The Fly and the Bumblebee: Not a Love Story

The Toronto Entomologists' Association invites some great speakers. Yesterday we were treated to nine grad student presentations on topics ranging from the identification of syrphid fly pollinators to the diversity of aquatic invertebrates. Especially, it seemed, there's an awful lot of fly research going on in southern Ontario.

Most interesting to me was a paper given by Joel Gibson of Carleton University on the fly family Conopidae, some members of which are parasitic on bumblebees. Actually the technical term for these Thick-headed Flies is parasitoids. A parasitoid is any parasite that spends an extended time in the body of its host and is so large that as it grows it ultimately kills that host. Parasitoids, you could say, are parasites that are rather more like predators than like ordinary parasites.

Right: A conopid fly of the genus Physocephala, known to be parasitoids of bumblebees. Note that unlike most flies, the antennae are quite long, making it a more effective wasp mimic.

Adult Thick-headed Flies are rather elegant-looking creatures, with a slim, wasp-like body and black-and-yellow waspish colors. They visit flowers where they feed on nectar.

And it is presumably around flowers where they encounter unsuspecting bumblebees. The fly is said to be able to alight on the flying bumblebee and deposit an egg into its body. When the fly larva hatches, it burrows deep into its host and begins to feed on the bumblebee's tissues.

In some areas, we were told, 40% to 50% of the bumblebees are carrying conopid larval parasitoids. The infected bees become feeble and unable to fly. They may be found walking on the ground, their abdomens swollen by the mature fly larva that has consumed most of their internal organs. Eventually the bee dies and the Thick-headed Fly larva pupates inside.

Thick-headed Fly parasitoids are yet another pressure on bumblebee populations. The fly is, no-doubt, something that bumblebees have evolved to cope with over time. But when combined with habitat destruction and widespread pesticide use, this highly effective parasitoid is just one more potentially dangerous stressor.

[Photo by James Lindsey, CC license]