Beyond the obvious: a realistic and holistic approach reveals multiple disorders caused by reduced-risk pesticides on individuals and colonies of bumble bees

There is growing evidence of the negative impacts of prolonged exposure to sublethal concentrations of pesticides that are considered safe for bees, such as biopesticides, herbicides, and fungicides. In this study, we investigated the effects of four different types of pesticides on individuals and colonies of Bombus terrestris , using a holistic approach. Bees were orally and chronically exposed to either pure honey syrup (control, CTRL) or honey syrup contaminated with the neonicotinoid acetamiprid (ACE), the herbicide glyphosate (GLY), the fungicide metalaxyl-M (MET), or the biopesticide sweet orange essential oil (OEO). Sublethal effects were observed for all pesticide treatments at both individual and colony levels; however, lethal effects were only observed in colonies treated with OEO, likely due to the higher concentration tested for this product. Pesticide-exposed colonies experienced negative impacts on mass gain (ACE, OEO, GLY), food ingestion (all pesticide treatments), and number of eggs (all pesticide treatments). Pesticide-exposed individuals showed negative impacts on food ingestion (OEO). The walking behavior of workers was affected by pesticide exposure at the colony level, and by social isolation when treated individually. Overall, our results demonstrate that chronic exposure to pesticides considered safe for bees can cause detrimental sublethal effects on bumble bees, potentially contributing to the decline of pollinators. We also highlight the importance of considering the social context when assessing pesticide toxicity in social insects.