The commercial bumblebee industry drove a western species, Bombus occidentalis, to the brink of extinction in the 1990s. A recent study, however, suggests the bees are staging a comeback. 

"The population seems at least to be re-emerging where it hadn't been seen in the last 10 years," James Strange, one of several co-authors and a researcher at the U.S. Department of Agriculture's (USDA) Pollinating Insects Research Unit at Utah State University, said in a news release. "There is some resilience in the population of Bombus occidentalis. They do seem to be coming back."

Bombus occidentalis is one of 30 species of bumblebees living in western North America, and was among the most common variety in the Pacific Northwest until the 1990s. Researchers believe the common beekeeping disease, Nosema, may have played a role in the species' decline.

The Nosema parasite can be found in the gut of infected bees and rapidly spread within a hive, wiping out entire colonies. While there are no identifiable symptoms, brown spotting may appear on the outside of the hive. However, researchers are still unsure if Nosema is the sole factor in the bee's decline. 

"When we try to raise the bees in captivity, they die, so we can't do a lot of experimental work to show that this is really the thing [killing bees]," Dr. Strange explained. "We have a lot of correlation, but we can never get the species without the pathogen. We can't clean this pathogen out."

The study's findings also raise the question of why Bombus occidentalis population declines appear to have suddenly stopped. Evolution may have taken over, equipping bees with an increased resistance, or the pathogen's virulence may have simply declined. 

Their findings were recently published in the Journal of Insect Science. 

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