Despite their collective efficiency and order, beehives are often plagued by scourges that would rival a medieval city. Varroa mites, deformed wing virus, and intestinal fungi are just a few of the worst. Now researchers have identified a new enemy that ought to strike fear in the hearts of honey bees: A tiny fly that lays its eggs in the bee abdomen, giving rise to maggots that wiggle out near the victim's head. So far, the infection rate does not appear to be high enough to cause problems for hives, but experts are casting a wary eye on the fly. "It's certainly worth a lot more attention," says Dennis vanEngelsdorp of the University of Maryland, College Park.
The parasitism was discovered by accident. In 2008, John Hafernik, an entomologist at San Francisco State University in California, was looking for insects to feed to praying mantises he had collected for a class. He scooped up some dead honey bees that were lying under a light outside his building on campus and left several of the corpses in a vial on his desk. About a week later, Hafernik noticed maggots in the vial. "I knew there was something strange going on," he recalls. After the maggots matured into flies, entomologist Brian Brown from the Natural History Museum of Los Angeles County in California identified the insects as Apocephalus borealis, a kind of scuttle fly. The flies are native to North America and were known to parasitize bumble bees, but they had not been seen afflicting honey bees.
When Hafernik and his students collected more dead bees under the light outside the building, they found that the vast majority had been parasitized by the scuttle fly. In a clear plastic box in the lab, they observed the flies chasing live honey bees and laying eggs in them. After a week, up to a dozen larvae squirmed out near the bee's head. In the wild, as the larvae grow inside them, infected bees abandon the hive at night, head for bright lights, and then die stumbling on the ground.
The problem was not unique to the campus; the researchers found fly-parasitized bees in three out of four honey bee hives sampled in the San Francisco Bay Area , they report online today in PLoS ONE. The good news is that when Hafernik's group examined a hive that had been set up near the entomology building a few years ago, only about 5% to 15% of the forager bees were infected—not a level that would threaten the hive. For individual bees, of course, being parasitized is bad news. "It's a death sentence," Hafernik says. "We don't find bees that are surviving." In addition, the flies appear to be able to transmit deformed wing virus, which is fatal, and the deadly fungus Nosema ceranae, which causes bee diarrhea.
It's not clear when or how the fly might have jumped from bumble bees to honey bees. Because the fly is present across the continent, the next step is to figure out where it is parasitizing honey bees. DNA analysis of commercial hive samples suggests that the flies are present in South Dakota and the Central Valley of California. (Honey bees are trucked between these two locations.) The distribution of the flies in Europe or Asia is unknown. "Extensive surveys are now needed on the distribution of the flies in the global honey bee population," says bee pathologist Elke Genersch of the Institute for Bee Research in Hohen Neuendorf, Germany, who was not involved in the study.
The parasites conceivably might play a role in colony collapse disorder  (CCD), the sudden abandonment that has been resulting in the loss of 7% of hives a year in the United States. "Anything that further stresses the bee population and increases bee losses can contribute to CCD," says Eric Mussen of the University of California, Davis, who was not involved in the study. But given the infection rate observed in the San Francisco State University hive, the parasite "does not appear to be a dominant factor,"  he says. The situation could change if the flies are able to reproduce within bee hives and thus easily parasitize many bees, Genersch says. "Such a high host density might allow the fly population to explode."