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Approximate total insect species in the United States and Canada3 86,300

1Additional orders include Grylloblattodea (rock crawlers), Zoraptera (angel insects), and Mecoptera (scorpionflies).

2An approximation of the number of species in this order found north of Mexico. 3Authorities estimate that 10-25% of all species are still unknown to science.

Approximate total insect species in the United States and Canada3 86,300

1Additional orders include Grylloblattodea (rock crawlers), Zoraptera (angel insects), and Mecoptera (scorpionflies).

2An approximation of the number of species in this order found north of Mexico. 3Authorities estimate that 10-25% of all species are still unknown to science.

The majority of parasites (also called parasitoids) are very small wasps in several families of the order Hymenoptera. Another important group is tachinid flies (order Diptera,family Tachinidae). Parasites are about the same size as their hosts, or smaller. All parasites undergo complete metamorphosis. In most cases, the adult female lays eggs on, within, or near a host insect of the appropriate stage.When the egg hatches,the parasite larva consumes its host. Each parasite larva can attack only one host. A parasite larva that feeds and develops within its host is called an endoparasite (figure 7), whereas one that feeds while attached to the outside of its host is called an ectoparasite (figure 8). Endoparasites are much more common than ectopara-sites.When a single parasite develops within a single host, it is called a solitary parasite; when two or more individuals of the same species can develop within one host, the species is said to be a gregarious parasite.

Figure 7. Endoparasites develop inside the body of the host insect.

Different parasites attack different stages of the host.There are egg parasites, nymphal parasites, larval parasites, pupal parasites, and adult parasites. Parasites are generally host specific, attacking only a single insect species or a group of closely related species. Most types of insects are attacked by one or more species of parasite. In fact, many parasites themselves are parasitized by other species, called hyperparasites.

Section 2 contains more details on specific predatory and parasitic insects. Many of these insects are covered in even greater detail in the companion publication Biological Control of Insects and Mites (NCR 481), which includes descriptions and color photographs of many types of natural enemies.

Pathogens are microorganisms that cause diseases.The most common types of insect pathogens are bacteria,fungi, nematodes, viruses, and protozoans. Some insect diseases are highly lethal, killing a large portion of the insect population. Other diseases are less lethal but may retard insect development, shorten the insect's life, or prevent reproduction. Most insect pathogens are pathogenic only to insects, frequently to very small groups of closely related species. Microorganisms that are pathogenic to a specific pest are often harmless to other insects, including predators and parasites.

Often, specific conditions are necessary for an insect pathogen to reduce a pest population effectively.These conditions vary with the type of pathogen and the host. Most fungal pathogens are only effective during periods of relatively high humidity, because it is only under these conditions that the fungi will produce spores and the spores will germinate. Viral pathogens are often most effective when the host population is very high, which facilitates spread from individual to individual within the population. Bacterial pathogens must be ingested to be effective, and therefore do not usually kill sucking insects such as aphids and whiteflies. Nematodes require a thin film of moisture on which to move, so they are not effective under dry conditions.

Figure 8. Ectoparasites feed externally, attached to the outside of the host.

Figure 8. Ectoparasites feed externally, attached to the outside of the host.

Because of these characteristics, the degree of control exerted by naturally occurring pathogens is often unpredictable. However, pest managers should be aware of the symptoms of insect diseases and should be able to assess their impacts on pest populations.

A few insect pathogens are available commercially.You can spray these on the crop using conventional pesticide spray equipment.When used in this fashion, insect pathogens are called microbial insecticides. These can be very effective at the time of application, but they usually do not persist in the greenhouse environment.Their advantages include wide availability from commercial sources, safety to humans, and safety to nontarget organisms, including beneficial insects. However, microbial insecticides have some disadvantages: they may be more costly to use than traditional pesticides; multiple applications may be necessary because residual activity is short; and if other types of insect pests are present, you may still need to suppress these with other controls. Microbial insecticides containing the bacterium Bacillus thuringiensis are discussed in Section 2 of this publication. More information on insect pathogens and microbial control can be found in the companion publication Biological Control of Insects and Mites (NCR 481) and in Alternatives in Insect Management (NCR 401).

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