prolonged and synergistic pesticide exposure can elevate insect mortality. However, such effects remain largely unstudied in non-target predatory carabid beetles. The carabid beetle Platynus assimilis was subjected to repeated oral and continuous contact exposure to low doses of prothioconazole (20 g·ha−1), lambda-cyhalothrin (0.4 g·ha−1), or their combination over a 64-day period. The food consumption rate, body mass, locomotor activity, and mortality were monitored throughout the experiment. All pesticide-treated groups showed significantly increased final mortality, with median lethal times (LT50) of 51.6 days for prothioconazole, 60.3 days for lambda-cyhalothrin, and 12.2 days for their combination. A significant synergistic effect on mortality was observed in the combined treatment group, with the highest synergistic ratio detected 20 days after the first exposure. Pesticide-treated beetles exhibited significant abnormalities in locomotor activity and body mass compared to the untreated group. These findings demonstrate that both time-cumulative mortality and potential synergistic interactions, reflecting field-realistic conditions, must be considered in ERA. Failure to do so may lead to an underestimation of pesticide toxicity to predatory carabids.