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The BasicsAn Overview of Biological Control History and ApplicationMark S. Hoddle, Department of Entomology, University of California, Riverside, CA 92521, USA History and Theory of Biological Control Biological control is the intentional use by humans of parasitoid, predator, pathogen, antagonist, or competitor populations to suppress a pest population, thereby making the pest less abundant and damaging than it would be in the absence of these organisms. Most applied biological control programs simply involve the addition of new species that fill an unoccupied niche (i.e., use the pest as a resource). Often, very few native natural enemies attack an adventive pest, and those that do are native generalist predators or herbivores that utilize other resources in the environment. Introductions of specialized natural enemies from the home range of the pest fill niches not occupied by local generalists. In most cases, native organisms in the upper trophic level are not adversely affected by the introduction of specialized natural enemies as they can utilize other resources for survival. With successful biological control programs, both the pest and natural enemy decline in abundance through time. Biological control has been used as a management tool for control of crop and forest pests, and for the restoration of natural systems affected by adventive pests. The development of the concept of biological control occurred with the gradual accumulation of biological and ecological knowledge of the natural world by humans. The first orchestrated biological control programs were run by Chinese and Yemenite farmers who encouraged native predatory ants to protect citrus and date trees from insect pests. By the 1700s, observations on the peculiar and mysterious production of flies and wasps from the larvae and pupae of various Lepidoptera (i.e., moths and butterflies) fostered an understanding of insect parasitism by hymenopterous and dipteran parasitoids. Investigations of diseases affecting economically important insects like silkworms were burgeoning at this time also. By the 1800s, the accrued body of knowledge on insect pathogen transmission and disease control, and observations of natural epizootics had scientists in Europe and the USA promoting the idea of mass rearing fungi for release against agricultural pests. The first field releases of mass reared fungi for control of weevils attacking sugar beet occurred in Russia in 1884. Around 1855, suggestions were being made by U.S. entomologists that insects be imported from Europe for the control of European weeds that were taking over valuable pasture land and were observed to lack herbivorous arthropods that were known to feed on these plants in Europe. By the 1880s biological and ecological knowledge of natural enemies (i.e., predators, parasitoids, and pathogens) attacking insects and weeds helped form the concept of “bug vs. bug” or “the parasite method” for pest management. Organized biological control programs (Prof. Harry S. Smith, an entomologist at University of California, Riverside, coined the phrase “biological control” in 1919) sponsored by fledgling institutions such as the U.S. Department of Agriculture (established in 1862) were launched. The first of these programs, the biological control campaign against the cottony cushion scale, Icerya purchasi Maskell, a devastating pest of California grown citrus, was launched in 1868 and was a spectacular success. This program is a premier example of pest regulation by upper trophic level organisms. Icerya purchasi, an insect native to Australia and New Zealand, had been accidentally imported into California on acacia trees in 1868. It rapidly spread to citrus trees causing massive crop losses and tree death. The non-pest status of I. purchasi in Australia supported the idea that in its home country this pest was controlled by natural enemies and these antagonists could provide similar pest suppression if they could be established in California. Foreign exploration (i.e., prospecting for natural enemies that co-evolved with the pest in its area of origin) in Australia and New Zealand for natural enemies of I. purchasi resulted in the importation of a predatory beetle (Rhodalia cardinalis [Mulsant]) and a parasitic fly (Cryptochaetum iceryae [Williston]). By 1888-89, I. purchasi was permanently reduced to densities no longer causing economic damage and these natural enemies were subsequently distributed to other countries with cottony cushion scale problems. Biological control of I. purchasi put California on an economic and agricultural trajectory that led to unprecedented prosperity, and pest control with natural enemies was enthusiastically endorsed by California citrus growers, who backed research programs with commodity funds. If the citrus industry had failed in California because of I. purchasi, this U.S. state would have had a radically different history and economic development. The cottony cushion scale project in California is considered the first scientific and institutionally backed biological control program and illustrates the concepts of: 1) adventive pest identification; 2) Foreign exploration for specialized natural enemies in the pest’s home range; 3) Importation and mass rearing of natural enemies; and 4) establishment, re-distribution, and impact monitoring of imported biological control agents. These basic practices, although