Biological control is the deliberate attempt by people to make practical use of the capacity of predation, parasitism, herbivory, and disease to restrain the growth of plant and animal populations. The ability to make practical use of these processes depends on an understanding of how pest population densities are controlled by natural enemies. Biological control also requires detailed knowledge of the biology of pests and their key natural enemies, because such knowledge often provides the means for their practical manipulation.
Predation by vertebrates on other vertebrates has long been part of human knowledge. Predation as a force affecting pest insects was recognized when people first saw individual acts of predation taking place on their crops. A close observer of an aphid colony, for example, cannot help but see the predatory action of ladybird beetles and cecidomyiid midge larvae as they devour their aphid prey. The predaceous effect of some species of ants on pest insects associated with citrus was recognized thousands of years ago by farmers in Yemen and China, who used the knowledge to suppress these pests by moving ant colonies into new orchards. In Europe during the Renaissance, the emergence of natural history as a subject worthy of observation and thought led keen-eyed naturalists to arrive at similar findings. The father of the classification of plants and animals, Caralus Linnaeus, observed in 1752 that "Every insect has its predator which follows and destroys it. Such predatory insects should be caught and used for disinfesting crop-plants." By the early 1800s, such observations led naturalists such as Erasmus Darwin and American entomologists such as Asa Fitch to suggest that predaceous insects should be used to suppress pest insects by making releases of the predators in places where they were lacking. These suggestions formed the fundamental basis for the modern use of augmentative biological control in greenhouses, vegetable production, and various outdoor crops.
The action of insect parasitoids on their hosts has no direct analogue among animals that people could easily observe before the invention of magnifying lenses. Consequently, the concept of parasitism took longer to become recognized. Because many parasitoids feed inside their hosts, their presence was not easily recognized, and the detection of parasitoids required that insects be either reared or dissected. By the 1600s, European naturalists were noticing the occurrence of parasitoids. Aldrovandi, for example, in 1602, reared tiny parasitoid wasps from the pupae of a nymphalid butterfly and recorded what he saw in woodcut print. Because he misunderstood the process, however, he wrongly concluded that the tiny wasps were an alternate adult form to the usual butterfly. The first person to publish a correct interpretation of insect parasitism was the English physician Martin Lister, who in 1685 noted that the ichneumon wasps seen emerging from a caterpillar were a distinct kind of insect that originated from eggs inserted into the caterpillars. No one thought of any way to make practical use of such parasitoids, however, until 1855 when Asa Fitch proposed the importation of parasitoids from Europe to America to help suppress a nonnative invasive pest of wheat, the wheat midge Sitodiplosis mosellana. Fitch's ideas provided a clear plan for the modern practice of biological control through natural enemy importation, but they were not acted on for nearly 30 years. The first importations of exotic species of parasitoids between continents occurred in the 1880s, when Cotesia glomerata was brought to the United States from Europe to suppress Pieris rapae, a European pest of cabbage that had invaded North America in 1860.
The study of the diseases of insects started not for purposes of killing pest insects, but rather for protecting economically important species such as silkworms and honey bees. In the mid- to late nineteenth century, microscopes made it possible to observe bacteria and microscopic fungi, and the study of these organisms as pathogens of domesticated insects initiated insect pathology. The infectious nature of insect diseases was first demonstrated by Agostino Bassi of Italy, who in 1835 studied a fungal disease of silkworm larvae caused by the fungus Beauveria bassiana. Louis Pasteur continued work on silkworm diseases in France in the 1860s. The first attempt to use pathogens to destroy pest insects was made in 1884 by the Russian entomologist Elie Metchnikoff, who reared Metarhizium anisopliae, a fungal pathogen, and attempted to suppress the sugar beet curculio, Cleonus punctiventris, with application of the fungal spores. In 1911 the German scientist Berliner observed a bacterial disease of larvae of the flour moth, Anagasta kuehniella, and by 1938 this bacterium, Bacillus thuringiensis, was being marketed as a microbial pesticide for control of some species of caterpillars. These early efforts established the concepts that insects were subject to infectious diseases and that the causative agents could be reared in quantity artificially. Technical methods to use reared pathogens to reliably infect insects in crops, hence to achieve biological control, came later.
Although humans have known for millennia that insects damage and even kill plants, the idea that specialized herbivorous insects could be manipulated to suppress plants considered to be weeds is a relatively new concept. The first person to suggest such use was Asa Fitch. In 1855, Fitch noted that some European plants that had invaded North America, such as toadflax (Linaria vulgaris), had no American insects that fed on them. He suggested that importation of insects from Europe might help suppress these invasive plants. In 1863 this concept was implemented when a scale insect was moved from northern to southern India for the purpose of damaging an invasive nonnative species of cactus
(Opuntia vulgaris). Two related cacti, O. stricta and O. inermis, were introduced as ornamentals to Australia and became highly invasive and damaging. By 1925, these plants occurred in dense stands over approximately 20 million hectares of land. The Australian government began a survey of South America (the home of these cacti) looking for specialized insects attacking the ornamentals and a moth, Cactoblastis cactorum, was released into Australia in 1926. By 1932, the cacti were killed over most of the infested area and native vegetation and crops were able to reclaim the cleared ground.
These early suggestions and projects laid out the concepts that many plants are limited in number by specialized insects and that plants moved to new regions often become separated from these specialized insects because they are not moved along with the seed or nursery stock used to import the plant.
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