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Pesticide application can artificially select for resistant pests.
In this diagram, the first generation happens to have an insect with a heightened resistance to a pesticide (red).
After pesticide application, its descendants represent a larger proportion of the population because sensitive pests (white) have been selectively killed.
After repeated applications, resistant pests may comprise the majority of the population.
Pesticide resistance is the adaptation of pest species targeted by a pesticide resulting in decreased susceptibility to that chemical. In other words, pests develop a resistance to a chemical through selection; after they are exposed to a pesticide for a prolonged period it no longer kills them as effectively. The most resistant organisms are the ones to survive and pass on their genetic traits to their offspring.

More specific definitions of pesticide resistance often apply to particular classes of pesticides. Manufacturers of pesticides tend to prefer a definition that is dependent on failure of a product in a real situation, sometimes called field resistance. For example, the Insecticide Resistance Action Committee (IRAC) definition of insecticide resistance is 'a heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest species'.

Pesticide resistance is increasing in occurrence. In the 1940s, farmers in the USAmarker lost 7% of their crops to pests, while since the 1980s, the percentage lost has increased to 13, even though more pesticides are being used. Over 500 species of pests have developed a resistance to a pesticide. Other sources estimate the number to be around 1000 species since 1945.

Rachel Carson predicted the phenomenon in her 1962 book Silent Spring.

Factors

Pesticides that fail to break down quickly and remain in the area contribute to selection for resistant organisms even after they are no longer being applied.

In response to pesticide resistance, farmers may resort to increased use of pesticides, exacerbating the problem. In addition, when pesticides are toxic toward species that feed on or compete with pests, the pest population will likely expand further, requiring more pesticides. This is sometimes referred to as pesticide trap, or a pesticide treadmill, since farmers are continually paying more for less benefit.

Insect predators and parasites which live on other insects generally have smaller populations and are therefore much less likely to develop resistance than are the primary targets of the pesticides, such as mosquitoes and those that feed on plants. This can compound the pest problem because these species normally keep pest populations in check. But resistant predators of pest species can be bred in laboratories, which can help keep pest populations down.

The fewer sources of food a pest has the more likely it is to develop resistance, because it is exposed to higher concentrations of pesticides and has less opportunity to breed with populations that have not been exposed. Other factors in the speed with which a species develops resistance are generation time and fecundity (shorter generations and more offspring lead to resistance more quickly).

Examples

Resistance has developed in a variety of different pest species:Resistance to insecticides was first documented by A. L. Melander in 1914 when scale insects demonstrated resistance to an inorganic insecticide. Between 1914 and 1946, 11 additional cases of resistance to inorganic insecticides were recorded. The development of organic insecticides, such as DDT, gave hope that insecticide resistance was an issue of the past. Unfortunately, by 1947 housefly resistance to DDT was documented. With the introduction of every new insecticide class – cyclodienes, carbamates, formamidines, organophosphates, pyrethroids, even Bacillus thuringiensis – cases of resistance surfaced within two to 20 years.
  • In the US, studies have shown that fruit flies that infest orange groves were becoming resistant to malathion, a pesticide used to kill them.




  • In England, rats in certain areas have developed such a strong resistance to rat poison that they can consume up to five times as much of it as normal rats without dying.


  • DDT is no longer effective in preventing malaria in some places, a fact which contributed to a resurgence of the disease.


  • In the southern United States, the weed Amaranthus palmeri, which interferes with production of cotton, has developed widespread resistance to the herbicide Roundup.


Multiple resistance

Multiple resistance is the phenomenon in which a pest is resistant to more than one class of pesticides. This can happen if one pesticide is used until pests display a resistance and then another is used until they are resistant to that one, and so on. Cross resistance, a related phenomenon, occurs when the genetic mutation that made the pest resistant to one pesticide also makes it resistant to other pesticides, especially ones with similar mechanisms of action or ones in the same class.

Physiology

Frequently a pest becomes resistant to a pesticide because it develops physiological changes that protect it from the chemical. In some cases, a pest may gain an increased number of copies of a gene, allowing it to produce more of a protective enzyme that breaks down the pesticide into less toxic chemicals. Such enzymes include esterases, glutathione transferases, and mixed microsomal oxidases. Alternately, the number of biochemical receptors for the chemicals may be reduced in the pest, or the receptor may be altered, reducing the pest's sensitivity to the compound. Behavioral resistance has also been described for some chemicals; for example, some Anopheles mosquitoes developed a preference for resting outside that prevented them from coming in contact with pesticide sprayed on interior walls.

Blowfly maggots produce an enzyme that confers resistance to organochloride insecticides. Scientists have researched ways to use this enzyme to break down pesticides in the environment, which would detoxify them and prevent harmful environmental effects. Later they discovered a similar enzyme produced by soil bacteria that also breaks down organochloride insecticides but which works faster and remains stable in a variety of conditions. The product, called Landguard is used in Australia to decontaminate spray equipment, soil and water after pesticide spraying and spills.

Management

Pest resistance to a pesticide is commonly managed through pesticide rotation, which involves alternating among pesticide classes with different modes of action to delay the onset of or mitigate existing pest resistance. Different pesticide classes may have different effects on a pest. The U.S. Environmental Protection Agency (EPA or USEPA) designates different classes of fungicides, herbicides and insecticides. Pesticide manufacturers may, on product labeling, require that no more than a specified number of consecutive applications of a pesticide class be made before alternating to a different pesticide class. This manufacturer requirement is intended to extend the useful life of a product.LOSERTankmixing pesticides is the combination of two or more pesticides with different modes of action in order to improve individual pesticide application results and delay the onset of or mitigate existing pest resistance.

Another strategy is to avoid using pesticides more often than necessary.

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