Biological control or biocontrol is a method of controlling pests, whether animals, weed plants, or pathogens of animals or of plants by using other organisms. It relies on predation, parasitism, and herbivory, but typically involves an active human management role. It can be a component of integrated pest management (IPM) programs. Invertebrates and other macroorganisms are registered as biological control agents by the authorities in the US and Europe very differently from microorganisms, which are registered as biopesticides.
There are three basic strategies for biological control: classical (importation), where a natural enemy of a pest is introduced in the hope of achieving control; inductive (augmentation), in which a large population of natural enemies are administered for quick pest control; and inoculative (conservation), in which measures are taken to maintain natural enemies through regular reestablishment.
Natural enemies of insects play an important part in limiting the densities of potential pests. Biological control agents such as these include predators, parasitoids, pathogens, and competitors. Biological control agents of plant diseases are most often referred to as antagonists. Biological control agents of weeds include seed predators, herbivores, and plant pathogens.
Biological control can have side-effects on biodiversity through attacks on non-target species by any of the above mechanisms, especially when a species is introduced without a thorough understanding of the possible consequences.
History
Ancient China
The first report of the use of an insect species to control an insect pest comes from "Nanfang Caomu Zhuang" (南方草木狀 Plants of the Southern Regions) (), attributed to Western Jin dynasty botanist Ji Han (嵇含, 263–307), in which it is mentioned that "Jiaozhi people sell ants and their nests attached to twigs looking like thin cotton envelopes, the reddish-yellow ant being larger than normal. Without such ants, southern citrus fruits will be severely insect-damaged". The ants used are known as huang gan ("yellow citrus") ants (Oecophylla smaragdina). The practice was later reported by Ling Biao Lu Yi (late Tang dynasty or Early Five Dynasties), in Ji Le Pian by Zhuang Jisu (Southern Song dynasty), in the Book of Tree Planting by Yu Zhen Mu (Ming dynasty), in the book Guangdong Xing Yu (17th century), Lingnan by Wu Zhen Fang (Qing dynasty), in Nanyue Miscellanies by Li Diao Yuan, and others.
Modern era
Modern biological control techniques started to emerge in the US in the 1870s, when the Missouri State entomologist C. V. Riley and the Illinois State entomologist W. LeBaron began distributing parasitoids to control crop pests in their states. The first international shipment of an insect as a biological control agent was made by Charles V. Riley in 1873, shipping the predatory mites Tyroglyphus phylloxera to France to help fight phylloxera that was destroying grapevines there. The United States Department of Agriculture (USDA) initiated research in classical biological control following the establishment of the Division of Entomology in 1881, with C. V. Riley as Chief. The first importation of a parasitoidal wasp into the United States was that of the braconid Cotesia glomerata in 1883–1884, imported from Europe to control the invasive cabbage white butterfly, Pieris rapae. In 1888–1889 the vedalia beetle, Novius cardinalis, was introduced from Australia to California to control the cottony cushion scale, Icerya purchasi. The scale insect was threatening the newly developed citrus industry in California, but by the end of 1889, the vedalia beetle had already reduced the cottony cushion scale population. This success led to further introductions of beneficial insects into the US.
The first reported case of a classical biological control attempt in Canada involves the parasitoidal wasp Trichogramma minutum. Individuals were caught in New York State and released in Ontario gardens in 1882 by William Saunders, a trained chemist and first Director of the Dominion Experimental Farms, for controlling the invasive currantworm Nematus ribesii. Between 1884 and 1908, the first Dominion Entomologist, James Fletcher, continued introductions of other parasitoids and pathogens for the control of pests in Canada.
In 1905 the USDA initiated its first large-scale biological control program, sending entomologists to Europe and Japan to look for natural enemies of the spongy moth, Lymantria dispar dispar, and the brown-tail moth, Euproctis chrysorrhoea, invasive pests of trees and shrubs. As a result, nine parasitoids (solitary wasps) of the spongy moth, seven of the brown-tail moth, and two predators of both moths became established in the US. Although the spongy moth was not fully controlled by these natural enemies, the frequency, duration, and severity of its outbreaks were reduced and the program was regarded as successful. This program also led to the development of many concepts, principles, and procedures for the implementation of biological control programs.
The term 'biological control' was first used by Harry Scott Smith at the 1919 meeting of the Pacific Slope Branch of the American Association of Economic Entomologists, in Riverside, California. It was brought into more widespread use by the entomologist Paul H. DeBach (1914–1993) who worked on citrus crop pests throughout his life.
