Bumblebees are dying out because they're too fat to mate

A queen bumblebee can birth males—called drones—on her own. But only after a male fertilizes her eggs can she produce female bees. This is crucial, because drones are essentially layabouts. It’s the women, the worker bees, that do all the foraging to sustain a colony. So inside a mating cage, Strange places a queen bumblebee, and on the other side a drone infected with Nosema bombi. Already this fungus has spreads down the bee’s throat, has rooted itself in the gut where the spores “proliferate like crazy,” according to Strange.

There the fungus swells in the soft tissue between the bumblebee’s organs until the drone grows so plump it can’t bend its abdomen to mate with the queen. Without fertilization, the queen can only birth more males. Without females, future colonies starve.

It’s devastating for bees, and perplexing for scientists, because the oddest part of Nosema bombi is that it’s lived alongside bumblebees for centuries.

“We still don’t know why this fungus went from being not so bad, to really bad for certain bees,” Strange told me.