What is a parasitoid?

A parasitoid is an organism that has young that develop on or within another organism (the host), eventually killing it. Parasitoids have characteristics of both predators and parasites.

In general, parasitoids share the following features:

• Parasitoids are usually smaller than their selected host.
• Parasitoids are very selective and only attack a particular life stage of one or several closely related species.
• Adult parasitoids are generally either nectar feeders or predators.
• Only female parasitoids are involved in finding and using the host.
• Generally the host does not die until the young are fully grown and ready to become adults.
• Parasitoids can sometimes prevent larval hosts (e.g. caterpillars) from developing, until the parasitoid is fully developed.

Which invertebrates are parasitoids?

Parasitoids include species of wasps, flies (e.g. tachinid flies), beetles and worms (e.g. gordian worms).

What do parasitoids target?

Parasitoids target other invertebrates, although a few (e.g. a species of rove beetle) are known to infect snake eggs . Parasitoids usually target certain groups, for example spider wasps infect spiders and cuckoo wasps infect other wasps. Some even specialise in targeting hosts that are already infected.

How do parasitoids infect their hosts?

Parasitoids infect their hosts using one of the following three methods:

1. Eggs are laid in, on or near host eggs and young.
2. Eggs are laid on plant species visited by the host.
3. Eggs are laid in, on or near adult hosts, which have been stung and paralysed.

Parasitoid larvae hatch and grow by feeding on the bodily fluids or the internal organs of the host (usually the non-vital parts first). The host dies when the fluid has been sucked dry or its internal organs cease to function.

• Predators and parasitoids are extremely important in keeping the large populations of plant-eating insects in check. Research has shown that in response to an attack by insects, some plants actively attract predators and parasitoids by releasing chemicals.
• Predators and parasitoids control populations of pest species. Many species of predators and parasitoids are reared by laboratories to control pests of economically important crops.
• Parasites can control pest animal populations by spreading diseases. For example, in Australia parasites such as mosquitos and fleas helped spread myxomytosis and the calicivirus that have been successful in reducing European rabbit numbers.
• Predators, parasites and parasitoids influence nutrient recycling by contributing dead animals and faeces to the decomposition zone.

References

Blouin-Demers G. and Weatherhead, P.J. 2000. A novel association between a beetle and a snake: Parasitism of Elaphe obsoleta by Nicrophorus pustulatus. Ecoscience 7(4): 395-397

Feener Jr. D. H. 1997. Diptera as parasitoids. Annual Review of Entomology. 42: 73-97.

Gullan, P.J. and Cranston, P.S. 2004. Chapter 12 pp. 324-351 in The insects: an outline of entomology. Blackwell Publ.

Lemckert, F. 2000. An observation of predation on an adult toadlet (Uperoleia laevigata) by a dragonfly nymph, Herpetofauna (Sydney) 30(1): 50.

New, T.R. 1991. Insects as predators. UNSW Press, Sydney.

Robertson, J G M. 1989. Predation by a carabid beetle on Australian frogs, Herpetological Review 20(1) 1989: 7-8

Thompson W M. 1996. Plants and Parasitoids of Herbivorous Insects: A Mutually Beneficial Relationship.

Walter D.E. and Proctor H.C. 1999. Mites: Ecology, evolution and behaviour. UNSW Press, Sydney.