Equivocal Evidence for Colony Level Stress Effects on Bumble Bee Pollination Services

Climate change poses a threat to global food security with extreme heat events causing drought and direct damage to crop plants. However,by altering behavioural or physiological responses of insects, extreme heat events may also affect pollination services on which many crops are dependent. Such effects may potentially be exacerbated by other environmental stresses, such as exposure to widely used agro-chemicals. To determine whether environmental stressors interact to affect pollination services, we carried out field cage experiments on the buff-tailed bumblebee (Bombusterrestris). Using a Bayesian approach, we assessed whether heat stress (colonies maintained at an ambient temperature of 25 ◦C or 31 ◦C) and insecticide exposure (5 ng g–1 of the neonicotinoid insecticide clothianidin) could induce behavioural changes that affected pollination of fababean (Viciafaba). Only the bumble bee colonies and not the plants were exposed to the environmental stress treatments. Bean plants exposed to heat-stressed bumblebee colonies(31◦C) had a lower proportional podset compared to colonies maintained at 25◦C. There was also weak evidence that heat stressed colonies caused lower total bean weight. Bee exposure to clothianidin was found to have no clear effect on plant yields, either individually or as part of an interaction. We identified no effect of either colony stressor on bumblebee foraging behaviours. Our results suggest that extreme heat stress at the colony level may impact on pollination services. However, as the effect for other key yield parameters was weaker (e.g.,beanyields), our results are not conclusive. Overall,our study highlights the need for further research on how environmental stress affects behavioural interactions inplant-pollinator systems that could impact on crop yields.