The observation that nonnative insects and plants could be suppressed by importing missing specialized natural enemies from their homelands led to the first successful method for practical use of biological control. This approach is called classical biological control (because it was the first deliberate, successful application of biological control as a technology), or importation biological control, or simply natural enemy introduction.
After World War II, the chemical industry began the rapid development and marketing of chemicals to control pest insects by poisoning them. Pesticides became very popular and were used on a large scale in the second half of the twentieth century, such that the frequent application of insect-killing poisons to crops became routine. Widespread pesticide use led to a substantial reduction in the level of natural control provided by predators and parasitoids of pest insects, necessitating the further use of pesticides to suppress pest insect populations. However, many pests became resistant to one or more pesticides. This resistance sparked an interest in restoring natural control by reducing the use of insecticides in crops and making their use less damaging to natural enemies by manipulating their timing, placement, or formulation. The effort to restore natural control while making judicious use of pesticides formed the basis of the integrated pest management (IPM) movement in the late 1950s. Efforts to restore and protect natural controls by removing damaging influences such as pesticides are referred to as conservation biological control. A more recent, and less successful, mode of conservation biological control has been the attempt to increase natural enemy numbers by actively providing them with better food sources or habitats. Ideas that have been investigated include a variety of vegetation manipulations in or near crop fields, including ground covers between crop rows and unmowed field borders, where flowering plants provide nectar and pollen for natural enemies.
In the 1970s farmers in Europe producing vegetables in greenhouses were also interested in enhancing natural control of pests such as whiteflies in tomato and cucumber crops, because frequent development of pesticide resistance had rendered pesticides alone unreliable. The desire to reduce or even completely avoid the use of insecticides in tomato and cucumber crops was further stimulated in the 1980s when growers began the practice of placing colonies of bumble bees inside their greenhouses for crop pollination. Inside greenhouses, however, there were very few natural enemies because natural enemy immigration from outdoors is difficult in the indoor, sealed crop environment. Placing whitefly parasitoids, such as Encaria formosa, or predatory mites, such as Phytoseiulus persimilis, in the crop shortly after planting allowed natural control to develop and act on incipient pest populations. Once there was demand by growers for natural enemies, it became possible for specialized businesses (insectaries) to rear and sell natural enemies. This kind of pest management is called augmentative biological control, because the goal is to augment or initiate a natural enemy population.
In some crops, pests that feed directly on the edible part of the plant may not be adequately controlled by natural enemies before damage occurs. For example, cabbageworm larvae feed on cabbage heads and codling moth (Cydia pomonella) larvae burrow into apples. A faster acting form of biological control that can be applied when and where needed may be necessary in these situations. Microbial pesticides can be used in this way and were developed to meet immediate needs without resorting to disruptive chemical pesticides. The most successful of these products are those containing B. thuringiensis, a bacterium that produces toxic proteins that kill insects within a few days of ingestion. There are many subspecies of this bacterium that can be used to control the larvae of some moths, butterflies, beetles, and flies.
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