If a bumblebee is attacked by a thick-headed fly, it’s doomed. The fly will lay an egg inside it and the larva will eat it alive. And if that wasn’t an ignoble enough fate, the larva also forces the bee to burrow into the ground. The soil is warm and safe, and makes for a better nursery for the developing fly. And the bee? The bee is as good as dead. For its last act, it might as well dig its own grave.
There are around 800 species of thick-headed flies or conopids, and they’re all parasites. They use the hard tips of their abdomens like can-openers to prise apart the body segments of bees and wasps, so they can lay an egg inside. They even do this while flying. A conopid can chase down a bee, grab it in mid-air, open it up, and implant it with an egg, without ever touching the ground.
The fly maggot takes just under two weeks to kill its host, first by draining nutrients from its bodily fluids and then by actually eating it. Shortly, after, it forms a pupa and transforms into an adult.
In 1994, Christine Muller discovered that the vast majority of infested bumblebees bury themselves. As soon as she put them on soil, they started to dig. This behaviour didn’t matter to the bees, but it was critical for the flies.
Conopids have yearly life cycles. The adults emerge in the spring after spending the winter as pupae, hibernating inside their dead hosts. If the host dies in the open, the developing fly faces months of cold, dehydration, fungi, and even other parasites. If the host dies underground, the fly is sheltered and more likely to survive.
These kinds of manipulations are common in the world of parasites, many of which commandeer the brains and bodies of their hosts to ensure their own survival. There are wasps that turn caterpillars into head-banging zombie bodyguards, and fungi that make ants climb to the ideal locations for spores to grow. In this case, a fly turns a bee into a shovel.
But not all bees make equally good shovels.
In the summer of 2012, Rosemary Malfi at the University of Virginia collected three closely related species of bumblebees from a local field. She found that a quarter of them were parasitised by a single conopid species—a black, wasp-like insect called Physocephala tibialis.
The parasite forced all three species of bumblebee to dig, but with varying degrees of success. Around 70 percent of the two-spotted or common eastern bumblebees dug their own graves when infected, but only 18 percent of the brown-belted bumblebees did so.
This isn’t a case of resistance in the classical sense. Host insects often have defences that stop parasitic flies and wasps from implanting them with eggs. If that fails, their immune system can sometimes destroy the developing larva. Some species can even self-medicate (with booze, no less) to cure themselves. These countermeasures can force parasites to be very specific, to only target hosts whose defences they can overcome.
It’s possible that the brown-belted bees in Malfi’s study use one or more of these countermeasures, but they could also protect themselves by resisting manipulation. If they don’t dig their own graves, they’d make poor winter homes for a conopid maggot, and a poor choice of target for a conopid adult. Perhaps they defend themselves from parasites not by being inhospitable hosts, but by being incompetent ones.
PS: Carolyn Beans has written a good post on one of Malfi’s earlier studies on conopid flies. Check it out.
Reference: Malfi, Davis & Roulston. 2014. Parasitoid fly induces manipulative grave-digging behaviour differentially across its bumblebee hosts. Animal Behaviour. http://dx.doi.org/10.1016/j.anbehav.2014.04.005