Assessment of gustatory responses to different sugars in harnessed and free-moving bumblebee workers (Bombus terrestris)

For bumblebee colony survival, sugar responses are crucial as nectar is the main carbohydrate source and flower choice is likely determined by sugar composition. This study used a bioassay both with harnessed and with free-moving workers of the bumblebee Bombus terrestris to study the gustatory response to the 3 major plant sugars by both groups. In harnessed workers of B. terrestris, a concentration of 5.5% of fructose and glucose was required to induce the proboscis extension reflex in 50% of the workers, whereas for sucrose, a much higher concentration of 40% was needed. In contrast, free-moving workers given a choice between 30% glucose, 30% sucrose, 30% fructose, and water showed a strong preference for sucrose (66% of individuals) compared with 18% for glucose and 16% for fructose; water was never chosen. Familiarization with 30% fructose provoked a significant increase in preference toward fructose, indicating plasticity. In addition, by amputation of the tarsi, it was found that tarsi plays a role in the sugar response with especially the foreleg tarsi being involved in the response to fructose. Our results demonstrated that sugar response is different in free-moving versus harnessed bumblebee workers and that tarsi plays a role in sugar perception