Effects of pesticide exposure on the morphological development of the bee brain and the consequences for learning performance

Abstract

How pesticides contribute towards insect pollinator declines is a question of global ecological and economic importance. In social bees, a critical but understudied step in evaluating pesticide risk is to understand how contaminated food brought back to the colony can affect developing individuals. Within social bee colonies individuals are likely to be exposed to pesticides during two principal developmental stages - brood (larval & pupal) and early adulthood. Understanding how the developmental timing of pesticide exposure affects adult phenotypes is important in revealing the stages at which workers are most vulnerable to exposure and thus uncovers windows of susceptibility in colony development. This thesis investigates how exposure to a neurotoxic pesticide in a major insect pollinator ¬– the bumblebee (Bombus terrestris audax), at different key developmental stages affects learning behaviour at two adult ages, and links variation in learning performance with volumetric changes in brain development. In Chapter 2 I investigate the olfactory learning behaviour of individuals, finding that older bees have reduced learning performance, whether exposed during brood or early-adult development. In chapter 3 I present the development of a new method for exploring morphological variation in tiny soft tissue structures using micro-Computed tomography and image analysis. In Chapters 4 and 5 I apply this new protocol to investigate the volume of the major brain neuropils of individual workers that were experimentally treated and assessed for learning performance in Chapter 2. I find that exposure during either developmental phase can impede growth of the structures associated with olfactory learning and that these reductions in volume correlate with impaired learning performance. My findings show that in response to pesticde exposure during brood development or early adulthood, bumblebee colonies can produce workers with developmentally and functionally impaired brains. This is worrying as it suggests that social bee colonies foraging on pesticide contaminated food may be producing a behaviourally impaired workforce.