Weakened by inbreeding and disease, bumblebees have died off at an astonishing rate over the past 20 years, according to a study that found some US populations plummeting by more than 90%.
The findings are of serious concern, the BBC reports, because the bees play a crucial role in pollinating crops such as tomatoes, peppers and berries.
The study's leader, University of Illinois entomologist Sydney Cameron, says similar declines have been noted in Europe and Asia.
"The decline of bumblebees in the US is associated with two things we were able to study: the pathogen Nosema bombi and a decline in genetic diversity," Mr Cameron says.
"We are not saying Nosema is the cause; we don't know. It's just an association. There may be other causes."
Nosema bombi has also afflicted European bumblebees.
Honey bees also vanishing fast
The study examined eight species of North American bumblebees and found the "relative abundance of four species has dropped by more than 90%, suggesting die-offs further supported by shrinking geographic ranges".
"Compared with species of relatively stable population sizes, the dwindling bee species had low genetic diversity, potentially rendering them prone to pathogens and environmental pressures."
Their cousins, the honey bees, have also experienced catastrophic die-offs since 2006, in a phenomenon known as "colony collapse disorder", the causes of which have yet to be fully determined.
Bumblebees also make honey but it is used to feed the colony, not farmed for human consumption.
The findings of the three-year study, reported by AFP, were published in the Proceedings of the National Academy of Sciences.
