<b>Pesticide-exposed bees fail to thermoregulate leading to cold colonies with consequences for offspring development</b>

Insects face multiple environmental stressors, including widespread pesticide exposure under climate change. Thermoregulation is a critical adaptive response to changing environmental temperatures, but if pesticide exposure were to impair this homeostatic process, reproduction and population viability may be at risk. This is of significant relevance to the ecologically important social insects, as multiple individuals must act collectively to maintain brood temperature for the successful rearing of offspring. In this study, we tested if pesticide exposure affects the ability of bumblebee individuals to maintain body temperature, whether such impact translates to reduced colony-level function (brood care and nest thermal homeostasis), and if this consequently impacts offspring development. Conducting three complementary experiments exposing bumblebees to a neonicotinoid pesticide under different temperature challenges, we show: first, that pesticide-exposed individuals cannot maintain a stable thorax temperature especially at lower temperatures. Second, individual reductions in body temperature are accompanied by behavioural changes and that pesticide-exposed colonies fail to maintain appropriate brood temperatures. Third, the impairment to brood thermoregulation (and not the pesticide toxicity to offspring <i>per se</i>) leads to delayed pupal development and reduced adult population growth rate. We provide a valid mechanistic explanation for why terrestrial insects requiring brood thermoregulation have shown signs of decline over the past few decades. With pesticide exposure landscapes being commonplace, and extreme weather events forecasted to become more frequent, our findings have concerning implications for the ability of populations to adequately respond, persist, and grow under current pesticide use regimes with ramifications on pollination services.