What's killing bumble bees? Some species' numbers down 96 percent.

Bumble bees – ace field hands pollinating apple orchards, wildflowers, and backyard clusters of Coreopsis – appear to be facing hard times in the US.

Nearly 10 percent of wild bumble-bee species have undergone serious declines in number and in geographic range, according to a new study – the first attempt at gauging the health of native bumble-bee populations nationwide.

Depending on the species, the bees' ranges have dropped as much as 87 percent below their historic extent, with some of those declines occurring within the past 20 years. Meanwhile, their relative abundance compared with estimates of their historic levels have plunged by as much as 96 percent.

Several factors could be driving the declines, according to a team of federal, state, and university researchers conducting the survey. These factors range from a parasite thought to have been introduced with the arrival of bumble bees bred in Europe to pollinate hot-house crops, to habitat fragmentation and declining genetic diversity among the affected species.

Bumble bees "are incredibly resilient," especially when their numbers are set against the range of stresses their populations already have endured, particularly habitat loss, notes Sydney Cameron, an entomologist at the University of Illinois at Urbana-Champaign who led the team.

Still, the trend is troubling, she says.

"There's a threshold, and above that threshold? Bang, that's it," Dr. Cameron says. "We just don't know what the thresholds are for these species."

The loss of a bee species or two wouldn't bring the end of the world, since other species could fill the deceased's ecological role to some extent, notes Robbin Thorp, a bee biologist at the University of California at Davis.

But the study represents a wake-up call, says Dr. Thorp, who was not involved in the project.

"Not all bumble bees are being affected by this," he says, but the few species that are affected "are really a kind of environmental warning."

Several studies over the past decade of wild bumble-bee populations at specific locations in the US and Canada helped shine a spotlight on the issue in North America.

Researchers say that these represent suggestive snapshots of the status of bumble-bee populations. But until now, no one had taken a comprehensive look and trends as well as the correlation those trends may have with potential causes.

Cameron's team selected eight species of wild bumble-bees that historically were widespread. They picked four species whose populations appeared to be stable or growing, and four that had been identified in local studies as showing declines.

Team members conducted three years of extensive field surveys that yielded nearly 17,000 bee specimens, including the species the scientists were examining, to build a picture of contemporary populations.

To estimate historic ranges and relative abundance, the researchers built a database containing information on 78,000 bumble-bee specimens -- these included nearly 74,000 from the eight species in question. The data were drawn from collections and records in natural-history museums around the country.

The researchers also undertook genetic studies of the bees as well as of the suspected parasite as they explored factors contributing to the declines they noted.

Among the findings, the team noted that the four declining species -- three of which are very closely related genetically -- had significantly lower genetic diversity than the four more-robust species. Moreover, higher numbers of individuals in the declining species carried the parasite in question, Nosema bombi. In some cases, dramatically high numbers.

Cameron recalls bumble-bee specimens taken from one canyon in Utah in which 100 percent of the bees from the declining species carried the parasite, while few if any specimens from other bumble-bee species in the same canyon carried the organism.

What isn't clear, she says, is whether the reduced genetic diversity, or some combination of that and other environmental factors, have left the declining species more vulnerable to stresses, including the parasite, or whether the parasite's arrival triggered the species' declines.

Yet the parasite appears to be emerging as a lead suspect, based on genetic studies the team conducted on the bees.

Low genetic diversity among the decliners would usually suggest that they inhabit highly fragmented habitats and are becoming increasingly inbred; they don't mix with other populations of their species. Noticeable genetic differences among these isolated populations and their less-isolated relatives would begin to show up.

Indeed, even though a species might range across the entire eastern US, one would expected to see some differences in the genetic make-up of a population within that species from one region to the next, Cameron says. It's as though species develop a regional genetic "accent."