Many researchers feared that the Western bumblebee was headed toward extinction.
Many researchers feared that the Western bumblebee was headed toward extinction. (Stephen Ausmus, U.S. Department of Agriculture)

An elusive bumblebee with a distinctive white rump hadn't been seen on Colorado's Front Range since 2009, when an entomologist spotted a single specimen outside Boulder.

But this summer, a crew of student researchers captured more than a dozen Western bumblebees, suggesting the insect is coming back.

Many researchers feared that the Western bumblebee, or Bombus occidentalis, was headed toward extinction as are other bee species in the U.S.

Their outlook changed in 2012, when a team of two University of Colorado at Boulder researchers and five undergraduate assistants found five in Boulder County.

The group was in the third year of a five-year census of bumblebee species along the Front Range.

The discovery of the insects in multiple colonies has researchers cautiously optimistic the Western bumblebee is making a comeback along the Front Range.

Two decades ago, the bee was found in wide swaths of the Western U.S. and Canada. Since then, its numbers have dropped precipitously.

"The neat thing about that is we found them at four different sites so we know there are multiple colonies out there," said CU biologist Carol Kearns, who is one of the census project's leaders. "We're pretty excited."

Kearns said leading bee researchers hypothesize that Western bumblebees imported to Europe years ago picked up a non-native gut parasite. When the queens made their way back to the U.S., they probably brought the parasite with them.

As to why they might be making a comeback now, the bees could be developing a resistance to those parasites, Kearns said, "but that's just speculation."

Ben Bruffey, an evolutionary and ecology biology student at CU who was involved with the study, said the research will give a broader understanding of bumblebee numbers along the Front Range.

"It's really important research because pollinators are an indicator species," Bruffey said. "If their populations are declining, it could mean the environment is becoming harder to live in."