Within the past 20 years abundances of the bee species Bombus occidentalis, B. affinis, B. pensylvanicus, and B. terricola have come down by up to 96 percent.
The finding is based on a new study 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.
The discovery makes it harder to pinpoint pesticides or climate change as a cause for the bug die-off, because those factors wouldn't clarify why other bumblebee species in the same areas have survived.
One possibility is that the four species in crisis may all be infected with the invasive Nosema bombi fungus, which was found in larger quantities on the dying bumblebees than on relatively healthy species studied by Cameron's team.
And bees reared in Europe, where the fungus is more dangerous, were imported to California in the early 1990s—right before the bees began to die off. But the link isn't sure yet.
"We're at this frustrating stage where there's a lot of circumstantial evidence to say that these species are declining because of this pathogen," Cameron said. "But we don't have undeviating evidence. We have no cause and effect."