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Parasite Forces Host To Dig Its Own Grave

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

More on parasites:

Parasites Make Their Hosts Sociable So They Get Eaten

Sexually Transmitted Virus Sterilises Insects, Turns Them On

Zombies Snipers At The Doorstep

9 thoughts on “Parasite Forces Host To Dig Its Own Grave

  1. I LOVE these guys!

    I stumbled (partly by accident) across them researching Tachinid flies with their amazingly divers methods of parasitization.

    They’ll always have a special place in the part of my brain I reserve for nature’s delightful wrongness. Great article!

  2. Will Holz – I’m mildly intrigued by your use of the word “wrongness”. Not whether it’s a proper word – I think it might be, and if not should be – but more in the vein of, if hominids could get cows to fully optimally provide for themselves during their early lives, and then when they arrived at peak condition for being processed into beef, fully process themselves into various cuts and quantities of meat for consumption as hominid food, don’t you think most hominids would be attracted to the rightness in all that?

  3. Are you accusing cows of gustatory mimicry, Avattoir?


    And it’s effective too! Look at what a scary large percentage of the land mammal biomass cows are now.

    So much about the world is delightfully wrong if you look at it right . . . thanks for that!

  4. Are these flies found in europe or the USA. I have seen flies tracking bumblebees. They fly about 6 cm always behind the bee as it moves from flower to flower, just like a missile. Then attack! Fantastic – anyone got any video of this

  5. @Will Holz
    They have a special place “in” your brain?

    Careful, sir, of the soft ground and the old shovel, you may have an urge.

  6. @ Chris: They are both in Europe and the U.S., and many other parts of the world! Other flies use bumblebees as hosts, too – tachinids and phorids. It sounds like what you saw could definitely be a conopid.

    We just published a follow-up to this study in Oecologia. It details differences in the immune responses of the same bumblebee hosts and how these differences affect developmental success of the same parasitoid conopid fly. http://link.springer.com/article/10.1007/s00442-015-3292-8.

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