Pesticide resistance

Pesticide resistance is the evolution 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 artificial 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 USA 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 resistance 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 pesticide use decreases populations of species that feed on or compete with pests, their populations 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 off of other insects are much less likely to develop resistance than insects such as mosquitoes and those that feed on plants, which can compound the 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 less 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 is 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:
 * The first known case of pesticide resistance occurred with DDT in Sweden in 1946 shortly after the chemical was first used.


 * 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 Hawaii and Japan, the diamondback moth developed a resistance to Bacillus thuringiensis about three years after it began to be used heavily.


 * 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.

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 an enzyme that protects it from the chemical. Such enzymes include esterases, glutathione transferases, and mixed microsomal oxidases. In some cases, a pest may increase the numbers of copies of a gene that produces a protective enzyme that break down pesticides into less toxic chemicals, so that the pest produces more of the enzyme. Also, the number of biochemical receptors for the chemicals may be reduced in the pests, reducing their sensitivity to the compounds.

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 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.

Tankmixing 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.