Methods have been developed to rear a variety of species of predators and parasitoids at commercial levels. Augmentative biological control is based on the user purchasing and releasing the natural enemies needed for his crop. This approach to pest control began in green house grown tomatoes with Encarsia formosa Gahan, a parasitoid of the greenhouse whitefly (Trialeurodes vapor ariorum [Westwood]), which was first reared commercially by English growers in the 1920s. Modern augmentative biological control began in the 1970s when Dutch greenhouse tomato growers revived E. formosa rear ing as a commercial activity because whiteflies in their greenhouses had developed pesticide resistance. From 1970 to 2006, the number of commercial insectaries pro ducing parasitoids and predators for pest control grew to several dozen firms, which collectively produce about 100 species of natural enemies for use in greenhouses or related facilities. Encarsia formosa and the predatory mite Phytoseiulus persimilis Athias Henriot make up most of the sales. Natural enemy releases are used in greenhouses, plant conservatories, mushroom houses, and animal hold ing buildings such as dairies, hog rearing facilities, poultry barns, and zoos.
Outdoor releases of several species of predators and parasitoids developed independently of the indoor applications described above and mainly focused on egg parasitoids in the genus Trichogramma (Hymenoptera: Trichogrammatidae). These parasitoids are released to suppress pest weevils and caterpillars in cotton, corn and sugarcane, especially in China, Russia, and tropical sugar producing countries. Predators of mealybugs for release on citrus crops in parts of California have been reared by a growers' cooperative since 1926. Several species of predatory phytoseiid mites are released for control of pest spider mites in strawber ries, outdoor foliage plant production, and other high value crops.
There are two approaches to augmentative biological control: inoculative and inundative releases. Most indoor releases of natural enemies are inoculative, consisting of a small release early in the crop cycle that is intended to suppress the pest after the natural enemy's numbers have increased naturally through reproduction in the crop. Cost of this approach is minimized because smaller num bers of the natural enemy are needed. In contrast, inundative biological control consists of frequent, large releases throughout the crop cycle, with control coming from the attacks of the released individuals. Little repro duction in the crop is expected. Because much higher numbers are released, only natural enemies with very low production costs are economical for use in this way. This is the approach behind Trichogramma releases, given that these parasitoids can be reared very inexpensively.
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