Understanding and encouraging parasitic wasps and parasitoids in field crops

There are thousands of species of parasitoids. Most species are wasps, but there are also families of parasitoid flies and beetles.

Adult parasitoid wasps are usually small and hard to identify, requiring magnification.

In some cases, it is possible to identify when the host has been parasitised; for example, mummified aphid nymphs and adults, or by changes in egg colouration. This can provide an insight into the proportion of parasitised pests and whether or not chemical treatments are needed.

There is a huge number of parasitoid species. These coexist with hosts in diverse ways.

The larvae of parasitic Hymenoptera are usually endoparasites (the egg is laid on or inside the host and the larvae consume it, usually killing it); such insects are called parasitoids. There are also examples of ectoparasites, which feed outside of the host’s body, and some that parasitise plants (for example, gall wasps). In a few cases, there are species with plant-feeding larvae (for example, Agaonidae and Cynipinae) or with predatory larvae.

Adults feed on nectar, pollen and honeydew, although females of at least 17 families also feed on their hosts, consuming body fluids. The amount and quality of adult food can, in some cases, extend the adult’s lifespan and increase fecundity.

Adult parasitoids often emerge in areas devoid of their hosts. They use chemical and physical parameters to locate their host’s habitat and, eventually, the hosts. This is followed by a process to determine host suitability and acceptance.

Parasitoids overwinter within their host, either coinciding their development with that of the host, or altering the host’s development for their own purpose.

*(abundant species in brackets).

*(abundant species in brackets).

Almost every life stage of each insect species is parasitised by one or more wasps; even parasitic wasps can be parasitised by hyperparasitoids. Typically, each parasitoid species attacks a specific host species or closely related group of species. They have developed venoms to immobilise their host and an ovipositor that can place eggs precisely within or on the host.

Parasitoids are important in the control of most pests, with those from the Ichneumonidae and Braconidae (superfamily Ichneumonoidea) being the most important, comprising over 3,000 species. The other common superfamily in agricultural crops is the Chalcidoidea, most of which are parasitoids, often of eggs.

Parasitoids can control the sex of their offspring and, consequently, the number of fertile females and infertile males. Some species can also produce eggs that continue to divide after laying, producing more than one larva. These attributes give them the ability to respond to their environment by controlling their reproductive output.

In some cases, parasitoids provide sufficient control to prevent pest outbreaks. However, it may be necessary to adopt specific crop management practices to aid them.

Parasitoids are vulnerable to the direct and indirect effects of pesticides. This is because of their small size, dependence on pests as hosts and preference for floral resources that are usually located on the top of the canopy. Only spray when thresholds are exceeded and avoid drift into uncropped areas.

Parasitoids may benefit from supplies of nectar provided by simple open flowers, such as those on buckwheat and umbellifers. Alternative host species may be needed outside the crop growing season.

Parasitoids are weak flyers. To improve parasitism rates for those species that overwinter in the soil (for example, parasitoids of pollen beetle), adopt minimum tillage and plant crops near to the previous year’s location.