The Climate Crisis Is Cooking Baby Bumblebees In Their Nests

Whether they live in the Arctic or the tropics, bumblebee babies appear to have the same nest temperature requirements

I recently shared a study with you that documented a disturbing decline in insect-pollinated wildflowers (more here). There are a variety of threats to flowering plants, and foremost amongst them is the sharp decline in native pollinating insects. The most obvious threats to pollinating insects are pesticides, the decline in flowering plants, and climate change, so I thought it might be useful to share a recent study about some of the native insects that pollinate flowering plants: bumblebees.

Bumblebee numbers are declining around the globe, and a new study uncovers yet another threat to their continued survival: the impending global climate collapse. As you all know, the runaway climate crisis is rapidly increasing the planet’s temperatures and now these temperatures have reached the point where nesting bumblebees are struggling to cope. An international team of researchers came to this conclusion after reviewing 180 years of the scientific literature. They identified increasing heat as a likely driver of the decline in global bumblebee populations because it interferes with these insects’ ability to construct a liveable home where healthy larvae can develop.

“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 study’s lead author, ecologist Peter Kevan, a Professor Emeritus of Environmental Sciences at the University of Guelph, said in a statement.

Globally, here are more than 250 species of bees that are classified in the genus Bombus, with 49 species of bumblebees in the United States (2 species are extinct). They primarily occur in higher altitudes or latitudes in the Northern Hemisphere, although a few lowland species are also found in South America. Despite living in a variety of different environments around the world, it may be surprising to learn that all bumblebees all share one critically important feature regardless of they occur: the optimal temperature of their nests must be between 28–32 degrees Celsius (82.4-89.6 degrees Fahrenheit).

“It’s remarkable that all the way from the high Arctic to the tropics, bumblebees seem to have the same sort of nest temperature requirements,” Professor Kevan stated.

Further, the consistency of this particular environmental demand that is shared amongst so many bumblebee species suggests temperature has limited evolutionary plasticity in these insects, so it’s likely that bumblebees would find it hard to adapt to rising environmental temperatures, and would struggle to remain within their thermal neutral zone — which requires minimal metabolic expenditure to for them to live in.

“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.”

“We can assume that the similarity reflects the evolutionary relatedness of the various species,” Professor Kevan added.

As the temperature rises, bumblebees experience heat stress.

“Excessively high temperatures are more harmful to most animals and plants than cool temperatures,” Professor Kevan explained. “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.”

Professor Kevan and collaborators’ literature review indicated that bumblebees seem able to survive at temperatures of up to 36 degrees Celsius, and develop normally between 30–32 degrees Celsius — although it is possible that this optimal temperature might differ somewhat between species and biogeographical conditions.

Not surprisingly, bumblebees have developed some behaviors that allow them to thermoregulate — but even these adaptations may not provide them with the needed relief as temperatures rise.

It’s important to point out that bumblebees are social insects that form colonies consisting of a single queen and her offspring. Although these colonies comprise individual bees, it acts like a ‘superorganism’ where its reproductive fitness depends upon the survival and reproduction of the collective rather than individual bees. For example, even if one individual bumblebee deals better with heat than another, the entire colony still suffers when it becomes too hot to raise healthy larvae.

“The effect of high nest temperatures has not been studied very much, which is surprising,” Professor Kevan explained. “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.”

There are published studies of honeybees, Apis mellifera, widely domesticated pollinator species that are native to Afro-Eurasia, reporting that higher nest temperatures harm the honeybee queens’ physical strength and reproductive ability so they produce worker bees that are smaller and in physically poorer condition. If heat has similar effects on bumblebees, as is likely, it is reasonable to assume that global warming is leading to the decline of bumblebees.

Yet, because these important gaps exist in our current knowledge of bumblebee biology and ecology, Professor Kevan and collaborators mentioned in their review specific unexplored avenues of bumblebee research that are important to understand specifically how nest morphology and the various properties of nest materials affect nest temperatures and the ability of bumblebees to thermoregulate. The researchers then remarked that it is necessary to understand how different colonies cope with the same conditions and how different species cope with different conditions — including identifying whether some bumblebee species have broader thermal neutral zones, thereby granting these species more temperature flexibility.

To conduct such studies, Professor Kevan and collaborators helpfully suggested using ground-penetrating radar to study ground-nesting species, whilst flow-through respirometry analysis of nests at different temperatures may help scientists measure the stress experienced by the bee colonies inside.

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

Source:

Peter G. Kevan, Pierre Rasmont, and Baptiste Martinet (2024). REVIEW: Thermodynamics, thermal performance and climate change: temperature regimes for bumblebee (Bombus spp.) colonies as examples of superorganisms, Frontiers in Bee Science 2 | doi:10.3389/frbee.2024.1351616