Bombus occidentalis used to be the most common bumble bee species in the Pacific Northwest, but in the mid 1990s it became one of the rarest. The reason or reasons for the species decline remain unsolved, but one possibility points to a fungal pathogen known as Nosema bombi.

Whatever the causes were for the decline, an article in the Journal of Insect Science offers some good news for the bee.

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

The researchers sampled sites from the San Juan Islands and the Olympic Peninsula to northern Idaho and northeast Oregon. They excluded the dry central region of Washington because Bombus occidentalis is not usually found in such areas. Its natural habitat is typically wetter areas.

In addition to the wild populations, Bombus occidentalis was once used to pollinate crops in greenhouses.

“They were raised in large numbers commercially and shipped around to greenhouses,” said lead author Paul Rhoades, a graduate research fellow at the University of Idaho, who notes that the Nosema bombi pathogen might have been in the bees’ semen and transmitted within the bee colonies. “Some of the individuals might have escaped and spread the parasite,” he said.

The bee is no longer used commercially in greenhouses, but in the wild it pollinates a large variety of plants. It is a generalist, pollinating a range of crops including pumpkins, raspberries, apples, cherries, and canola, among others.

According to Dr. Strange, the Nosema parasite might have been a factor in the bee’s decline, but whether it’s the sole factor remains an open question. Trying to better understand the impact of Nosema on Bombus occidentalis is difficult, he said, because laboratory studies are hard to do.

“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],” he said. “We have a lot of correlation, but we can never get the species without the pathogen. We can’t clean this pathogen out.”

“If you look at [bumble bee] populations up in Alaska, they have Nosema, but they are present in fairly high abundance,” Rhoades said. “There’s also evidence that there’s genetically distinct populations that are in northern British Columbia and Alaska compared to the northwestern part of the United States and southern British Columbia.”

The study’s findings raise the question of why the population decline appears to have stopped. Dr. Strange’s answer is “evolution.”

The pathogen’s virulence may have declined. If the pathogen wipes out its host, “that’s clearly not in the best interest of the pathogen,” he said.

Another possibility is that some individuals of Bombus occidentalis are able to resist the pathogen, “and that’s probably more of what’s going on,” said Strange.

So what will the future of Bombus occidentalis in the Pacific Northwest look like? It’s a fairly large concern, according to Rhoades.

“What we seem to detect is that there are bees in northern Idaho and some in the Olympic Peninsula, and there are very few if any in the intervening region. That raises the problem of a lack of gene flow between these populations. If there aren’t immigrants, there’s a whole host of problems associated with the lack of genetic diversity and inbreeding.”