Biological control, also called biocontrol, is a way to manage pests like insects, mites, weeds, or harmful organisms that affect plants and animals. It uses other living things to control pests through methods such as hunting, parasitism, eating plants, or other natural processes. People often help manage these processes actively. Biological control is a key part of integrated pest management (IPM) programs. In the US and Europe, invertebrates and other large organisms are registered as biological control agents differently than microorganisms, which are registered as biopesticides.
There are three main strategies for biological control: classical (importation), where a natural enemy of a pest is brought in to control it; inductive (augmentation), where a large number of natural enemies are added quickly to reduce pests; and inoculative (conservation), where steps are taken to keep natural enemies alive by regularly reintroducing them.
Natural enemies of insects help keep pest populations low. These biological control agents include predators, parasitoids, disease-causing organisms, and competitors. For plant diseases, biological control agents are often called antagonists. For weeds, agents include organisms that eat seeds, animals that eat plants, and disease-causing organisms.
Biological control can sometimes harm other species that are not the target pests, especially when a new species is introduced without fully understanding the possible effects.
History
In 1919, Harry Scott Smith first used the term "biological control" during a meeting of the Pacific Slope Branch of the American Association of Economic Entomologists in Riverside, California. Later, Paul H. DeBach, an entomologist who studied citrus crop pests, helped make the term more widely known. However, people had used biological control methods for many centuries before this. The earliest known example comes from a Chinese text called "Nanfang Caomu Zhuang" (Plants of the Southern Regions), written around 304 AD by Ji Han. The text describes how people in Jiaozhi used ants to protect citrus fruits from insect damage. These ants, called huang gan ants (Oecophylla smaragdina), were sold with their nests attached to twigs. This practice was also mentioned in later writings by other Chinese scholars.
Biological control methods became more organized in the 1870s. In the United States, scientists like C. V. Riley and W. LeBaron began moving parasitoids (insects that lay eggs inside pests) between states to control crop pests. In 1873, Riley sent predatory mites to France to help fight grapevine phylloxera, a pest destroying grapevines. The United States Department of Agriculture (USDA) started research on biological control in 1881 after creating the Division of Entomology, with C. V. Riley as its leader. In 1883–1884, the USDA imported a parasitoidal wasp called Cotesia glomerata from Europe to control the cabbage white butterfly. In 1888–1889, the vedalia beetle (Novius cardinalis), a type of lady beetle, was brought from Australia to California to control the cottony cushion scale, a pest harming citrus crops. This effort was very successful, and the beetle helped reduce the pest population by the end of 1889.
In 1905, the USDA began its first large-scale biological control program. Scientists were sent to Europe and Japan to find natural enemies of invasive pests like the spongy moth and the brown-tail moth. They introduced nine parasitoids for the spongy moth, seven for the brown-tail moth, and two predators for both. Though the spongy moth was not completely controlled, the program reduced the frequency and severity of its outbreaks. This effort also helped develop many ideas and methods used in biological control today.
Prickly pear cacti were brought to Queensland, Australia, as ornamental plants starting in 1788. By 1920, they had spread over 25 million hectares, growing by 1 million hectares each year. Methods like digging, burning, and crushing the cacti did not work. In 1926–1931, scientists introduced two control agents: the cactus moth (Cactoblastis cactorum) and the scale insect Dactylopius. Millions of cactus moth eggs were released, and by 1932, most prickly pear areas had been destroyed.
In Canada, the first recorded use of classical biological control involved the parasitoidal wasp Trichogramma minutum. In 1882, William Saunders, a scientist and first director of the Dominion Experimental Farms, caught the wasps in New York and released them in Ontario gardens to control the invasive currantworm. From 1884 to 1908, James Fletcher, the first Dominion Entomologist, continued introducing other parasitoids and pathogens to manage pests in Canada.
Types of biological pest control
There are three main ways to use natural enemies to control pests: importation (also called classical biological control), augmentation, and conservation.
Importation involves bringing a pest's natural enemies from another place where they already live. This method was sometimes used without proper research, and in some cases, the introduced species became pests themselves. For a natural enemy to control a pest well, it must be able to spread and adapt to changes in the environment. It should also be able to survive even if the pest is not around for a while and should be able to quickly find and attack pests when they appear.
One of the first successful examples was using the vedalia beetle (Rodolia cardinalis) to control the cottony cushion scale (Icerya purchasi) in Australia. This method was later used in California with the same beetle and a parasitoidal fly (Cryptochaetum iceryae). Other successes include using Neodusmetia sangwani to control Antonina graminis in Texas in the 1960s.
