University of California Davis, California, United States
Abstract: Heat waves are an increasingly common type of extreme weather event across the globe and are projected to increase in frequency and severity in the coming decades. Pollination interactions between insects and flowering plants are vulnerable to heat waves due to combined effects of extreme heat on the foraging behavior of ectothermic insect pollinators and on the flowers at which they forage. Studies of extreme heat on pollinating insects mostly document effects on confined individuals in lab studies. However, studies of foraging at real flowers under heat wave conditions are needed to understand how foragers respond to heat directly, and to reveal potential indirect effects through changes to flower rewards and other traits.
The goal of this study was to partition the impacts of heat waves on bumble bee foraging mediated directly through air temperature at the time of foraging and indirectly through changes in floral rewards.
Our approach applied a crossed design of experimental heat treatments and baseline control temperatures to 1) flowers during development and 2) flowers and bees during a foraging period. Treatments were applied using growth chambers and experimental foraging arenas containing flowering Brassica plants connected to active bumble bee colonies.
We showed that temperatures simulating a moderately severe heat wave directly impacted foraging bumble bees by reducing the proportion of successful foraging trips, the duration of foraging trips, as well as the numbers of plants and flowers visited during foraging trips. The simulated heat waves indirectly affected bee foraging through reduced nectar production that limited foraging trip duration. Our experimental results provide a mechanistic link between realistic heat waves, flowering plants, and bee pollinators. They suggest in situ conditions from heat waves could have profound negative consequences for bumble bee colony persistence by diminishing the rate of resource return. The same heatwaves also may decrease pollination services by changing flower visitation.
