Biological control

Biological control, biocontrol, or biological pest control, is the reduction of pest populations by using natural enemies. It is important because crop pests become resistant to chemical pesticides.

Natural enemies of insect pests include predators, parasitoids, and pathogens. Biological control agents of weeds include herbivores and plant pathogens. Predators, such as birds, lady beetles and lacewings, are free-living species that eat many prey during their lifetime.

Parasitoids are species whose larvae develop on or in a single insect host, ultimately killing or fatally infecting the host. Most have a very narrow host range. Many species of wasps and some flies are parasitoids.

Pathogens are disease-causing organisms including bacteria, fungi, and viruses. They kill or debilitate their host and are relatively specific.

Three strategies

There are three basic types of biological control strategies; conservation, classical biological control, and augmentation.

• Conservation: Avoid using pesticides, which kill the natural enemies of the pests. Encourage the growth of those things which attack the pests. Use companion planting where possible. In China, the mosquito fern has been used for at least a thousand years, as a companion plant for rice crops. It hosts a special cyanobacteria that fixes nitrogen from the atmosphere, and also blocks out light from competing plants (but not the rice, which grows taller).

• Classical biological control: This is the introduction of natural enemies to a new area where they did not originate or do not occur naturally. Thereafter, the predator lives as a natural part of the habitat, reproducing and killing the pest species. This method is especially effective when the pest is itself an invasive or introduced species in the area. Free of its natural enemies, the pest multiplies to huge numbers. The introduction from its homeland of an enemy or two can be very successful. One example is the use of Larinus planus to control the Canada thistle.

• Augmentation: This is the release of numbers of natural enemies at specific times. These predators are natural parts of the ecosystem, but are released in such numbers as to overwhelm the pest at some critical time. Stocks of the predator are got from commercial suppliers.

An early example

A Chinese text from 304 AD, Records of the plants and trees of the southern regions, by Hsi Han, describes mandarin oranges protected by large reddish-yellow citrus ants. The ants attack and kill insect pests of the orange trees. The citrus ant (Oecophylla smaragdina) was rediscovered in the 20th century, and now is again used in China to protect orange groves.

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