Chapter 1: The Thermal Struggles of Bumblebees

Bumblebees, as vital pollinators, face a significant challenge as they warm up when carrying heavy loads of pollen. This could lead to serious consequences, especially in a warming climate.

Like many insects, a bumblebee's body temperature is largely influenced by its surroundings. These cold-adapted creatures have a unique ability to warm themselves by shivering during chilly mornings, allowing them to fly in cooler temperatures and at higher altitudes. Their ability to tolerate cold enables them to remain active for longer periods throughout the year, allowing them to begin pollination earlier than other insects and to engage with a diverse array of flowers.

While foraging, bumblebees can carry pollen loads that may weigh up to 80% of their body weight. Recent studies indicate that this demanding activity can significantly elevate their body temperature. However, like all animals, bumblebees have a threshold beyond which their bodily functions can be compromised. The thermal stress associated with transporting heavy pollen loads raises urgent questions about the impact of climate change on these essential agricultural pollinators. How well can bumblebees cope with rising temperatures?

This bumblebee (Bombus impatiens) was intently foraging for nectar on Chromolaena odorata. Bumblebees are active throughout much of the year and pollinate a wide variety of plants. (Credit: Bob Peterson / CC BY-SA 2.0)

In a laboratory study involving 91 common eastern bumblebee workers (Bombus impatiens), researchers discovered that these insects begin to overheat at temperatures ranging from 41.3° to 48.4°C, which are already being reached in various regions globally. Consequently, the thermal burden from pollen could limit the foraging efficiency of bumblebees on hot days.

According to Malia Naumchik, the lead researcher and an ecologist who conducted this study while an undergraduate at North Carolina State University, “The temperature increase from carrying pollen may push bumblebees into dangerously high temperature ranges.” After graduating, she is now pursuing a Master’s degree in ecology.

Naumchik and her mentor, entomologist Elsa Youngsteadt, an assistant professor, noted that it is expected for bumblebee body temperatures to rise in relation to their pollen loads. They found that for every milligram of pollen carried, bumblebee body temperatures increased by 0.07°C, resulting in fully loaded bees being 2°C warmer than their unladen counterparts. This suggests that on hot days, carrying a full load of pollen could place bumblebees at risk of exceeding their lethal temperature limits.

“This has significant implications for the relationship between bumblebees and climate change,” Naumchik stated. “As temperatures rise, the viable temperature range for these bees may significantly decrease.”

How might this impact bumblebee behavior?

“We must understand how bumblebees could alter their behaviors to assess the effect on their pollen collection and pollination rates during warmer days,” Youngsteadt elaborated.

“Whether they opt to carry smaller pollen loads or forage for shorter durations, these changes could lead to reduced pollen availability for their colonies and a decline in plant pollination,” speculated Youngsteadt.

Pollen serves as a crucial food source at all life stages of bumblebees. In the spring, newly emerged queens need to consume it themselves before providing it to their sister workers. These workers subsequently take over the feeding of the colony, larvae, and future queens. If pollen collection is insufficient, bumblebee colonies may struggle to survive, jeopardizing future populations and the species as a whole.

Bumblebee populations and their habitats are already in decline, posing a threat not only to these insects but also to the plants they pollinate and to agricultural productivity. Key crops like tomatoes, cucumbers, squash, and peppers heavily rely on bumblebee pollination.

“This is particularly crucial as bumblebees deliver essential ecosystem services and are vital pollinators for agriculture, particularly in the U.S. and Europe.”

Source: Malia Naumchik and Elsa Youngsteadt (2023). Larger pollen loads increase risk of heat stress in foraging bumblebees, Biology Letters 19:20220581 | doi:10.1098/rsbl.2022.0581

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Originally published at Forbes.com on 20 May 2023. Keep up with my writing with my free newsletter. (Usually emailed on the last Friday of the month.)