Thiamethoxam reduces fitness of the painted lady butterfly, Vanessa cardui.

The recently reported insect declines are concerning due to their critical role in ecosystem functionality and human food security. There is consensus that widespread environmental pollution is a critical underlying factor. Mounting data suggest that neonicotinoid insecticides can have detrimental effects on pollinating insects. However, whether these chemicals affect lepidopteran fitness remains poorly understood. Here, we show that exposure to neonicotinoid contaminated feed can reduce fitness in painted lady butterflies, Vanessa cardui. In the laboratory, larvae and adults were chronically exposed to three environmentally relevant concentrations of thiamethoxam (0.1, 1.0 and 10 ppb), representative of residue levels commonly detected in foliage and nectar. Then, lethal (survival) and sublethal (consumption, developmental time, pupal and adult mass, oviposition and egg hatching rates) parameters were assessed and used to model population growth. Although survival was unaffected, thiamethoxam exposure significantly increased pupal mass, adult consumption, male adult mass and developmental time. Further, viable offspring were reduced by 75 % at the highest tested concentration (10 ppb). Despite the absence of lethal effects, our data demonstrate significant sublethal impacts of thiamethoxam on fitness, providing a plausible mechanistic explanation for ongoing insect population declines. These findings highlight the urgent need to integrate fitness-relevant traits into ecotoxicological assessments to accurately assess the potential risks of agrochemical pollution to insects and the broader environment.