Bumblebee returns to areas where it once was a key crop pollinator. (REUTERS)

A North American bumblebee species that all but vanished from about half of its natural range has reemerged in Washington state, delighting scientists who voiced optimism that the insect might eventually make a recovery in the Pacific Northwest.

Entomologists and bee enthusiasts in recent weeks have photographed several specimens of the long-absent western bumblebee (Bombus occidentalis) buzzing among flower blossoms in a suburban park north of Seattle.

The sightings are evidence of western bumblebee colonies in the area, although it hardly proves the species will thrive, said Rich Hatfield, a biologist for the Oregon-based Xerces Society for Invertebrate Conservation.

B. occidentalis is one of four wild North American bumblebee species whose populations began to plummet two decades ago, while honeybees — commercially bred, for the most part — have undergone less precipitous declines, Hatfield said.

Scientists have cited a number of likely factors for bumblebee declines, including parasites, pesticides and habitat fragmentation, though a fungus spread from Europe is now a leading theory. Until the mid-1990s, the western bumblebee was among the most common bees in the western United States and Canada, where it was valued as a key pollinator for tomatoes and cranberries. It has since virtually disappeared from about half its historic range, although its population remains relatively robust in the Mountain West.

The discovery of the bees near Seattle might mean that a population resistant to the fungus has emerged, Hatfield said.