The alfalfa weevil (Hypera postica) caused serious damage to forage crops, but its population dropped by 75% in the Northeastern United States 20 years after natural enemies were introduced. Alligator weed, brought to the U.S. from South America, grows in shallow water and harms navigation, irrigation, and flood control. In Florida, the alligator weed flea beetle and two other biological controls reduced the plant’s spread. Another aquatic weed, giant salvinia (Salvinia molesta), was controlled in warm climates using the salvinia weevil (Cyrtobagous salviniae) and the salvinia stem-borer moth (Samea multiplicalis). In Zimbabwe, these controls reduced the weed by 99% in two years.
Small, commercially raised parasitoidal wasps (Trichogramma ostriniae) provide limited control of the European corn borer (Ostrinia nubilalis), but careful use of the bacterium Bacillus thuringiensis is more effective. Combining the release of Trichogramma brassicae (an egg parasitoid) with Bacillus thuringiensis subspecies kurstaki (a larvicide) reduces pest damage better than insecticides.
The Levuana moth (Levuana iridescens), a major pest of coconuts in Fiji, was controlled in the 1920s through a classical biological control program.
Augmentation involves adding more natural enemies to an area to help existing ones control pests. In inoculative release, small numbers of natural enemies are released at regular intervals to allow them to reproduce and maintain long-term control. This helps keep pest populations low. In inundative release, large numbers are released to quickly reduce a pest population that is already causing harm. Augmentation can work well but depends on how well the natural enemies and pests interact.
An example of inoculative release is using the parasitoidal wasp Encarsia formosa in greenhouses to control greenhouse whiteflies. The predatory mite Phytoseiulus persimilis is used to control two-spotted spider mites. Inundative releases of Trichogramma wasps are used to control harmful moths. New methods, like using drones, are now being tested for these releases. Trichogramma wasps find moth eggs using chemical cues called kairomones found on moth scales. For vegetable or field crops, recommended Trichogramma release rates range from 5,000 to 200,000 per acre (1 to 50 per square meter) per week, depending on pest levels. Entomopathogenic nematodes (nematodes that kill insects) are released in millions or even billions per acre to control soil-dwelling pests.
Conservation involves protecting natural enemies already in an environment. These enemies are adapted to the habitat and target pests, making conservation simple and cost-effective. For example, growing nectar-producing plants around rice fields supports parasitoids and predators of planthopper pests. This method reduced pest numbers by 10- to 100-fold, leading to less insecticide use and higher crop yields. In England, predators of aphids were found in tussock grasses along field edges, but they moved too slowly to reach field centers. Planting a 1-meter-wide strip of tussock grass in field centers helped predators overwinter and control aphids more effectively.
Cropping systems can be changed to support natural enemies, a practice called habitat manipulation. Features like shelterbelts, hedgerows, or beetle banks provide homes for beneficial insects like parasitoidal wasps. Simple actions, such as leaving fallen leaves or mulch, provide food and shelter for worms, insects, and small mammals. Compost piles and wood stacks support invertebrates and small mammals. Long grass and ponds help amphibians. Leaving dead annual plants in autumn allows insects to use hollow stems for winter. In California, prune trees are sometimes planted in grape vineyards to provide better winter habitat for a key grape pest parasitoid. Artificial shelters like wooden boxes or flowerpots can attract natural enemies. For example, upside-down flowerpots filled with straw or wood wool attract earwigs, and plastic bottles with cardboard rolls attract green lacewings. Birdhouses help insect-eating birds nest, and choosing the right opening size attracts specific bird species.
In cotton farming, replacing broad-spectrum insecticides with selective methods like Bt cotton (a type of genetically modified cotton) can help natural enemies by reducing insecticide exposure. These natural enemies can control pests not affected by the Bt protein. However, in some cases, reduced prey quality or abundance from Bt cotton may lower natural enemy populations. The number of pests eaten or parasitized in Bt and non-Bt cotton is often similar.
Biological control agents
Predators are animals that live freely and eat many prey throughout their lives. Since many major crop pests are insects, many predators used in biological control are insect-eating species. Lady beetles, especially their larvae that are active from May to July in the northern hemisphere, eat many aphids and also consume mites, scale insects, and small caterpillars. The spotted lady beetle (Coleomegilla maculata) also eats the eggs and larvae of the Colorado potato beetle (Leptinotarsa decemlineata).
The larvae of many hoverfly species mainly eat aphids, with one larva eating up to 400 aphids in its lifetime. Scientists have not studied how well they work in commercial crops.
The running crab spider (Philodromus cespitum) eats many aphids and helps control pests in European fruit orchards.
