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PARASITES

Parasites may be insects (wasps, flies, and beetles), mites, or nematodes. Parasites are usually free-living adults which lay eggs on or within a living host which is larger and/or stronger than themselves. The immature(s) gradually feed on host tissues until the host is killed. Parasitic immatures can complete development in one host. Because they are extremely specialized, they often only attack one or a few closely related species of insect. They DO NOT harm humans or their pets.

Some parasites have extremely complex and wondrous life cycles. For example, a eucharitid wasp that is a parasite of ants lays her eggs on the leaves of trees. The eggs hatch into a mobile immature that is able to crawl about on the leaf surface. In the spring, worker ants climb into the trees in search of aphids and other insects for food. The parasite larva attaches itself to any worker ant that comes close and, when the worker ant returns to its nest, it carries along the parasite. Once in the nest, the parasite drops off and attaches itself to a larval ant. The wasp larva feeds on the ant larva, eventually killing the ant. After emergence from the pupa, the adult wasp flies out of the ant nest to lay her eggs on leaves once more.

Other types of parasitic wasps control aphids. The female lays an egg inside an aphid. The activity of the immature wasp within the aphid causes it to form a stiff, immobile form called a "mummy." Homeowners should leave mummies alone, and not wash them off the plant, because the new adult wasp will emerge and attack more aphids.

Gardeners may encounter caterpillars, such as hornworms, suddenly decorated with white egg-like structures. These are actually wasp pupae within silken cocoons. The caterpillar should be left alone, because it will soon die, and the wasps will emerge and attack more caterpillars.

Sometimes homeowners may find wasps with long projections or what they may call "stingers" at the end to their abdomen. These are members of another group of parasitic wasps called "ichneumonids." The tail is a long tube used for laying eggs, or "ovipositor." They use the ovipositor to lay eggs in insect larvae found feeding deep within plants, or even wood. They are completely harmless to humans.

Bee flies are examples of flies that are parasites of other insects as larvae. The adult flies mimic bees and may be mistaken for the predatory flower flies discussed in the previous section. The larvae attack the eggs or immatures of grasshoppers, beetles, moths, bees, and wasps.

Nematodes are hair-like worms found naturally in the soil. Many are microscopic in size and vary in life-style. The parasitic forms generally feed on insects that are found in the soil during one or more stages of their life cycle, such as white grubs, root maggots or weevils. Some forms are available commercially for insect control.

The advantages of using a parasite to control an insect is that they tend to be very specific, and attack only one or a few closely related species. Some have short life spans, and can build up to high numbers quickly. An example of a successful use of parasites is to control flies in dairy barns. The parasitic wasp lays an egg in the pupa of the fly, and thus prevents emergence of the adult. Dairy farmers must change their management practices in order to maintain the wasps, however. Because the wasps emerge from the fly pupae, the farmer must be aware of where flies pupate and reduce insecticide use in those areas. Fly predators may be found in those areas as well.

DISEASES

All the different disease organisms, including viruses, rickettsia, bacteria, protozoa and fungi, attack insects. Some disease organisms have been grown commercially, and sold over-the-counter for insect control. A classic example is the bacteria, Bacillus thuringiensis (also called Bt). This bacteria produces a toxin which disrupts the gut of the insect that eats it. Commercial preparations are available from several nurseries and garden supply companies for the control of various pests with chewing mouthparts, especially Lepidoptera. A related bacteria, Bacillus popillae causes milky spore disease of Japanese beetle larvae and other white grubs. Unfortunately, it seems to be adversely effected by the high temperatures found in Arizona.

Although they have yet to be used commercially, insect viruses can control pest insects successfully. They are passed from insect to insect in much the same way as between humans, but are specific to insects.

Fungal pathogens are also known to attack insects, but are often more difficult to grow commercially than bacteria. Fungi generally require high relative humidity to germinate and to infect the target insect, but do not need to be consumed to be effective.

CONCLUSION

Indirect evidence suggests biological control agents may be extremely important for keeping pest insects in check. When an insect is introduced into a new area and leaves its natural enemies behind, it often will become a serious pest. There are many examples of insects that are of little or no importance in their native land, that devastate our crops when introduced here. One such example is the Russian wheat aphid. In Eastern Europe, where it is native, the Russian wheat aphid is of little concern. But once it was accidently introduced to North America, populations exploded and it became a major pest in the mid-west and western wheat producing areas. In order to control the outbreak, scientists visited areas where the pest was native in search of predators and parasites that fed on the aphid. These insects were gathered and shipped back to the United States where, after a period of quarantine, they have been released.

There are disadvantages of using solely biological control, however. First of all, the insect pests do not necessarily disappear quickly. We have begun to expect instant results in our world of microwave ovens and one-hour-photo shops, but often biological control agents require weeks, months or sometimes even years to bring populations of pests under control. And a "good" predator or parasite never completely wipes out its host or prey because it would go out of business. The idea is to hold pest numbers below the level of damage that can be tolerated by the consumer or gardener. Biological control agents are often successful at that level.