Some of the more spectacular diseases are the viral infections of caterpillars that leave thousands of flaccid, black caterpillars hanging dead from tree branches, and the fungal diseases of grasshoppers and flies that cause the insects to climb up the plants, where they die clinging to the very top. Some pesticides have adverse effects on these pathogens of insects. In particular, many fungicides suppress important fungal pathogens of insects. Parasites of insects live in or on the bodies of their hosts, where they feed during at least part of their life cycle. Many insect parasites are referred to as parasitoids because they consume a large portion of their host, eventually killing it.
Most insect parasites are flies, bees and wasps.
There are at least six important families of fly parasites and 10 families of important wasplike parasites. In just one family of parasitic wasps, the ichneumons, more than 3,100 species have been identified in North America. Most ichneumon species are specialized to parasitize only one or a few host insect species. Insect parasites are very susceptible to insecticides, and some may also be killed by other pesticides, such as fungicides and herbicides. The final groups of natural enemies are the predators.