FRESNO, California (AP) _ Northern California scientists say they have found a possible explanation for the honey bee die-off: A parasitic fly that hijacks the bees' bodies and causes them to abandon hives.

The symptoms mirror colony collapse disorder, in which all the adult honey bees in a colony suddenly disappear. The disorder continues to decimate hives in the U.S. and overseas.

The disease is of great concern, because bees pollinate about a third of the United States' food supply. Its presence is especially alarming in California, America's top producer of fruits and vegetables, where bees play an essential role in the $2 billion almond industry and other crops.

The latest study, published Tuesday in the science journal PLoS ONE, points to the parasitic fly as the new threat to honey bees. It's another step in ongoing research to find the cause of the disease.

Researchers haven't been able to pin down an exact cause of colony collapse or find a way to prevent it. Research so far points to a combination of factors including pesticide contamination, a lack of blooms _ and hence nutrition _ and mites, fungi, viruses and parasites.

Interaction among the parasite and multiple pathogens could be one possible factor in colony collapse, according to the latest study by researchers at San Francisco State University. It says the phorid fly, or apocephalus borealis, was found in bees from three-quarters of the 31 hives surveyed in the San Francisco Bay area.

Scientists say the fly deposits its eggs into the bee's abdomen, causing the insect to walk around in circles with no apparent sense of direction. The bee exhibits zombie-like behavior, said lead investigator John Hafernik. The infected bee leaves the hive at night and dies shortly thereafter.

The combination of a parasite, pathogens and other stressors could cause die-off, Hafernik said. The parasitic fly serves as a reservoir that harbors pathogens _ honey bees from parasite-infected hives tested positive for deformed wing virus and other pathogens, the study found.

``We don't fully understand the web of interactions,'' Hafernik said. ``The parasite could be another stressor, enough to push the bee over tipping point. Or it could play a primary role in causing the disease.''

Hafernik stumbled onto the parasitic fly by accident. Three years ago, the biology professor looked for something to feed a praying mantis. He found some bees outside his classroom, placed them in a vial and forgot about them. When he looked at the vial a week later, he found dead bees surrounded by small fly pupae. A parasitic fly was feeding on the bees and had killed them, he said.

The fly is a known parasite in bumble bees. Scientists used DNA barcoding to confirm the parasite in the honey bees and bumble bees was the same species.

The fly might have recently expanded its host presence from bumble bees to honey bees, Hafernik said, making it an emerging threat to agricultural pollinators. The fact that honey bees live in large colonies placed in close proximity to one another and beekeepers frequently move the hives throughout the country could lead to an explosion of the fly population, he said.

The fly, which is found all over North America, could also become a threat to native bees.

Hafernik plans to expand his research to other parts of the country and to study the parasite's impact on agriculture in California's Central Valley.

Since it was recognized in 2006, colony collapse has destroyed colonies at a rate of about 30 percent per year, according to the U.S. Department of Agriculture. Before that, losses were about 15 percent per year from a variety of pests and diseases.