Insecticide exposure during brood or early-adult development reduces brain growth and impairs adult learning in bumblebees

For social bees, an understudied step in evaluating pesticide risk is how contaminated food entering colonies affects residing offspring development and maturation. For instance, neurotoxic insecticide compounds in food could affect central nervous system development predisposing individuals to become poorer task performers later-in-life. Studying bumblebee colonies provisioned with neonicotinoid spiked nectar substitute, we measured brain volume and learning behaviour of 3 or 12-day old adults that had experienced in-hive exposure during brood and/or early-stage adult development. Micro-computed tomography (µCT) scanning and segmentation of multiple brain neuropils showed exposure during either developmental stage caused reduced mushroom body calycal growth relative to unexposed workers. Associated with this, was a lower probability of responding to a sucrose reward and lower learning performance in an olfactory conditioning test. Whilst calycal volume of control workers positively cor...