Predators, parasites and parasitoids

Many animals are predators, parasites or parasitoids, using other animals as food. They use a great diversity of hunting strategies and behaviours to capture or feed on their prey. Invertebrate predators, parasites and parasitoids play an important role in keeping many animal populations under control.

A predator is an organism that captures and eats another (the prey). This act is called predation.

In general, predators share the following features:

• They are usually larger than their prey, or overwhelm their prey by attacking in large numbers like ants.
• Most do not have specific prey and feed on a wide range of animals which, in some cases, they consume in large numbers. Lady beetles for example, can devour several hundred aphids in a day.
• Some are specific, targeting a particular prey group. For example. feather-legged assassin bugs are known to feed only on ants.
• Death to the victim is usually immediately after capture.
• Adults or young may be predatory. In a number of cases the young are predatory but the adults are not. Adults of alderflies or dobsonflies do not eat but their aquatic larvae are fierce predators.

Which invertebrates are predators?

Predation in insects is common. The adults and young of groups such as mantids, dragonflies and damselflies, lacewings, scorpionflies and alderflies are entirely predacious. Even some moths, a group dominated by plant-feeding species, will bear offspring that are voracious predators.

There are also many non-insect invertebrates that are predators. The main group being the spiders, which are specialist predators. Spiders exploit an immense diversity of habitats and have developed some of the most sophisticated trapping and hunting techniques of any animal group. Other predatory invertebrates include scorpions, centipedes, predatory mites, predatory snails and slugs, nematodes and planarian worms.

What do invertebrate predators feed on?

Invertebrate predators usually feed on other invertebrates, however some feed on fish, frogs, small mammals and birds, in the case of certain large spiders and centipedes. Generally, predators kill to feed themselves but in some cases they hunt for their offspring. Wasps and ants primarily feed on plant liquids as adults but their offspring require solid foods. The adults capture prey and deliver it to their young.

Invertebrate predators usually eat their prey while it is still alive. They do this either with biting and chewing mouthparts or by piercing the body and sucking out the victim's internal fluids. The latter may also involve injecting a cocktail of salivary secretions that act to paralyse the prey and liquefy its tissue.

How do invertebrates capture their prey?

The methods by which invertebrate predators catch prey are incredibly diverse and in some cases very complex.

Invertebrate predators are either passive or active:

• Passive predators tend to sit and wait for prey to come close. Mantids wait poised for long periods of time for prey to come within the grasp of their powerful forelegs.
• Active predators are those that search or hunt for their prey. Dragonflies and Robber flies use speed and agility to pluck prey from the air, while some spiders construct casting nets that are thrown over prey as they fly past.

Some active predators specialise in capturing and eating other predators. The Fringed Jumping Spider specialises in eating other spiders. It will vibrate the web of the intended prey in an attempt to imitate a victim caught in it. The web-building spider comes to investigate what they believe is food, to find they themselves are on the menu.

A parasite is an organism that lives at the expense of another organism - the host.

In general, parasites share the following features:

• Parasites are usually smaller than their host.
• Parasites use both invertebrate and vertebrate hosts.
• Adult parasites may live on the host (e.g. lice), in the host (e.g. tapeworms) or feed on a host occasionally (e.g. mosquitoes).
• Parasites generally do not kill the host but may harm the host indirectly by spreading pathogens. This may affect the host's behaviour, metabolism or its reproductive activity.
• Many parasites have hooks, claws or suckers to attach to their host.
• Generally parasites have either a sucker (e.g. leeches) or piercing and sucking type mouthparts (e.g. fleas) for feeding.
• Both adults and young can be parasitic. In some cases the young are parasites but the adult is not.

Which invertebrates are parasites?

Many invertebrate groups have parasitic members. Some well known parasites are fleas, ticks, parasitic mites, leeches, worms (e.g. round worms) and some parasitic flies (e.g. mosquitoes). Stylops are parasites of wasps, bees and bugs. The female is larvae-like and spends its entire lifecycle within the host. The winged, free-living males locate females by scent and mate with the small portion of the female protruding from the host.

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.