How do neonicotinoids affect social bees? Linking proximate mechanisms to ecological impacts

Neonicotinoid insecticides are among the most widely used agrochemicals globally. Given the critical importance of bees for biodiversity, crop productivity, and human health, recent decades have seen substantial interest in the impacts of neonicotinoids on bees and the pollination services they provide, both among researchers and the public. Social bees (e.g. honey bees, stingless bees, and bumble bees) are especially important for supporting crop productivity, yet significant knowledge gaps remain in understanding the impacts of neonicotinoids on colony health. Here, we review current knowledge on how neonicotinoid exposure affects social bees, including how direct effects on individuals can impact colony performance, as well as the potential roles of colony state and collective behaviour in modulating these effects. We focus in particular on challenging aspects of predicting real-world impacts of neonicotinoids (including stressor interactions, life history, complex routes of exposure, and taxonomic considerations) that have practical relevance for regulatory decisions, and highlight current knowledge gaps. Finally, we provide an overview of emerging issues and methods, and highlight the potential for mutually beneficial outcomes for crop production and biodiversity conservation.