Identification

Parasitic wasps, such as ichneumon wasps, braconid wasps and chalcid wasps, comprise many species that differ widely in their appearance and size. The smallest species develop as larvae inside the eggs of other insects.

Most parasitic wasps develop as larvae inside the larval or nymphal stages of insects or other invertebrate animals. Some attack the pupal stage and a few feed inside the bodies of adult insects.

Some female parasitic wasps have a long sting-like egg-laying structure on the rear end of the abdomen. This is particularly true of parasites whose prey insect lives in a concealed situation, such as inside a plant stem.

Size

Adult parasitic wasps range in size from less than 1mm long up to 35mm.

Distribution

Parasitic wasps occur throughout Britain.

Status

Parasitic wasps are not as well studied as most other insects, so relatively little is known about the biology of many species. There are in excess of 4000 species in Britain and new species continue to be recorded.

Habitat preference

Found in all land and freshwater habitats.

Where to find in them in the garden

Adult parasitic wasps sometimes visit flowers to feed on nectar and pollen. Aphids killed by parasitic wasps become abnormally swollen and straw brown in colour. Such dead aphids can be found on many plants in late summer. Dying caterpillars of large cabbage white butterfly may be found surrounded by sulphur yellow silk cocoons spun by fully fed larvae of the parasitic wasp, Cotesia glomerata.

Role in the garden

Parasitic wasps, like predators and diseases, are one of the means by which the populations of the animals they parasitise are controlled. During the early stages of the parasite’s development, it causes no obvious harm to the host animal, which carries on feeding and growing. However, in the later stages of the parasite’s development, it destroys its host’s vital organs and kills it. Some parasitic wasps are being reared and can be purchased for the biological control of certain glasshouse pests. These include Encarsia forma for glasshouse whitely and Aphidius species for aphids.