It’s not often that bumblebees make it onto the BBC panel show Have I Got News For You?. But drowning bumblebees obviously tickled someone’s funny bone, and I have to admit that the story (as presented) did seem fairly hilarious. But as usual the detail is crucial, and the show omitted some very important details.
Did two Canadian biologists simply decide one day to drown some bumblebees to see what happened? No they did not, and had they done so, I can tell you the likely result: a lot of dead bees.
Instead they asked a much more interesting question, and one that can only become more important as climate change continues. The question is: lots of insects survive the winter below ground, so what happens if it rains a lot? Do they simply drown?
A common strategy, in grasshoppers and leafhoppers for example, is to overwinter as eggs. Another possibility, as in many ground-nesting solitary bees, is to survive the winter as pupae, and both those dormant stages are known to be highly tolerant of flooding. But invertebrates that survive the winter as adults, such as many spiders and ground beetles, are generally not tolerant of being submerged; if they are threatened by flooding, they simply clear out and return when the danger is over.
Some other invertebrates have a somewhat ambivalent relationship with water. Earthworms, for example, breathe through their skin and depend completely on remaining moist, but they don’t seem too keen on actual submersion. Everyone used to assume that worms found on the surface after heavy rain had emerged from their burrows to avoid drowning, but worms (not having any lungs) can’t literally ‘drown’ in water like you or I would, and can even survive in well-aerated water for a few days. Current thinking is that earthworms looking for food or new habitat simply find it easier to travel across the soil surface when it’s wet.
But bumblebees are unusual, at least among insects, in surviving the winter below ground, as adults. But the adults that do this, and the only bumblebees that survive the winter, are mated queens; female workers and males all die before the winter. They also survive in an unusual dormant state, known technically as diapause. Diapause is induced by low temperatures, and in an earlier experiment (actually designed to look at effects of pesticides on overwintering bees), the researchers had accidentally allowed containers housing diapausing bumblebee queens to fill with water. After draining off the water, they were surprised to find the bees were still alive, which is how they came to ask: if we put queen bumblebees into diapause at 4 °C, can they then survive being submerged in water (in their experiments, for 7 days)?
And the answer is, yes they can. Queens kept underwater for 7 days survived just as well as control bees that were kept dry at 4 °C. We don’t know how they do this, but it’s likely that they can keep water out by closing their breathing spiracles, while reducing their metabolic rate so they need very little oxygen.
This is all slightly surprising, since bumblebee queens are known to avoid flood-prone soils, preferring to overwinter in sloping, sandy and well-drained ground. Yet this is obviously not always enough and, should the worst happen and their burrows are inundated, bumblebee queens have evolved the ability to survive submersion. In other words, the real story is more interesting than the one on Have I Got News For You?, even if not as funny.