In the mid-1880s, southern California's developing citrus industry experienced devastating losses from an introduced pest, cottony cushion scale. Growers tried every available chemical control known at the time, even fumigation with hydrogen cyanide, but nothing provided sufficient control; many growers removed their citrus groves because the damage was so serious. After determining that the scale insect was native to Australia and New Zealand, the U.S.D.A. sent an entomologist to that area to look for effective natural enemies. The entomologist found a small lady beetle, the vedalia beetle, which he sent to California. It rapidly reproduced in infested citrus groves and brought the cottony cushion scale under complete and lasting control. This was the first highly successful case of controlling an alien pest by introducing its natural enemies from a foreign land, a technique now known as classical biological control.
Agents of biological control (natural enemies) of insects include predators, parasitic insects, and insect pathogens. Predators may be insects or other insectivorous animals, each of which consumes many insect prey during its lifetime. Predators are often large, active, and/or conspicuous in their behavior, and are therefore more readily recognized than are parasites and pathogens.
Parasites (also called parasitoids) of insects are other insects which lay their eggs in or on the host insect. When the parasite egg hatches, the young parasite larva feeds on the host (the pest) and kills it. Usually that one host is sufficient to feed the immature parasite until it becomes an adult. Many parasites are very specific to the type of host insect they can attack, and they are not harmful to humans. Although insect parasites are very common, they are not well known because of their small size. One of the smallest, Trichogramma, is only about the size of the period at the end of this sentence.
Insects, like other animals, are subject to attack by disease organisms. Insect pathogens include viruses, bacteria, fungi, nematodes, and other microorganisms that cause insect diseases. Disease epidemics among insects are not commonly encountered in nature except when insect populations are very large or when environmental conditions favor the growth of the disease organism. Nevertheless, insect pathogens are very important in the constant suppression of pest populations. Also, certain insect pathogens have been very successfully manipulated to achieve biological control of specific pests. For example, different strains of the bacterium Bacillus thuringiensis, commonly known as "Bt", are marketed to control many insects including various caterpillars such as cabbage loopers and gypsy moth larvae, mosquitoes, and Colorado potato beetles. Many insect pathogens attack only one species or a limited group of insects and therefore are unlikely to harm non-target species such as beneficial insects, humans, livestock, wildlife, or plants.
There are three broad approaches to biological control. Importation of natural enemies is conducted by federal and state agencies to find better beneficial natural enemies and permanently establish them into new areas. Conservation of natural enemies improves the effectiveness of natural enemies through farming and gardening practices that provide necessary resources for their survival and protect them from toxins and other adverse conditions. Augmentation of natural enemies temporarily increases the numbers of natural enemies through periodic releases, thereby increasing the overall numbers of natural enemies and improving biological control.
Mahr, D. L. and N. M. Ridgway. 1993. Biological control of insects and mites: An introduction to beneficial
natural enemies and their use in pest management. NCR Publ. No. 481, 91 pp.
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