Scientists call it the Beepocalyspe. (OK, not scientists, but I like to call it that.) In late 2006, whole hives of honey bees began dying overnight for reasons that are still unclear. Scientists called it colony-collapse disorder (CCD), and it’s as scary as it is mysterious. Adult bees simply leave the hive, ostensibly in search of pollen, only to die somewhere in the open. Reported death rates in bee colonies in the U.S. were 29% in 2009 and rose to 34% in 2010. (Data from the Department of Agriculture’s CCD Progress Report—download a PDF here.) It’s still unclear what’s behind CCD—recent studies have suggested that it might be due to a combination of viral and fungal infections—but there’s no doubt about the impact that sustained bee loss would have on the agricultural sector. About 130 crops in the U.S.—worth some $15 billion a year—depend on pollination from the honeybee alone in the U.S., and it’s scary to think what might happen to the world food supply if CCD can’t be curbed.

Get ready for more bad news—it’s not just the honeybees that are disappearing. North American bumble bees have been steadily dwindling, vanishing from their long-established habitat. Bumble bees aren’t as well-known as honeybees, but they’re important pollinators as well, especially for tomatoes and berries. While there have been anecdotal reports from beekeepers and other observers about population declines for bumblebees, however there hasn’t been the same concerted effort to track bumble bees. But a new paper published in the January 3 Proceedings of the National Academy of Sciences (PNAS) contains the results of a multiyear study of bumble bee populations, and the numbers are sobering. According to the study, written by a team of scientists including entomologist Sydney Cameron of the University of Illinois, the relative abundance of four species of bumble bees over the past few decades has dropped by more than 90%—and those disappearing species are also suffering from low genetic diversity, which makes them that much more susceptible to disease or any other environmental pressures. (Download a PDF of the paper here.)

As with CCD, it’s not clear why bumble bee populations seem to be declining. Cameron and her colleagues note the possible role of a parasite called Nosema bombi that commonly found in Europe but which hasn’t been fully studied among North American bumble bees. It’s not a smoking gun, but the PNAS paper found that declining populations of bumble bee species were associated with high levels of parasite infection, while stable species were less likely to show frequent infection. Still, the authors write that the parasite could simply be more common in species that are declining—correlative, rather than causative.

Indeed, while studies like this one can help us get a grip on the problem of declining species, it’s still not clear what’s causing it—which makes the vanishing that much more eerie. On a day when authorities are trying to figure out why thousands of birds fell from the sky and thousands of fish died in the rivers in Arkansas, the bee study is a reminder of all the ways we may be impacting the natural world for the worse—without even knowing it.

More from TIME on bees:

Why We Should Care About Dying Bees

Postcard from Hughson

New Clues in the Mass Death of Bees