What Do Bumble Bees Do with Their Dead?

By Posted on 4 min read 445 views

Like all living things, bumble bees eventually die. A queen bumble bee lives for about a year. She hatches around late summer, hibernates over the winter, and emerges in spring to start her own nest of bees. By the end of the summer, she dies. Worker bees and male bees live for only several weeks. It’s a short life but a very busy one.

A dead bumble bee. Rest in peace, sweet one. Photo source: Shutterstock.

This week for my #BeeFactFriday I tweeted that there are “undertaker bees” that remove dead bees from the nest. I’ve seen them do this. In the lab where I did my graduate studies, we had colonies of bumble bees which we placed in wooden boxes with a clear plastic top so we could see inside. These boxes were connected to a wooden, glass-topped tunnel that led to a screened flight cage–sort of like a gazebo where the bees could fly around. One day I saw a worker bumble bee carry a dead bee in her mandibles (mouthparts). She carried the dead bee through the nest, down the tunnel, and then dropped it inside the flight cage! An undertaker bee in action.

It makes sense that bumble bees would get rid of dead bees in their nest. Corpses take up precious space in a busy home, they could carry disease or parasites, and in my experience, dead bumble bees stink! Bumble bees like to keep a clean, healthy nest (for instance, they do all their pooping outside), and dead bees would be something they would want to get rid of.

After I tweeted my bee fact, I wondered: What, if anything, do we know about these undertaker bees? Has anyone done any research?

I came across a study that was published last year by a team of scientists from the United States and New Zealand. They focused on the species Bombus impatiens, the common eastern bumble bee, which is found across most of North America. I also worked with the common eastern bumble bee for my own research.

Bombus impatiens, the common eastern bumble bee, on a thistle. Photo source: iNaturalist.

The idea behind the scientists’ experiment was simple: place dead bees into bumble bee nests and see what the bees do. The scientists mounted a video camera above each nest box so they could record the behaviour of the bees.

The scientists had three colonies of bumble bees, all in their own nest boxes. To tell each bee apart, they labelled each worker bee by gluing a numbered plastic disc to their thorax (the middle part of their body). Where did they get the dead bees? They plucked some of the worker bees from the colonies and froze them. Poor bees! But it was the most humane way to sacrifice them while still keeping them intact.

Coloured, numbered discs were glued to the thorax of the dead bees, too, although a different colour was used from the live bees. Then, every so often the scientists placed five dead bees into each of the three nests. If a bee pulled on a dead bee with her mandibles, she was considered an “undertaker” and her “tug time” (the amount of time she spent tugging on the corpse) was recorded. A corpse was considered to be removed from the nest when it was tugged past the threshold of the nest entrance.

A worker bumble bee with the yellow label 22 drags the corpse with the blue label 8 through the nest. Photo source: Insectes Sociaux journal.

When there were no corpses in the nest, the scientists watched and classified other behaviours. “Nursing” was when bees fed larvae (baby bees), kept the baby bees warm, made honeypots out of wax, and inspected the larvae and honey pots. “Guarding” was when bees perched near the entrance of the nest, fanned their wings (to keep the nest temperature steady), or patrolled the nest. “Foraging” involved going outside the nest to collect food (pollen and/or nectar). Bees who sat still in the nest and seemed to do nothing were labelled, “inactive.”

So, what did the scientists find? About a third of the bees in a nest participated in corpse-tugging at some point. “Guard bees” tended to spend more time tugging corpses compared to the other tasks of nursing, foraging, or being inactive. These more consistent corpse-tugging “undertaker” bees tended to be bigger than other bees, and the bigger bees tended to be more successful in fully removing dead bees from the nest. Interestingly, on more than one occasion, one bee was seen dragging a corpse to the back of the nest, away from the entrance, digging a hole, and placing the dead bee inside the hole. Why did she do this, instead of taking the dead body outside the nest? The scientists don’t say in their report whether these corpses were left buried or whether they were eventually dumped outside the nest. Still, pretty peculiar that only one bee did this.

The team of scientists point out that it makes sense that guard bees would act as undertaker bees. Guards might be more likely to notice changes inside the nest, such as the presence of a dead body. Dead bee bodies probably give off chemical cues, such as a distinctive smell, that the bees can detect. So maybe there aren’t really “undertaker bees” after all, but instead, corpse disposal is just one task of guard bees.

Reference

Walton, A., Jandt, J. M., & Dornhaus, A. (2019). Guard bees are more likely to act as undertakers: Variation in corpse removal in the bumble bee Bombus impatiens. Insectes Sociaux, 66, 533-541. https://doi.org/10.1007/s00040-019-00718-8