Honey bees and bumble bees are very different creatures, and we need more research on bumble bee biology to support threatened populations
When two live bumble bee colonies showed up on my lab bench this spring, I wasn’t entirely sure what I was getting myself into. Having never worked with bumble bees before, aside from occasionally moving stray bees off the sidewalk when they were too cold to fly, I had ordered these colonies from BioBest to prepare myself for an upcoming bumble bee research project. Now that I’ve had a glimpse into their world, I am absolutely hooked.
After spending so many years working with honey bees, bumble bees seem like fuzzy aliens. When I opened their box, their nest smelled weird, kind of musty, but in a good way. They scream at you if you bump their nest even slightly, using their wing muscles to generate an alarmingly high-pitched collective whine. And their brood cells and honey pots are arranged haphazardly, with what looks like more madness than method to an untrained eye. Their wax comes off their abdomen like a grease, rather than neat little flakes like honey bees, and it shows.
Most strikingly, while they still have four wings, an interest in flowers, and that unmistakably cute bee face, they operate with a tenacity and fervor that honey bees should be jealous of. Last year, for example, I captured a few bumble bees for practicing dissections, and those bees were not happy about being in a cage. Whereas honey bees might aimlessly scurry around in confusion, I could hear these bumbles chomping at the cage bars for hours, desperate to get out and back to their chores, like a truly wild animal.
Bumble bees make honey bees look like sheep, but that doesn’t mean they are indestructible. While I have focused on studying how extreme temperatures affect honey bee fertility for the last four years, bumble bees experience these conditions too. Yet, we know remarkably little about how their fertility may hold up in the face of these same challenges.
A dim prognosis
We have good reason to think that their outlook, at the population level, is not good. In a paper evaluating long-term population data for 66 bumble bee species in Europe and North America, authors Peter Soroye, Tim Newbold, and Jeremy Kerr describe how the growing frequency of hotter temperatures predicts whether a given bumble bee population will persist or not.1 That is, heat events explain patterns of local bumble bee extinction.
While warming temperatures also mean that the bees’ habitable range is expanding to higher latitudes, we can’t count on climate change being a net neutral phenomenon. Some bumble bee species will persist and move into their newly available habitats (hopefully with wildflower abundance still in sync with their life cycle), and others will cease to exist, but no new species will magically evolve to fill that void in our lifetime.
As Dave Goulson, author of “Silent Earth: Averting the Insect Apocalypse,” writes, “The trouble is, this time climate change is happening very fast, and it is occurring in a world when natural habitats are already badly degraded. As a result, most butterflies and bumblebees don’t seem to be moving north.” Research by Jeremy Kerr and colleagues agrees with this view, showing that over the last century, bumble bee populations have, by and large, been compressed from the south and failed to move poleward.2
It’s getting hot in here
Sperm of many animals are sensitive to heat, and one reason why heat events are so predictive of local bumble bee extinction could be through reductions in fertility. Research conducted on fruit flies in the tropics, for example, shows that the temperature at which the flies’ fertility is impaired predicts their population distributions better than their lethal thermal limits.3
We have a reasonably good idea of what those fertility limits are for honey bees, but not bumble bees. What’s more, there are around 250 species of bumble bees worldwide (around 50 in the United States), and different species likely have different temperature sensitivities, too. As far as I can tell, only one research article has yet investigated the impact of heat on fertility of different bumble bee species, and the results were not hopeful.
That work was done by Baptiste Martinet, a researcher at the University of Brussels in Belgium, and one of my collaborators. Martinet and colleagues tested how simulated heat waves affected male fertility of three different bumble bee species4 — Bombus terrestris, B. magnus, and B. jonellus, the latter two being cold-adapted species whose populations are in decline, whereas B. terrestris is an abundant warm-adapted species. The researchers found, perhaps not surprisingly, that the fertility of the two cold-adapted species suffered significantly from heat exposure, whereas B. terrestris males were more tolerant to the heat.
This year, I am aiming to add data from a fourth species — B. impatiens — which is why I walked into the lab to a buzzing benchtop last March. Normally, B. impatiens queens would be just awakening from their winter dormancy, with several weeks until their first clutch of workers would emerge to form a nascent colony. Thanks to the commercialization of bumble bees for greenhouse pollination, though, well-established colonies with ~200 individuals are available from BioBest and other producers year-round.
The great escape
To be clear, B. impatiens is not endemic to British Columbia, where I live; their native range is southeastern Canada and the eastern U.S., but they were brought to the West Coast to facilitate greenhouse pollination in the late 1990s, when availability of B. occidentalis, a species native to the West, was suddenly limited. While there has been speculation that the demise of B. occidentalis was caused in part by the unregulated harvesting of wild queens for commercial rearing, this has not yet been officially recognized.
As the story inevitably goes, despite promises of being contained in greenhouses which would theoretically prevent their dispersal, somewhere along the way, mated queens escaped. This is no surprise to me, as bumble bees seem happy to ….