Rising temperatures are overheating bumblebee nests

A recent study led by the University of Guelph in Canada has found that global warming is increasingly posing a significant threat to bumblebee populations worldwide. 

According to the experts, the perils that rising temperatures pose to bumblebee nest viability and larvae development should urgently be taken into account in order to protect these insects.

Decline in bumblebee populations 

“The decline in populations and ranges of several species of bumblebees may be explained by issues of overheating of the nests and the brood. The constraints on the survival of the bumblebee brood indicate that heat is likely a major factor, with heating of the nest above about 35 degrees Celsius being lethal, despite the remarkable capacity of bumblebees to thermoregulate,” explained Peter Kevan, an expert in environmental sciences at Guelph.

Optimal temperature range for bumblebee nests

Bumblebees, diverse in species and widespread across various climates, have consistently shown an optimal nest temperature range between 28 and 32 degrees Celsius. This similarity across species suggests a limited ability to adapt to rising temperatures outside of their thermal neutral zone, where they can maintain body temperature without expending much energy. 

“Excessively high temperatures are more harmful to most animals and plants than cool temperatures. When conditions are cool, organisms that do not metabolically regulate their body temperatures simply slow down, but when temperatures get too high metabolic processes start to break down and cease. Death ensues quickly,” Kevan said.

Through an extensive review spanning 180 years, the researchers have found that bumblebees can survive temperatures up to 36 degrees Celsius and thrive best around 30-32 degrees Celsius, though variations exist depending on species and geographic conditions. However, their innate ability to thermoregulate may not suffice against the escalating challenges posed by global warming.

Collective health of the bumblebee colony

The study also emphasizes the concept of the bumblebee colony as a “superorganism,” highlighting that the reproductive success of the colony hinges on the collective health and survival of its members, not just individual bees. High nest temperatures, therefore, jeopardize the entire colony’s future by potentially reducing the health and viability of new offspring.

“The effect of high nest temperatures has not been studied very much, which is surprising,” Kevan remarked. “We can surmise that nest temperatures above the mid-30s Celsius would likely be highly detrimental and that above about 35 Celsius death would occur, probably quite quickly.”

Elevated nest temperatures 

Parallel findings in honeybees have shown that elevated nest temperatures can weaken bee queens and result in smaller, less robust worker bees. A similar impact on bumblebees could directly contribute to their decline, suggesting an urgent need for further research.

The scientists advocate for deeper exploration into the understudied areas of bumblebee ecology such as nest morphology, material properties, and temperature regulation strategies. Innovative methods like ground-penetrating radar and flow-through respirometry analysis might reveal how different colonies and species manage thermal stress, potentially uncovering broader thermal neutral zones in some species that could confer greater resilience.

“We hope that future scientists may take the ideas we present and apply them to their own research on bumblebee health and conservation,” Kevan concluded.

Bumblebee nests 

Bumblebee nests are unique and fascinating structures usually found underground or in dense grass. Unlike honeybees, which build extensive hives, bumblebees often repurpose abandoned rodent burrows or thick grass clumps to create their homes. These nests are less populous, typically housing anywhere from 50 to 400 bees, which is considerably smaller than honeybee hives.

The queen bumblebee initiates the nest by laying the first batch of eggs, which she incubates with her body. As these hatch and mature into worker bees, they take over the duties of expanding the nest, foraging for food, and caring for subsequent broods laid by the queen. The interior of a bumblebee nest contains wax pots built by the workers for storing food and rearing young bees.

These nests have a relatively short lifespan. They are usually active for only a few months each year, from spring to late summer. At the end of the season, new queens are produced, which leave to mate and then find a place to hibernate through the winter. 

The original nest is abandoned and generally does not get reused the following year. This cycle highlights the bumblebee’s vital role in pollination and the ecosystem, despite their nests being less permanent than those of some other bee species.

The study is published in the journal Frontiers in Bee Science.

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