substantially refined, still form the foundation upon which current biological control programs are built. The addition of exotic natural enemies to the environment has three impacts on the target system: 1) The number and function of food web links that connect the pest to other members of the community are permanently changed; 2) Natural enemies can radically alter pest densities and population dynamics; and 3) reduction in pest density by natural enemies changes the community structure of the affected system. In successful biological control programs, reduction of pest densities and recovery of adversely affected flora or fauna leads towards a system with better ecological balance and community structure. Following establishment, successful natural enemies can provide enduring pest control, they can replicate and disperse without continued human management, they can persist when pest populations are stabilized at very low densities, and the technology is often relatively cheap to implement. Successful biological control programs against noxious insects, mites, weeds (aquatic and terrestrial), plant pathogens, and vertebrates are extensive, and well-documented examples exist (see Recommended Background Reading). Types of Biological Control Four types of biological control are recognized: (1) Classical biological control; (2) Augmentative biological control; (3) Inundative biological control, and (4) conservation biological control. Classical Biological Control: This practice involves prospecting for natural enemies in the home range of an exotic pest. In this instance the target pest is not native to the new area it has invaded. Consequently, the pest has left behind the guild of natural enemies that utilized it as a resource in it home range and in the new environment without specialized natural enemies the population of the invader reaches damaging levels. The classical approach involves the prospecting for, importation, safety testing, release and impact assessment of exotic natural enemies for control of an exotic pest that has extended its home range. Augmentative Biological Control: When natural enemies are missing from an agricultural setting (e.g., greenhouses or row crops immediately after planting), or are too scarce to provide control, their numbers can be increased by making releases of agents purchased from insectaries. This approach is termed augmentative biological control and either inoculative or inundative approaches are used. Inoculative augmentative releases are those in which small numbers of natural enemies are introduced early in the crop production cycle and subsequent control occurs as natural enemies reproduce and feed on the pest. This approach has been used to control T. vaporariorum on greenhouse grown tomatoes with E. formosa. Inundative Biological Control: Inundative augmentation or mass releases of insectary reared natural enemies are used when natural enemy reproduction is expected to be insufficient to suppress pest population growth, and control is achieved by released natural enemies and to a limited extent their offspring. Inundative biological control is the approach used for rapid pest suppression on greenhouse grown ornamentals or vegetable because of low tolerances for aesthetic damage and arthropod contamination of the saleable product. Preventative inundative natural enemy releases are started early in the growing season when pest densities are well below those that cause economic damage, and pest population growth is prevented from reaching damaging levels. Inundative natural enemy releases can not be used in “curative” manner to reduce pest populations to non-damaging levels once they have reached economically injurious densities because natural enemies can not immediately control pest populations in a manner similar to insecticides. Conservation Biological Control: With this approach, resident natural enemy populations in agricultural settings can be preserved and enhanced with simple cultural techniques. Such practices may involve provisioning natural enemies with flowers that produce nectar. Nectar and other carbohydrate sources such as honeydew from softscales can significantly enhance the longevity and fecundity of predacious beetles, lacewings, and parasitic wasps thereby maximizing their impact against pest populations. Alternatively, refuges in the form of unsprayed plants, or deliberate provision of alternative hosts that feed on deliberately sown and managed grasses, trap crops, windbreaks, or hedgerows can significantly enhance and maintain natural enemies in and around crops. Recommended Background Reading Bellows, T.S. Jr and Fisher, T.W. (1999). Eds: Handbook of Biological Control. Academic Press, San Diego, 1046 pp. DeBach, P. (1964). Eds: Biological Control of Insect Pests and Weeds. Reinhold Publishing Corporation, New York, 844 pp. DeBach, P. (1974). Biological Control by Natural Enemies. Cambridge University Press, Cambridge, 323 pp. Gurr, G. and Wratten, S. (2000). Eds: Biological Control: Measures of Success. Kluwer Academic Publishers, Dordrecht, 429 pp. Van Driesche, R.G. and Bellows, T.S. Jr. (1993). Eds: Steps in Classical Arthropod Biological Control. Thomas Say Publications in Entomology: Proceedings, Entomological Society of America, Lanham, 88 pp. Van Driesche, R.G. and Bellows, T.S. Jr. (1996). Biological Control. Chapman and Hall, New York, 539 pp. |
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