Some types of entomopathogenic nematodes are important predators of insect and other invertebrate pests. These nematodes form a special stage called the infective juvenile. These stages move through soil and infect insect hosts. Once inside the insect, they move to the insect’s body fluid, recover from their inactive state, and release bacteria. The bacteria grow, produce toxins, and kill the host. Phasmarhabditis hermaphrodita is a tiny nematode that kills slugs. Its life cycle includes a stage in the soil where it connects with bacteria like Moraxella osloensis. The nematode enters the slug through the back part of its body, feeds and reproduces inside, but the bacteria kill the slug. This nematode is sold in Europe and applied by watering it onto moist soil. Entomopathogenic nematodes do not last long because they are not resistant to high heat or dry conditions. The type of soil they are used in can also affect how well they work.
Predators used to control spider mites include the mites Phytoseiulus persimilis, Neoseilus californicus, and Amblyseius cucumeris, the midge Feltiella acarisuga, and the ladybird Stethorus punctillum. The bug Orius insidiosus has been used successfully to control the two-spotted spider mite and the western flower thrips (Frankliniella occidentalis).
Predators like Cactoblastis cactorum (mentioned earlier) can also destroy invasive plants. For example, the poison hemlock moth (Agonopterix alstroemeriana) eats poison hemlock (Conium maculatum). During its larval stage, the moth eats only poison hemlock, and hundreds of larvae can live on one plant, killing large areas of the plant.
For rodent pests, cats are effective biological control when combined with reducing hiding places. Cats can prevent rodent populations from growing too large, but they cannot eliminate severe infestations. Barn owls are sometimes used as biological rodent control. While there are no studies showing how well barn owls work, they are known to eat rodents and can be used instead of or along with cats. They can be encouraged to live in an area by placing nest boxes.
In Honduras, where the mosquito Aedes aegypti spread dengue fever and other diseases, biological control was tried by adding copepods, baby turtles, and juvenile tilapia to wells and tanks. This helped eliminate mosquito larvae.
Even some arthropods that are mainly predators of animals also eat flower nectar and pollen. A study found that adult Adalia bipunctata (a predator that controls Ephestia kuehniella) can survive on flowers but cannot complete its life cycle. A meta-analysis was done to see if this pattern exists in other species. In some cases, flower resources are needed. Overall, flower resources (or sugar water) help predators live longer and reproduce more, which can affect how well biological control works.
Parasitoids lay their eggs on or inside an insect host, which the developing larvae use as food. The host eventually dies. Most parasitoids are wasps or flies and have very specific host preferences. Important groups include ichneumonid wasps, which mainly use caterpillars; braconid wasps, which attack caterpillars and other insects like aphids; chalcidoid wasps, which parasitize insect eggs and larvae; and tachinid flies, which attack a wide range of insects. Parasitoids work best when their hosts have few places to hide.
Parasitoids are widely used in biological control. Commercially, there are two types of systems for raising them: one produces many parasitoids daily for short periods, and the other produces fewer over longer periods. Production must match the timing when hosts are available at the right life stage. Large facilities produce year-round, while some produce only seasonally. Rearing facilities are often far from where parasitoids are used, and transporting them can be a problem. Heat or vibrations during transport can harm parasitoids.
Encarsia formosa is a small parasitoid wasp that attacks whiteflies, which damage crops by causing wilting and black sooty mold. It works best on small infestations and provides long-term protection. The wasp lays its eggs in young whitefly scales, turning them black as the larvae develop. Gonatocerus ashmeadi (Hymenoptera: Mymaridae) was introduced to control the glassy-winged sharpshooter (Homalodisca vitripennis) in French Polynesia and reduced the pest population by about 95%.
The eastern spruce budworm is a harmful insect in fir and spruce forests. Birds naturally control it, but the wasp Trichogramma minutum has been studied as an alternative to chemical controls.
Recent studies are exploring ways to control urban cockroaches using parasitic wasps. Since cockroaches often live in sewers and hard-to-reach areas, using wasps that actively hunt them may help reduce their numbers.
Pathogenic microorganisms include bacteria, fungi, and viruses. They kill or weaken their hosts and are usually specific to certain species. Some microbial diseases occur naturally and can be used as biological pesticides. These outbreaks often happen when insect populations become very dense.
The use of pathogens to control aquatic weeds was unknown until a 1972 proposal by Zettler and Freeman. Before that, no biological control methods had been used against water weeds.
