Parasites and flower choice of bumblebees

In a field experiment, we tested whether workers of bumblebees,Bombus pascuorumandB.humilis, parasitized by larvae of conopid flies,Physocephala rufipesandSicus ferrugineus, differ in their flower choice from unparasitized ones. We collected workers at random in the field and immediately tested them in experimental arenas that offered the choice of a reference plant (red clover,Trifolium pratense) versus a test plant (from five species). The choices of 396 workers were analysed with logistic regression models (logit analysis). We performed all tests in the same field and at the same time where the workers were foraging naturally. On average, the parasitized bees were less likely to visit the reference plant. In addition, they were more likely to switch plant species even after the first visit in the experimental sequence.