The heap of dead bees was supposed to become food for a newly captured praying mantis. John Hafernik, a biology professor at San Francisco State University, had collected the belly-up bees (Apis mellifera) from the ground underneath lights around the university campus. “But being an absent-minded professor,” he noted in a prepared statement, “I left them in a vial on my desk and forgot about them.” He soon got a shock. “The next time I looked at the vial, there were all these fly pupae surrounding the bees,” he said. A fly (Apocephalus borealis) had inserted its eggs into the bees, using their bodies as a home for its developing larvae. The pile of dead bees ended up revealing a previously unrecognized suspect in colony collapse disorder—a mysterious condition that for several years has been causing declines in U.S. honeybee populations, which are needed to pollinate many important crops. It turns out that the parasitic flies that had attacked Hafernik’s bees have been taking over the bodies of honeybees in other parts of the country. A detailed description of the newly documented relationship was pub­lished online in January in PLoS ONE.

Hafernik believes that the fly, which also parasitizes bumblebees and paper wasps, may have only recently begun attacking honeybees. “Honeybees are among the best-studied insects in the world,” Hafernik said. “We would expect that if this has been a long-term parasite of honeybees we would have noticed.”

The fly lays eggs in a bee’s abdomen. Several days later the parasitized bee bumbles out of the hive—often at night—on a solo mission to nowhere. Such bees often fly toward light and wind up unable to control their own body. After the bee dies, as many as 13 fly larvae crawl out from the bee’s neck.

The team members found evidence of the fly in 77 percent of hives they sampled in the San Francisco Bay Area, as well as in some hives in California’s agricultural Central Valley and in South Dakota. Earlier research had found signs that mites, a virus or a fungus, or a combination of these factors, might be responsible for the widespread colony collapse. In the case of the affected hives that Hafernik’s group studied, the bees—and the parasitizing flies and their larvae—contained genetic traces of a parasite and a virus that were previously implicated in colony collapse disorder. This double infection suggests that the flies might be spreading additional hive-weakening traits.

This article was published in print as "Body-Snatching Flies."