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Zombeeees!

Just in time for Hallowe'en!  I have photographic evidence that some of our bees have been taken over by parasitic phorid flies. These flies are a group of diverse animals, including wasps and nematode worms, described as "parasitoids." These are not your average parasites, which generally do not cause lethal damage to their host, although as in most areas of biology it is difficult to draw a solid distinction between the two.

It is generally in a parasite's best interest to keep its host alive, at least long enough for the parasite to complete its development and disperse to a new host--if the host dies, the parasite dies with it. Parasitoids, on the other hand, flat out kill the host. A famous example are the parasitoid wasps that lay their eggs inside the bodies of caterpillars; the wasp's larvae hatch inside the caterpillar and slowly devour it from the inside out. I'd link to a photo of this horrendous phenomenon, but those of you who know me personally know that I can't look at pictures of caterpillars. Makes my hands sweat just thinking about looking at one. Eww.

Apocephalus borealis is a phorid fly native to North America. It parasitizes various hymenopteran insects, including paper wasps and bumblebees. In January of this year a paper came out confirming that honey bees, Apis mellifera, are also parasitized by the fly. The authors speculate that the fly may be part of the melange of misfortunes resulting in Colony Collapse Disorder (CCD).

The really interesting thing, to me as a beekeeper, is that the samples analyzed were from the San Francisco Bay area. Not only that, but the authors are soliciting additional data from beekeepers and citizen scientists and have put together a cool Zombee Watch program. Hmm. I'm a scientist and a beekeeper in the greater SF Bay Area, so I thought I'd keep an eye out for any bees that were acting strangely as described in the paper. Come to think of it, last fall (November-ish, I think) we went through a period of about a week when bees would get into the house in the evening. It was clear that they were coming towards the light, but I couldn't figure out what they were doing flying around in the dark when they should have been back in their hive. At the time I didn't know to look for phorids, though.

One evening this past July, a few days before leaving on vacation, I noticed a bee on the screen door. She was obviously dying--hardly breathing, non-responsive to my breath or touch--and I thought it might be worthwhile seeing if she were parasitized. I didn't have time to do anything official according to the Zombee Watch protocol, so I just put her in a ziploc bag and forgot about her. A few weeks later I came across the bag again and--lo and behold!--the bee was dead and there were four pupae and four dead flies in the bag with her.

I finally got around to taking pictures of the bee corpse and her equally dead killers:

Dead honeybee with four pupae (bottom left) and four dead phorid flies (bottom right).

Flies and other holometabolous insects go through four distinct life history stages: egg, larva, pupa, and adult. The larva is a feeding stage (think caterpillar); in the case of flies the larva is the critter we call a maggot. After feeding for a certain amount of time the unwinged larva encloses itself into a cocoon and pupates. Inside the pupa the larva undergoes a drastic metamorphosis. The adult stage that emerges from the pupa looks entirely different from the larva:  it has legs and (usually) wings.

Empty pupae of the phorid flies

The adult phorid flies actually look kind of cool. If they weren't troubling my honeybees, I'd like them.

Adult phorid flies

The female phorid fly lays eggs inside the body of a live host. Maggots hatch out of the eggs and cause behavioral changes in the host. Parasitized honeybees abandon the hive and fly around at night, which is why they are easy to catch. They also get disoriented and walk around like, well, zombees. Eventually the fly larvae (maggots) burst out of the bee's body and pupate outside the bee. The host inevitably dies.

Now, isn't that a lot creepier than your average Hallowe'en tale?

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