Honeybees have gotten a lot of sympathy for their mysterious declines. But a recent study has shown that the honey-makers are spreading their worst diseases to their cousins the bumblebees.

A study published online this week in Nature clearly revealed that two pathogens that infect honeybees have crossed over into populations of wild bumblebees. "We've known there was spillover, but this study is the most thorough survey to date" showing the diseases are making their way into other species, says Dennis vanEngelsdorp, an entomologist and research scientist at the University of Maryland, who was not involved in the study.

Matthias Fürst of the Royal Holloway University of London and colleagues in the U.K. and Germany focused on two common and devastating honeybee diseases: deformed wing virus (DWV)—which causes wing and abdominal deformities in honeybees, especially those hosting the blood-sucking Varroa mite—and Nosema ceranae, a fungus that causes intestinal inflammation, among other problems. Both pathogens are known to have been major players in the collapse of honeybee colonies in North America and Europe.

The scientists first ran experiments to see whether bumblebees could contract both the virus and the fungus. They could. "That may not seem surprising," says Fürst, "but it is not easy for a pathogen to cross the genus boundary, so it wasn't a given."

Then the scientists sampled bumblebees from 26 sites across Great Britain and the Isle of Man. They looked to see whether the infections seen in the honeybees there had crossed over into the wild bees and, if so, how prevalent the infections were. (Although the study didn't absolutely confirm that the honeybees were the source and not the other way around, the high density and higher parasite counts in honeybees makes it a reasonable assumption.)

The researchers report that about 11 percent of the tested bumblebees were positive for DWV and 7 percent for the Nosema fungus. In comparison, about 35 percent of honeybees had DWV and some 9 percent had the fungus. These infections aren't just being passively carried around, says Fürst, but are active and replicating, meaning they are doing real damage to the insects hosting them. In the study, for example, bumblebees with DWV lived only a third as long as their healthy counterparts.

Pathogens Get Around

Emerging infectious diseases aren't a new thing in animals. The deadly rinderpest (a measles-like virus) was taking out huge numbers of cattle way back in the 1800s. But with today's more intensive land use and changes in rainfall patterns and global temperatures, deadly pathogens may become more widespread—and more likely to creep into new hosts.

For example, avian influenza, hosted naturally by certain wild bird populations, is now known to cross to domestic chickens, ducks, and turkeys, and through those birds to humans. But the spread of disease often goes in the other direction, from domestic animals to wild animals, as farms and ranches encroach farther into wild landscapes.

One of many examples: Conjunctivitus, or pink eye, can leap from goats and other livestock into wild bighorn sheep. The disease doesn't kill wild sheep outright, but there have been reports of the mountain inhabitants stumbling blindly to their deaths as a result of its effects. (See "The Threat of Emerging and Re-emerging Infectious Diseases to Wildlife.")

Deadly Decade

As with all livestock, diseases have always been a problem for managed bees. Put any kind of animal into unnatural conditions—packed together, moved around, and exposed to chemicals—and you are bound to see changes in their susceptibility to pathogens.

But in the past decade honeybees have seen unprecedented declines around the world. (See "Gold Dusters" and "The Plight of the Honeybee.") No single cause explains the massive losses of bees. Scientists believe that a complex mix of factors straining bees' immune systems are to blame—from parasites and pesticides to, in some areas, low-quality forage. That means there's no single solution.

Luckily for honeybees, says vanEngelsdorp, "because of their huge colonies, they can take a certain amount of insult and still survive." But other bee species are less hearty. "If you're a solitary bee, when you're dead, you're dead," he says. "And so are generations after you."

Rumbles Over Bumbles

The loss of honeybees and other pollinators like wild bumblebees could devastate agriculture. Pollinators are estimated to do nearly $200 billion in "work" around the world each year. Managed honeybees pollinate many of the most economically important crops, such as fruits and nuts, as well as foods like alfalfa that cattle eat.

Other insects, including wild bumblebees, are less well studied but no less important; they may pollinate as many as half of all animal-pollinated plants—and are much more efficient at the job than honeybees, boosting the output and size of fruits, for example.

The discovery that DWV and Nosema infect so many geographically separate wild bumblebee populations "adds to the urgency of the pollinator situation, the need to address it quickly and with common sense," says vanEngelsdorp. "Mostly we need to look at land use, at agricultural policy," to make sure crops like corn and soy (not good for bees) aren't replacing too much of the plant-diverse wild land that provides healthful nutrition for all pollinators.

Tough Row to Hoe

The planet has at least 20,000 species of bees. Past studies have revealed that a number of those bees share diseases known to honeybees, but no one knows how widespread the problem is across the vast sea of insect taxa.

Whatever the numbers, it's going to take heroic and unprecedented cooperation among beekeepers, farmers, the chemical industry, and policymakers to stop our valuable pollinators from dropping like flies. Says Fürst, "We have a long way to go."