Heat waves impair foraging initiation and directional movement toward a floral scent in the buff-tailed bumblebee...

Global climate change is disrupting key ecological processes and species interactions. In particular, the frequency and severity of heat waves have increased dramatically over the last decade. Bumblebees are key pollinators in natural and agricultural systems, representing great economic and biodiversity value. However, these insects are particularly vulnerable to heat stress, because they are exceptionally well adapted to cold environments. Previous studies showed that heat stress has negatively impacted bumblebee foraging in terms of flight performance and foraging success, but its effect on chemosensory orientation is still unknown. In this study, we experimentally investigated if heat wave-treated bumblebees have difficulties in sensing or locating the source of a synthetic floral blend. We found that the proportion of individuals that initiated foraging was significantly lower in the heat wave-treated group than in the control group. Moreover, heat wave-treated bees started foraging later and approached scent sources randomly, although they reached the first scent source with a latency comparable to that of control individuals. Contrary to our initial expectations, the heat wave treatment influenced the antennal response of bumblebees only in a body size-dependent manner. Our findings provide evidence that heat waves can reduce foraging activity and impair directional movements toward floral scents in buff-tailed bumblebees, and support the idea that climate change may be one of the most harmful anthropogenic factors affecting the foraging performance of this pollinator species.