There is a large number, and a diverse range, of species of parasitic wasp and some parasitic fly species which are natural enemies of apple and/or pear pests. They are commonly known as parasitoids, a term referring to any insect whose larvae develop by feeding on the bodies of other arthropods, usually insects. The larva or larvae eventually kill their host which provides all the parasite needs for its development.
Many parasitoids play only a minor part in regulating populations of their host. However, some species are important natural regulators of populations of their host. Insufficient attention is devoted to them.
- Adult parasitoids tend to attack a particular life stage or range of life stages of their host. Parasitoids are classified by the life stage of the host which is attacked (egg, larval, pupal or adult parasitoids).
- At oviposition the host is either killed or permanently paralysed or the host is not or only temporarily paralysed and continues feeding whilst the parasitoid remains quiescent as an egg or first instar larva until the host is big enough to support full development.
- Parasitoids are either solitary (one egg per host) or gregarious (two or more eggs per host).
Practically all apple and pear pest species are host to one or more parasitoids and it is probable that there are many unknown parasitoids. Some parasitoids are highly specialised attacking one or small group of closely related pest species. Others have a broader host range. They themselves are host to hyperparasitoids.
Exploitation of existing populations of parasitoids is an important task in Integrated Pest Management. The orchard habitat has to be made as suitable as possible for the particular species.
- For highly specialised parasitoids, adequate populations of the pest may have to be present. For this reason, exploitation of parasitoids is a more practical prospect for pest species which do not attack the fruit directly and which do not cause serious damage at low population densities and thus can be tolerated at low to moderate levels.
- Examples are such pests where parasitoids could or do act as key natural enemies include leafhoppers, leaf miners and apple leaf midge.
- Other important resources which contribute to the success of a parasitoid are suitable overwintering or shelter sites and or alternative hosts or food sources.
- Sensitivity to pesticides is also an important factor. Many species are likely to be highly sensitive to broad-spectrum insecticides and ways of avoiding harmful affects are crucial to the success of exploitation.
- Adult parasitoids are often the life stage most exposed to insecticides.
- Another option is the introduction of parasitoids into the orchard. This may be done by transferring plant material infested with parasitised insects from another orchard or by using parasites reared in mass culture by biological suppliers. However, this is usually uneconomic.