The real reason bumblebees are dying off is because they’re too fat to mate.
According to The Atlantic, the spread of a fungus called Nosema bombi is to blame. It infects male bumblebees-whose sole job it is to mate with the queen-by causing them to swell up until they’re unable to bend their abdomen enough to mate. The fungus doesn’t affect female bees who do all the pollinating.
Scientists are working hard to figure out how it happened and until they come up with a cure, laying off pesticides in your garden and your lawn would help the bees out.