Biological Control Agents: Parasites

Parasites, also called parasitoids, are insects that attack and kill hosts (often we call the hosts pests). An insect parasite develops as a larva in or on a single host; from an egg that is laid in, on or near the host; and pupates in, on or near the consumed (and killed) host; and is free-living as an adult. Although the adult female parasite attacks the host, it is her offspring that kill the host.

Unlike predators that kill many prey during the predators' lifetime, the parasite offspring almost always develop in only one host. Parasites usually are smaller than their hosts, and often a single host produces more than one parasite offspring. A female parasite may find and attack many hosts during her lifetime.

There are approximately 85-90 families of insects in several insect orders that are parasitic, but most are not applicable to biological control. Most of the parasites that are encountered or used for biological control belong to one of two orders -- the Diptera and Hymenoptera -- although there also are a very few parasites that belong to Coleoptera, Neuroptera, and Lepidoptera. The order Strepsiptera is an order of true zoological parasites whose parasitism differs, as the host is not killed. Far and away, the majority of parasitic species are Hymenoptera.

Many parasites have a narrow host range in the field -- often attacking only one or a few similar species in a given habitat. Other species are more polyphagous, attacking a broader range of species.