Prickly pear cacti had been introduced into Queensland, Australia as ornamental plants, starting in 1788. They quickly spread to cover over 25 million hectares of Australia by 1920, increasing by 1 million hectares per year. Digging, burning, and crushing all proved ineffective. Two control agents were introduced to help control the spread of the plant, the cactus moth Cactoblastis cactorum, and the scale insect Dactylopius. Between 1926 and 1931, tens of millions of cactus moth eggs were distributed around Queensland, and by 1932, most areas of prickly pear had been destroyed.
Types
There are three basic biological pest control strategies: importation, augmentation and conservation.
Importation
Importation or classical biological control is the introduction of a pest's natural enemies to a new locale where they do not occur naturally. Early instances were often unofficial and not based on research; some introduced species subsequently became serious pests themselves.
To be most effective at controlling a pest, a biological control agent requires the ability to colonise new areas and keep pace with changes to the habitat over space and time. Control is greatest when the agent can maintain its population during temporary absences of the target species, and can rapidly exploit pest outbreaks as an opportunistic forager.
One of the earliest successes was in controlling Icerya purchasi (cottony cushion scale) in Australia, using a predatory insect Rodolia cardinalis (the vedalia beetle). This success was repeated in California using the beetle and a parasitoidal fly, Cryptochaetum iceryae. Other successful cases include the control of Antonina graminis in Texas by Neodusmetia sangwani in the 1960s.
Damage from Hypera postica, the alfalfa weevil, a serious introduced pest of forage, was substantially reduced by the introduction of natural enemies. 20 years after their introduction the population of weevils in the alfalfa area treated for alfalfa weevil in the Northeastern United States remained 75 percent down.
Alligator weed was introduced to the United States from South America. It takes root in shallow water, interfering with navigation, irrigation, and flood control. The alligator weed flea beetle and two other biological controls were released in Florida, greatly reducing the amount of land covered by the plant. Another aquatic weed, the giant salvinia (Salvinia molesta) is a serious pest, covering waterways, reducing water flow and harming native species. Control with the salvinia weevil (Cyrtobagous salviniae) and the salvinia stem-borer moth (Samea multiplicalis) is effective in warm climates, and in Zimbabwe, a 99% control of the weed was obtained over a two-year period.
Small, commercially-reared parasitoidal wasps, Trichogramma ostriniae, provide limited and erratic control of the European corn borer (Ostrinia nubilalis), a serious pest. Careful formulations of the bacterium Bacillus thuringiensis are more effective. The O. nubilalis integrated control releasing Tricogramma brassicae (egg parasitoid) and later Bacillus thuringiensis subs. kurstaki (larvicide effect) reduce pest damages more than insecticide treatments
The population of Levuana iridescens, the Levuana moth, a serious coconut pest in Fiji, was brought under control by a classical biological control program in the 1920s.
Augmentation
Augmentation involves the supplemental release of natural enemies that occur in a particular area, boosting the naturally occurring populations there. In inoculative release, small numbers of the control agents are released at intervals to allow them to reproduce, in the hope of setting up longer-term control and thus keeping the pest down to a low level, constituting prevention rather than cure. In inundative release, in contrast, large numbers are released in the hope of rapidly reducing a damaging pest population, correcting a problem that has already arisen. Augmentation can be effective, but is not guaranteed to work, and depends on the precise details of the interactions between each pest and control agent.
An example of inoculative release occurs in the horticultural production of several crops in greenhouses. Periodic releases of the parasitoidal wasp, Encarsia formosa, are used to control greenhouse whitefly, while the predatory mite Phytoseiulus persimilis is used for control of the two-spotted spider mite.
The egg parasite Trichogramma is frequently released inundatively to control harmful moths. New way for inundative releases are now introduced i.e. use of drones. Egg parasitoids are able to find the eggs of the target host by means of several cues. Kairomones were found on moth scales. Similarly, Bacillus thuringiensis and other microbial insecticides are used in large enough quantities for a rapid effect. Recommended release rates for Trichogramma in vegetable or field crops range from 5,000 to 200,000 per acre (1 to 50 per square metre) per week according to the level of pest infestation. Similarly, nematodes that kill insects (that are entomopathogenic) are released at rates of millions and even billions per acre for control of certain soil-dwelling insect pests.
Conservation
The conservation of existing natural enemies in an environment is the third method of biological pest control. Natural enemies are already adapted to the habitat and to the target pest, and their conservation can be simple and cost-effective, as when nectar-producing crop plants are grown in the borders of rice fields. These provide nectar to support parasitoids and predators of planthopper pests and have been demonstrated to be so effective (reducing pest densities by 10- or even 100-fold) that farmers sprayed 70% less insecticides and enjoyed yields boosted by 5%. Predators of aphids were similarly found to be present in tussock grasses by field boundary hedges in England, but they spread too slowly to reach the centers of fields. Control was improved by planting a meter-wide strip of tussock grasses in field centers, enabling aphid predators to overwinter there.