Parasitoids

A parasite is an animal that lives in or on the body of another living animal (the host) at least during a part of its life cycle. It feeds on the tissues of the host. Insect parasites (parasitoids) have a larval stage that develops inside the body of its host. Usually they kill their host in the end.

Many insect parasitoids are wasps, but also some flies or other insects can be parasitoids.

Parasitoid
Bitter gourd leaf with a parasitoid wasp.

Depending on the host stage they attack, parasitoids can be called egg parasitoids, larval parasitoids or pupal parasitoids.

Egg parasitoids

Very small wasps, which are hardly visible with the naked eyes, can parasitize the eggs of bigger insects such as butterflies and moths. Parasitized eggs can be recognized as they are usually darker in color when compared to eggs that were not parasitized.

In a healthy rice field, usually a big number of stem borer eggs will be infested by egg parasitoids. The larva of the wasp develops inside the egg, feeding on its content. Finally, instead of a caterpillar, an adult wasp will emerge, which can start to infest eggs of other stem borer eggs.

A well know egg parasitoid is Trichogramma. It is possible to rear big numbers of these parasitoids and release them in a field. This has already been done successfully in the management of sugarcane stem borers.

Egg parasitoids are usually present in the field and play an important role in keeping pest populations at low levels. To protect these natural occurring egg parasitoids farmers should avoid the use of pesticides.

Larval / Pupal parasitoids

Many small wasps are predators of larval of pupal stages of pests. The female wasp actively searches the crop for suitable larvae and deposits one or more eggs in or on the body of its host. Most species of parasitoids will parasitize only one species or few related species of pests. Because they are so small, parasitoids go often unnoticed in the field. Careful monitoring is necessary to determine if parasitoids are present. Sometimes the percentage of larvae of a pest that are found to be parasitized can be surprisingly high.

Braconid wasp
Larval parasitoids are often small wasps such as this Braconid wasp (Apanteles sp.)