Within the past 20 years abundances of the bee species Bombus occidentalis, B. affinis, B. pensylvanicus, and B. terricola have plummeted by up to 96 percent. (Related: "Mystery Bee Disappearances Sweeping U.S.")

The finding is based on a new analysis of more than 73,000 museum collections of bumblebees, which showed where bees had been found over the last century, as well as collections of wild bees across the United States. The study looked at 8 of the 50 known bumblebee species in North America. (Get bumblebee wallpaper.)

"We found that yes, indeed, [these four species] are seriously declining, but there are some other species doing very well," said study co-author