Tiny parasitic flies are threatening honeybee populations by turning the bees zombie-like, lurching and staggering around and sometimes abandoning their hives, researchers say.

Such "zombees" have been seen on the U.S. West Coast and have recently been found in several Eastern states by volunteers helping track the spread of the zombie-bee cases as part of a program called ZomBee Watch, they say.

The flies, Apocephalus borealis, are suspected of depositing their eggs in the bees' stomachs, altering their behavior and causing them to fly erratically in atypical night flights.

The bees sometimes stagger or fly around porch lights, like moths, before falling to the ground dead, the researchers report.

The researchers say they're unsure if this is linked to colony collapse disorder -- believed caused by invasive mites and poisoning by pesticides -- that also sees worker bees abandoning their hives.

"We're not making a case that this is the doomsday bug for bees," says San Francisco State University biology professor John Hafernicki.

"But it is certainly an interesting situation where we have a parasite that seems to affect the behavior of bees and has them essentially abandoning their hive," says Hafernicki, who organized the ZomBee watch in 2012.

The parasitic flies were already known to affect bumblebees and yellow jackets, he says, but he confirmed their attacks on honeybees when he saw pupae emerging from a disoriented bee he had collected.

Scientists say they think the flies attack honeybees that are foraging, piercing their abdomens and depositing their eggs.

As the larvae hatched from the eggs grow, they feed off of the host bee's muscle and nervous system, eventually destroying the brain before they burst from the bee's body.

As worrisome as that may seem, some bee experts say that mites and pesticides are currently considered more serious threats to honeybee populations than a possible spread of the fly parasites.

"We have several other stresses on bees and we don't want any other stress like this one," says Ramesh Sagili, a professor of apiculture at Oregon State University. "We have to be cautious, but I'm not alarmed that this parasite is going to create a big problem."

Still, with a nationwide loss of bee colonies approaching 40 percent, the survival of bees is a concern, experts like San Francisco beekeeper Robert Mackimmie say.

"It's tough to be a bee these days," he says.