High temperature sensitivity of bumblebee castes and the colony-level costs of thermoregulation in Bombus impatiens

Tiffany Bretzlaff; Jeremy T Kerr; Charles-A Darveau

doi:10.1016/j.jtherbio.2023.103710

High temperature sensitivity of bumblebee castes and the colony-level costs of thermoregulation in Bombus impatiens

J Therm Biol. 2023 Oct:117:103710. doi: 10.1016/j.jtherbio.2023.103710. Epub 2023 Sep 12.

Authors

Tiffany Bretzlaff¹, Jeremy T Kerr², Charles-A Darveau³

Affiliations

¹ Department of Biology, University of Ottawa, 30 Marie Curie, Ottawa, ON, K1N 6N5, Canada. Electronic address: tbret032@uottawa.ca.
² Department of Biology, University of Ottawa, 30 Marie Curie, Ottawa, ON, K1N 6N5, Canada. Electronic address: jkerr@uOttawa.ca.
³ Department of Biology, University of Ottawa, 30 Marie Curie, Ottawa, ON, K1N 6N5, Canada. Electronic address: cdarveau@uottawa.ca.

PMID: 37716225
DOI: 10.1016/j.jtherbio.2023.103710

Abstract

Physiological thermal limits often reflect species distribution, but the role that ambient temperature (T_a) plays in limiting species within their thermal environment remains unclear. Climate change-linked declines in bumblebees, an important pollinator group, leave questions regarding which aspect of their physiology is hindered under high T_a. As a eusocial species, bumblebees utilize their ability to thermoregulate as a superorganism to maintain nest temperature (T_n) within a narrow thermal window to buffer developing larvae from developmental defects. Thermoregulatory behaviours, such as thermogenesis to warm up and fanning to cool down the nest, are energetically expensive and it is uncertain how successful large colonies are at maintaining T_n within its optimal range. Using a common bumblebee species, Bombus impatiens, our study first established the critical thermal limits (CTmax) of workers, queens, drones and larvae to determine which caste is most thermally sensitive to heat. We found that larvae had significantly lower heat tolerance than adults, highlighting the importance of colonial thermoregulation. We then measured the energy expenditure of large colonies under acute thermal stress (5-40 °C) using flow-through respirometry while simultaneously quantifying T_n. Colonies that experienced T_a at or below optimal T_n (≤30 °C) were successful at thermoregulation. At 35 °C and above, however, T_n increased despite high energetic costs to the colony. Together our results demonstrate that high T_a poses a risk to colonies that fail to buffer thermally sensitive larvae from changes in T_n.

Keywords: Bombus; Bumblebee; Castes; Colony; Heat tolerance; Metabolic rate; Thermoregulation.