Parasite May Help Explain the Decline of Honeybees

In the last year, American beekeepers lost more than 40 percent of their hives. Ali Jarekji / REUTERS

For years, a parasite known as Nosema has annoyed beekeepers in Europe and the United States, infecting adult honeybees and shortening their life span. Researchers didn't think, however, that it was capable of infecting very young, larval bees.

They were wrong. In a study published today in the journal PLOS ONE, University of California, San Diego, researcher James Nieh and colleagues found that the parasite can spread to larvae when even a small quantity of its spores make their way into the insects' food. They also discovered that the parasite, a type of fungus, decreases the bees' life span more than previously thought.

The parasite, the researchers say, lives inside honeybees' gut and is spread via fecal matter or mouth-to-mouth food-sharing. "If spores can stay in the larval gut, germinate and then affect adult longevity later, this can perhaps exacerbate the effects the pathogen already has on honeybees," says Zachary Huang, an entomologist at Michigan State University who wasn't involved in the study.

This spreading amongst larvae may even explain why Nosema can be treated (with an antibiotic) but bounce back. Beekeepers have witnessed this in the field and not understood why, according to the study.

The parasite may also have played a role in the decline of honeybees in Europe and the United States. In the last year, U.S. beekeepers lost more than 40 percent of their hives, according to a report released this month. This was also the first time on record the summer colony losses exceeded those during winter, a particularly confounding and worrisome finding, says Dennis vanEngelsdorp, an assistant professor of entomology at the University of Maryland and project director for the Bee Informed Partnership, which conducted the survey.

The parasite examined in this study, Nosema ceranae, is becoming more common, and now infects between 57 percent to 85 percent of the honeybees sampled from 2009 to 2011, Nieh says. In the spring of 2012, the average quantity of Nosema spores found in U.S. colonies was more than five times higher than what is was in 2010.

"Larval infection could also be a place for Nosema to 'hide out' in a colony, biding its time, because Nosema infection tends to become more severe as bees become old," Nieh says. "Larval infection could be a reservoir of infection inside a colony, even one that is treated for this infection."

The scientists want to find out if Nosema is being transmitted this way in the field, as this study only found that larval infection is possible in the lab. If this phenomenon is going on in the real world, beekeepers should be able to better contain the infection by treating larvae as well; currently, beekeepers feed adult insects an antibiotic to treat so-called nosemosis.

The active ingredient in the medication used to treat Nosema, called fumagillin, has been used to treat a related strain of the parasite for more than 60 years, Huang says. "It is not clear how long we can have it before [these parasites] become resistant," he says. There's already evidence that at low doses, the chemical makes the parasite create even more spores. "So it is a complex problem we have got."