Critical thermal limits of bumble bees (Bombus impatiens) are marked by stereotypical behaviors and are unchanged by acclimation, age, or feeding status

Abstract

Critical thermal limits often determine species distributions for diverse ectotherms and have become a useful tool for understanding past and predicting future range shifts in response to changing climates. Despite recently documented population declines and range shifts of bumblebees (genus Bombus), the few measurements of thermal tolerance available for the group have relied on disparate measurement approaches. We describe a novel stereotypical behavior expressed by bumblebee individuals during entry into chill coma. This behavioral indicator of minimum critical temperature (CTmin) occurred at ambient temperatures of 3-5°C (approximately 7-9°C core temperatures) and was accompanied by a pronounced CO2 pulse, indicative of loss of spiracle function. Maximum critical temperature (CTmax) was indicated by the onset of muscular spasms prior to entering an unresponsive state and occurred at ambient temperatures of approximately 52-55°C (42-44°C core temperatures). Measurements of CTmin and CTmax were largely unaffected by acclimation, age or feeding status, but faster ramping rates significantly increased CTmax and decreased CTmin This high-throughput approach allows rapid measurement of critical thermal limits for large numbers of individuals, facilitating large-scale comparisons among bumblebee populations and species - a key step in determining current and future effects of climate on these critical pollinators.