Parasitic wasps

There are around 2,400 species of solitary wasps recorded in Britain and Ireland, many are parasitic, and some are used in controlling pest insect species.

Female Ichneumonid wasp


The family Ichneumonidae contains the tiny parasitic wasps. There are over 60 000 species world wide, and over 2100 British species. All have long antennae with usually more than 16 segments, slender bodies, and very simple wing venation (see left and below). The adults usually feed on nectar from flowers and honeydew from aphids. The females usually have visible ovipositors.

Their larvae are usually parasitic in or on other insects; mainly butterflies, moths, sawflies and beetles, and are major agents of pest insect control. Few are parasitic on a single species; usually they parasitise a group of species. The female usually lays her egg inside the victim, but some species lay the egg on the surface of the victim's body.

female Ichneumonidae wasp ovipositing Ichneumonids often locate the host species by the smell from their frass (poo). Many of the host species are cryptically coloured, and some emerge to feed only at night, so smell is a good locator.

Rhyssa persuasoria, the sabre wasp, locates grubs of the wood wasp by detecting the presence of a particular fungus injected by the female wood wasp at the same time as she inserts the egg. The sabre wasp has an ovipositor almost 4 cm long, though her body length is 20 - 35 mm, making her the largest Ichneumonid in the U. K. It takes around 20 - 60 minutes to drill down into the 3 cm of wood to where the wood wasp larva is located. During this time the wasp is in a very vulnerable position with no protection from predators. Adults are black with white spots and red legs, and fly from June - September. A fully grown larva can reach 25 mm long.

Most overwinter as grubs in the host, or as pupae; just a few overwinter as adults, and of these it is only the females that do so. All the adult males die with the approach of cold weather.

On the left is a drawing of a female Ichneumonid ovipositing, and below left is a female showing her ovipositor. Below is Ichneumon suspiciosus showing the typical Inchneumonid body shape.

Ichneumon suspiciosus is black with a yellow patch near the top of the abdomen. The legs are black except for the tibia and tarsi which are yellow. Adult body length is 10 mm.

The larva is an internal parasite of caterpillars.

It is seen in the summer, often on umbellifers.

Ichneumon suspiciosus, parasitic wasp

On the right is the case of a Phryganidae caddis fly larva that has been parasitised by Agriotypus sp., an Ichneumonid wasp. The adult wasps swarm over streams in spring and mate. The female wasp then crawls down a plant stem, into the water and down to the stream bed where she searches under stones for a Phryganidae caddis fly larva.

She lays an egg inside the case. When the wasp hatches from the egg it starts to eat the body of the caddis fly larva. However, it does not eat the nervous system, as it does not want to kill the caddis larva.

Only once the caddis larva has fixed its case to a stone to prepare for pupation will the wasp kill it by eating all the caddis larva. Then the wasp makes its own silk cocoon inside the caddis case and emerges as an adult the following spring. A parasitised case is easy to recognise as it has a long thread hanging from it.

Agrotypus sp. Ichneuminid wasp parasitising a caddis fly larva


In England in 1920 Encarsia formosa, a tiny parasitic wasp, was used to control whitefly in greenhouses where tomatoes were grown. Then, with the abundance and ease of the use of new pesticides made in the 30s and 40s bio-control died out. It was revived by the Dutch greenhouse tomato growers in the 70s when their whiteflies had developed resistance to the currently available pesticides.

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