Parasitoids (Parasites)

Parasitoids are like the vampires of the insect world. As immatures, they obtain their nutrition by feeding in or on the body of another insect, ultimately killing it. The adults are typically-free-living and the females are responsible for finding host insects for their progeny. The two major groups discussed here are parasitic wasps and tachinid flies.

Parasitic Wasps

Parasitoid wasps comprise one of the most diverse and important groups of beneficial insects. Almost all insects are attacked by at least one species of parasitoid, and most by more than one. Some species attack only one insect host and many successful classical biological control programs have involved the introduction of highly specific parasitoids. Many species are large and colorful, but most of the economically important ones are small and very inconspicuous. For example, those attacking aphids are smaller than their aphid hosts, and those developing within a single moth egg or scale insect can be barely visible to the naked eye. Although reproduction is typically sexual, females can manipulate the sex of their progeny by controlling the fertilization of eggs; males are produced from unfertilized eggs and females from fertilized ones. In some species, all-female lines persist for many generations without sexual reproduction. The female uses an ovipositor to lay eggs in a host insect . In some species the ovipositor is held internally when not in use; in others it is not retractable and may be as long, or longer than, the entire wasp. Venom that serves to immobilize, paralyze or otherwise subdue the host may also be delivered via the ovipositor. Some female parasitoids also use the ovipositor to puncture a host and then feed on the body fluids before selecting other hosts for oviposition, thus causing two different types of mortality in the pest population. In some cases, the egg is laid externally on the body of the host and the larvae may also feed externally (ectoparasitism). More commonly, the larva develops and pupates within the host body, feeding selectively on the host’s internal tissues and leaving the digestive tract and nervous system to the very last (endoparasitism). Another important distinction is whether the host is allowed to develop and grow with the parasitoid larva inside it, or whether it is killed or permanently paralyzed by the attacking females so that it remains a static, rather than dynamic, food source for the developing larva.Greenbug Mummies from Lysiphlebus testaceipes- Photo by Phil Sloderbeck

Tachinid flies

This group represents a very large family of flies with over 1000 species in North America, all of which have a parasitic lifestyle. They vary considerably in appearance, but most have very bristled bodies and resemble house flies, although they can be substantially larger or smaller. The adult female typically lays an egg on the surface of the host insect cuticle and the hatching larvae then bores into the body of the host and develops internally. In other cases, the fly egg is consumed by the host insect in the process of feeding. Some species give birth to live larvae that are placed directly onto the host. A wide range of moth and butterfly larvae are attacked, and so are a number of beetle species. The host may be killed in the adult stage, but more commonly in the pupal stage. Tachinid Eggs on Armyworm - Photo by Bob Bauernfeind