Bees are of inestimable value as agents of cross-pollination (see pollination), and many plants are entirely dependent on particular kinds of bees for their reproduction (such as red clover, which is pollinated by the bumblebee, and many orchids). In many cases the use of insecticides for agricultural pest control has had the unwelcome side effect of killing the bees necessary for maintaining the crop. Such environmental stresses plus several species of parasitic mites devastated honeybee populations in the United States beginning in the 1980s, making it necessary for farmers to rent bees from keepers in order to get their crops pollinated and greatly affecting the pollination of plants in the wild. In 2006, commercial honeybee hives first suffered from "colony collapse disorder," which, for unclear reasons, left many bee boxes empty of bees after overwintering. Dead bees from affected colonies since have been to be infected with a fungus and a virus that, acting synergistically, are now believed to be the cause. Bee venom has been found to have medicinal properties. Toasted honeybees are eaten in some parts of the world.