Biology of parasitoids

A distinction is made between parasitoids solitary , of which a single larva develops at the expense of its host, and parasitoids gregarious in which several of them normally coexist on or inside it.

The ectoparasitoids consume their host from the outside. This one is most often sheltered, either inside a cocoon or a puparium, or within various plant tissues: stems, roots, fruits or galls. On the other hand, in Trombidiidae mites (figure 1), it is the larva which develops as an ectoparasite at the expense of a host which is free, but without killing it.

In endoparasitoids (Figure 2), larval development takes place entirely within the host. This implies certain adaptations, in particular at the respiratory and immune level. The embryonic chorion as well as the larval exuvia then remain attached to the posterior part of the body, forming a link between the larva of the parasitoid and the cuticle of its host.

The parasitoid must also be able to avoid encystment reactions from its host. In some Ichneumonidae and Braconidae, polydnaviruses are integrated into their genetic heritage in the form of proviruses. They replicate in the accessory glands of females and are injected at the time of spawning. They then act in synergy with the venom to neutralize the host's encystment reactions. These viruses are specific to each group of parasitoids and may even be absent or present depending on the strains considered within the same species of parasitoid.

Some additional definitions:

• parasitoids of parasitoids are hyperparasitoids (Figure 3);
• the larvae of some species sometimes develop into autoparasites . This is particularly the case with certain Aphelinidae: females develop as primary parasites of whiteflies or of cochineals mealybugs. , while males feed on hyperparasitoids on female (larvae), whether of their own species or 'a different aphelinide;
• we speak of superparasitism when excess eggs are deposited inside, on or near the same host;
• the multiparasitism occurs when the competition covers several types.

We also recognize two types of strategies for larval development:

• in idiobionts , development begins shortly after laying, as soon as the egg emerges. The emergence of the host takes place at the stage when the spawning took place. This type of development is very often found in ectoparasitoids, whose host is housed inside plant tissue. The females then deposit their eggs on hosts that have completed their larval development or in the nymph state. Oophagous parasites are also included in this case, but they are generally endoparasitoids;
• in koinobionts , a more or less extended period of time is established between the date of laying and the onset of larval development of the parasitoid. The female lays her eggs inside small hosts (embryos or young larval stages), which will complete their development before that of the parasitoid begins (photo). This presents, for the parasitoid, the advantage of being able to attack large hosts without having, for the female, to face their reaction. This type of development also allows certain parasitoids to attack hosts developing into endophytes and accessible only at the embryonic stage or in the young larval stages.

Finally, we distinguish the parasitoids according to the extent of the spectrum of their hosts:

• the general attack to various guests, can even belong to several orders of insects. However, in this case there is a certain specificity at the level of the habitat, the parasitized hosts live for example inside galls or stems of certain plants;
• specific parasitoids are said to be specialists; they can only attack a single genus or even a single host species. They are the preferred agents of biological control.

Bibliography

G. Delvare (2020) Order Hymenoptera (Hymenoptera). In: Insects of the World: Biodiversity, Classification, Keys for determining families. Aberlenc HP, Eds Museo-Quae