All across California, honeybees are flying away from their hives  and dying. Empty hives are causing a lot of worry about some important food crops. Bees give us a lot more than delicious honey. They are pollinators --they enable plants to produce the fruits and nuts we enjoy by carrying pollen from one plant or flower to the next. The wind pollinates oats, corn, and wheat, but many other plants (like apple and cherry trees and melon vines) depend on insects, bats, and birds. In the U.S., millions and millions of bees kept by human beekeepers fly around doing a lot of this important work for food crops. "Bees are worth protecting because their work adds so much to our diet," says Dr. Jeff Pettis of the Bee Research Laboratory. California's almond   crop alone depends on about half the bees in the country. But now the almond crop and many others could be in trouble with so many bees dying. Researchers at government and university labs all over the country are trying to figure out why so many bees are dying. However, bees are hard to study. Most die away from the hive, so researchers don't have dead bodies to examine. And when researchers return to a hive after two weeks, about half the bees they studied on their first visit will be dead, replaced by new ones in the natural life cycle of bees. "It isn't like studying a large animal like a cow that doesn't move around much and is easy to find out in the cow field," says Pettis. Researchers have some ideas about what could be affecting bee health. They could be sick from poisons widely used to kill insects, or they might not be getting enough good food to stay strong. Also, tiny insects called mites feed on bees. "A virus or bacteria could also be doing the killing." explains Pettis. Bees are hard to study because   _  .
they move around too much new bees soon replace all the bees studied by researchers they are too tiny they never return to hives