Relationships between parasitism, bumblebee foraging behaviour, and pollination service to T rifolium pratense flowers

1. Parasite effects on host behaviour frequently alter their hosts' trophic interactions. There are many compelling examples of such effects in herbivore‐based trophic interactions, but less attention has been paid to how parasite effects on host behaviour can alter mutualistic interactions. 2. Pollination mutualisms depend greatly on pollinator behaviour, and many pollinators are attacked by a wide range of parasites and parasitoids. 3. To investigate whether parasites affect pollination service via changes in host behaviour, natural variation in conopid fly parasitism was used to investigate the relationship between infection and Bombus impatiens Cresson behaviour foraging on arrays of Trifolium pretense L. flowers in the laboratory. The consequences of infection for seed set and seed mass were also examined. 4. Conopid parasitism was not related to any measured behavioural response; however, flowers visited by conopid‐parasitised bees set significantly heavier seeds than those visited by unparasitised bees. Larger bees were more likely to be parasitised, but the relationship between parasitism and seed set still held after accounting for body size. 5. The present results demonstrate that parasitoids may have positive impacts on per‐visit pollination, but, because larger bees were more frequently parasitised, parasitism could also affect pollination by removing the largest pollinators from the population.