Target pests
Botrytis cinerea causes disease on lettuce. Fusarium species and Penicillium claviforme cause disease on grape and strawberry. Trichoderma species cause disease on strawberry. Cladosporium herbarum causes disease on Chinese cabbage. Bacillus brevis causes disease on Chinese cabbage. Various yeasts and bacteria cause disease on many other crops. Sclerotinia sclerotiorum is controlled by several types of fungi that fight disease. Trichoderma hamatum prevents fungal pod infection on snap beans if it infects the plant at the same time or before the infection occurs. Cryphonectria parasitica, Gaeumannomyces graminis, Sclerotinia species, and Ophiostoma novo-ulmi are controlled by viruses. Powdery mildew and rust diseases are controlled by various Bacillus species and fluorescent Pseudomonads. Colletotrichum orbiculare can stop further infection if it is used to help plants fight off diseases throughout the plant by infecting the lowest leaf.
Difficulties
Many of the most important pests are non-native species that spread quickly and cause serious problems for farming, gardening, forests, and cities. These pests often arrive without the natural enemies that usually control them in their home environments. Without these natural enemies, pest populations can grow very large. Bringing in the natural enemies of these pests might seem like a good idea, but it can lead to unexpected problems. Rules to manage these introductions might not work well, and they could harm other plants and animals. Farmers and growers might also struggle to use these methods because they lack the knowledge to do so properly.
Biological control can change the environment by affecting other species through hunting, infection, competition, or other ways. Sometimes, the animals or insects used to control pests might also harm plants and animals that are not the target. For example, in the 1940s, parasitic wasps were brought to Hawaii to control a type of moth. These wasps are still there today, and their impact on the local ecosystem is still being studied. Before using any biological control method, scientists must carefully study how it affects the environment to understand its risks.
Vertebrate animals, like mammals and reptiles, usually eat a wide variety of food and are not often used for biological control because they can cause problems. One famous example is the cane toad, which was brought to Australia to control a type of beetle that harms sugarcane. In 1935, 102 toads were taken from Hawaii and released into sugarcane fields. The toads could not jump high enough to eat the beetles, which lived on tall parts of the plants. Instead, the toads thrived by eating other insects and spread quickly, taking over the homes of native frogs and toads. They also carried diseases that harmed native species. When threatened, the toads release poison from glands on their shoulders, which has killed some native animals that tried to eat them. However, recent studies suggest that some native animals are learning to avoid the toads or are becoming resistant to the poison over time.
A seed-eating weevil called Rhinocyllus conicus was brought to North America to control a type of thistle that grows in fields. However, the weevil also harms native thistle species, such as the Platte thistle, by choosing larger plants to feed on. This reduces the number of plants that can reproduce, which puts the species at risk of disappearing. A similar issue happened with another weevil, Larinus planus, which was used to control thistles but also harmed other plants, including one that is already endangered.
The small Asian mongoose was introduced to Hawaii to control rats. However, the mongoose is active during the day, while the rats are active at night. This made it easier for the mongoose to hunt native birds and their eggs instead of the rats. Now, both the rats and the mongooses threaten the survival of native bird species. This introduction happened without proper planning, and today, more careful study is needed before releasing animals into new environments.
The eastern mosquitofish, a type of small fish native to the southeastern United States, was introduced worldwide in the 1930s and 1940s to control mosquito populations and help reduce malaria. However, the fish have outcompeted native fish and frogs by eating their food and even their eggs and young. In Australia, scientists have discussed ways to control the spread of the mosquitofish. In 1989, researchers noted that managing the fish’s population is very difficult with current methods.
A challenge in using biological pest control is that some farmers may prefer using chemical pesticides, which are familiar to them. However, pesticides can cause problems, such as pests becoming resistant to them or harming the natural enemies of pests. This can lead to outbreaks of other pests that were not originally targeted, even in areas far from where the pesticides were used. One way to help farmers adopt biological control is by showing them simple experiments, such as observing how natural enemies hunt pests or how pests are affected by these enemies. In the Philippines, farmers were encouraged to avoid spraying insecticides for the first 30 days after planting crops. This led to a 33% reduction in pesticide use and changed how farmers viewed the use of insecticides.
Related techniques
A method related to biological pest control involves introducing sterile individuals into the population of a species. This method is often used with insects: many male insects are made sterile using radiation and then released into the environment, where they compete with non-sterile males for mates. Female insects that mate with sterile males will lay eggs that cannot develop into new insects, which reduces the population size over time. Repeating this process can lead to a large decrease in the population of the species. A similar approach has recently been used with weeds by releasing pollen that has been treated with radiation. This causes the seeds produced to be deformed and unable to grow into new plants.