Parasitica - (all Apocrita except for the Aculeata)

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A photograph of Rhyssa persuasoria, an Ichneumonid that is an ectoparasite on the Greater Horntail (_Urocerus gigas_).

Rhyssa persuasoria is a large Ichneumonid that is an ectoparasite on larvae of the Greater Horntail (Urocerus gigas). These impressive insects are completely harmless.

Most 'Parasitica' are very small, about the size of gnats, and often go unnoticed. They are called 'Parasitica' as a large majority of females seek out an invertebrate host to deposit their eggs. Females who lay eggs on insects in the open air have very small ovipositor; those which attack larvae underground, or are protected by some shelter, have long ovipositors.

On hatching, the larva gets nutrients from body of the victim, and only when the larvae reaches its final growing stage does it start to feed on the vital organs, resulting in the death of the parasitised invertebrate. There are many ways in which Parasitica may feed on a host. Parasitica that feed outside the body of the victim are called ectoparasitoids. Those that feed inside the body are called endoparasitoids.


The Ichneumonoidea are the largest group of the Parasitica, these account for almost half of the British Hymenoptera fauna. There are 3,200 British species of which there are two very distinct families the Ichneumonidae and the Braconidae.


Ichneumonidae are slender insects, with long antennae. They vibrate their antennae to locate their host larvae in which they usually only lay one egg. Ichneumonids include several conspicuous large species with very long ovipositors such as the Rhyssa persuasoria the ectoparasite of the horntail Urocerus gigas. Rhyssa is capable of locating the wood wasp through several centimetres of wood, and boring her long ovipositor to reach it. Occasionally, accidents can happen and the ichneumonid may miss the target host and the egg may stick on her own body, when the larva hatches it at once attacks its own parent.


Braconidae are smaller and inconspicuous. They are the parasitoids of a wide range of hosts in various life stages. Entomologists can tell them apart from Ichneumonidae as the Braconidae have three joints to their labial palps (the palps on their lower lip). Ichneumonidae have four joints to the palps of their lower lip.

Other Parasitica

There are many more Parasitica besides the Ichneumonoidea, in a number of families, accounting for over 2,300 of the British Hymenoptera. Most are small, feeding singly or gregariously and attacking hosts at a variety of life stages from egg to adult.

Some are known to introduce venom to the eggs of host to curtail host development while the larvae feed. Trissolcus basalis (Scelionidae) females will mark each host egg by scratching the egg surface with its ovipositor, thus showing that this egg is already occupied by her young. Some species will leave a chemical trace upon eggs already parasitised.

Hitch-hiking (phoresy) can be witnessed in the female Telenomus gracilis (Scelionidae). She seeks out moth pupae in spring and waits for the moth to emerge before flying aboard. Once aboard, she conceals herself in the dense hair on the moth's thorax, where she remains for 5-6 months until the moth lays eggs, after which she lays her eggs into these.

Most of the eulophid Parasitica (Eulophidae) have more than one generation a year. A few species have different seasonal colour forms. Some eulophids have different and complex mating strategies that can be used to separate the closely related species.

Some Parasitica have complicated life histories. A rare British species called Trigonalis hahnii (Famliy Triogonalyidae) has two potential life histories, depending on whether the egg is ingested by a caterpillar or a social wasp. If eaten by a caterpillar, the larva will fail to develop further unless the caterpillar is parasitized by an endoparasitoid, when the Trigonalis larva seeks out the endoparasitoid and enters its body as a hyperparasitoid. Alternatively, if the Trigonalis egg is consumed by a social wasp in the first instance, it will simply develop as parasitoid inside the wasp.

Finally, there are some 'Parasitica' who do not carry this name well. Many of these belong to the family Cynipidae. Some members cause the development of odd-looking growths called galls on plants. The female gall wasp oviposits in the host-plant, but the gall only develops after the larva has hatched. It is the plant's response to a salivary secretion from the feeding larva. (Galls formed by sawflies differ as the ovipositing females initiates the gall-formation).

Galls vary a great deal in size and structure and many species are easily recognised by the characteristic form of their gall e.g. the Robin's Pincushion (caused by Diplolepis rosae) and marble galls on oak (caused by Andricus kollari). The larva of the gall wasp obtains food and some protection within the gall, and the plant by producing the gall limits the amount and area of damage done by the gall wasp.

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