Gardeners may encounter caterpillars, such as hornworms, suddenly decorated with white egg-like structures. These are actually wasp pupae within silken cocoons. The caterpillar should be left alone, because it will soon die, and the wasps will emerge and attack more caterpillars. They use the ovipositor to lay eggs in insect larvae found feeding deep within plants, or even wood. They are completely harmless to humans.
Bee flies are examples of flies that are parasites of other insects as larvae.
The adult flies mimic bees and may be mistaken for the predatory flower flies discussed in the previous section. The larvae attack the eggs or immatures of grasshoppers, beetles, moths, bees, and wasps. Nematodes are hair-like worms found naturally in the soil. Many are microscopic in size and vary in life-style. The parasitic forms generally feed on insects that are found in the soil during one or more stages of their life cycle, such as white grubs, root maggots or weevils.