Predators and Parasitoids
Comparing parasitoids and predators
Whereas predators immediately kill or disable their prey, parasitoids kill pests more slowly. Some pests are paralyzed, while others may continue to feed or even lay eggs before succumbing to the attack. Parasitoids, however, often complete their life cycle much more quickly and increase their numbers much faster than many predators. Parasitoids can be the dominant and most effective natural enemies of some pest insects, but their presence may not be obvious (unless you dissect or rear samples of pest insects to see if any adult parasitoids emerge).
Parasitoids can be parasitized by other parasitoids. This phenomenon, known as hyperparasitism, is a natural occurrence, can be common, and may reduce the effectiveness of some beneficial species. Little can be done to manage hyperparasitism.
Parasitoids are often more susceptible to chemical insecticides than predators. Adult parasitoids are usually more susceptible than their hosts (pests). Immature parasitoids, especially if protected within the egg of their host or in their own cocoon, may tolerate pesticides better than adults, but immature parasitoids will usually die if their host is killed.
Insect parasitoids have an immature life stage (for example, eggs or larvae) that develops on or within a single insect host, ultimately killing the host, hence the value of parasitoids as natural enemies. Adult parasitoids are free-living and may be predaceous. Parasitoids are often called parasites, but the term parasitoid is more technically correct. Most beneficial insect parasitoids are wasps or flies, although some rove beetles (see Predators) and other insects may have life stages that are parasitoids.
Most parasitoids only attack a particular life stage of one or several closely-related insect species. The immature parasitoid develops on or within a pest, feeding on body fluids and organs, eventually leaving the host when it’s ready to turn into an adult (or shortly before). The life cycle and reproductive habits of beneficial parasitoids can be complex. In some species, only one parasitoid will develop in or on each pest while, in others, hundreds of young parasitoids may develop within the pest host. Overwintering habits may also vary. Female parasitoids may also kill many pests by direct feeding on the pest eggs and immatures.
Major characteristics of insect parasitoids:
- specialized in their choice of host (pest)
- smaller than host
- only the female searches for host (pest)
- different parasitoid species can attack different life stages of host (pest)
- parasitoid eggs are usually laid in, on, or near host (pest)
- immature parasitoids remain on or in host (host); adults are free-living, mobile, and may be predaceous
- immature parasitoids almost always kill host (pest)
Predators, such as lady beetles and lacewings, are mainly free-living species that consume a large number of prey during their lifetime. Predators of insects and mites include beetles, true bugs, lacewings, flies, midges, spiders, wasps, and predatory mites. Insect predators can be found anywhere on a plant, including below ground level. Some are extremely useful natural enemies of insect pests. Some predators are specialized in their choice of prey. Others are generalists, preying on other beneficial insects as well as pests.
Insect and Arthropod Predators
Insect predators can be found in almost all agricultural and natural habitats. Each group may have a different life cycle and habits. Although the life history of some common predators is well studied, information on the biology and relative importance of many predatory arthropods is lacking.
Major characteristics of arthropod predators:
- adults and immatures are often generalists rather than specialists
- they generally are larger than their prey (pests)
- they kill or consume many prey (pests)
- males, females, immatures, and adults may be predatory
- they attack immature and adult prey (pests)
Most beneficial predators will consume many pest insects during their life, but some are more effective at controlling pests than others. Some species may play an important role in the suppression of some pests. Others may provide good late season control, but appear too late to suppress the early season pest population. Many beneficial species may have only a minor impact by themselves but contribute to overall pest mortality. Often too, the role of the beneficial predators has not been adequately studied.
Surveys of agricultural systems give an indication of the potential number and diversity of predators in a crop. For example, over 600 species of predators in 45 families of insects and 23 families of spiders and mites have been recorded in Arkansas cotton. Eighteen species of predatory insects (not including spiders and mites) have been found in potatoes in the northeastern United States. There may be thousands of predators per acre, in addition to many parasitoids. Although the impact of any one species of natural enemy may be minor, the combined impact of predators, parasitoids, and insect pathogens can be considerable.
Hoffmann, M.P. and Frodsham, A.C. (1993) Natural Enemies of Vegetable Insect Pests. Cooperative Extension, Cornell University, Ithaca, NY. 63 pp.