When the majority of a colony's worker bees disappear and leave behind the queen, baby bees, and lots of food, that's called Colony Collapse Disorder (CCD). The mysterious phenomenon has caused enormous bee losses since at least 2006.
Researchers have identified a barrage of threats to bee health that could be responsible.
One factor is that bees are getting sick. New viruses and a new fungal gut parasite are killing bees worldwide. American foulbrood, the primary bacterial disease affecting US honeybees, has developed resistance to the antibiotic that beekeepers once used to prevent it.
Hives can also be invaded by parasitic mites. During the winters of 1995 through 2001 in particular, mites decimated bees across the northern US, wiping out entire beekeeping operations. Today many mites are pesticide-resistant.
Human activities can also stress bee colonies to the point of collapse. Bees, like humans, can't lead healthy lives consuming just one type of food, so as more farms grow miles and miles of a single crop, bees have to fly too far to get the nutrition they need.
Climate change and urban development further decimate bees' habitats.
Pesticides sprayed on crops also play a role. Some are poisonous to bees and have been directly linked to their population collapse, so the US Environmental Protection Agency (EPA) banned 12 pesticides that are harmful to bees this year.