Pesticide Resistance - Physiology and Behavior

Physiology and Behavior

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. Alternatively, 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. Still other mechanisms include increased rates of excretion of toxic molecules, their sequestration and storage inside of the insect body away from vulnerable tissues and organs, and decreased toxin penetration through the insect body wall.

Often, mutation in a single gene only leads to the development of a resistant organism. In other cases, multiple genes are involved. Resistant genes are usually autosomal. This means that they are located on autosomes (as opposed to sex chromosomes). As a result, resistance is inherited similarly in males and females. Also, resistance is usually inherited as an incompletely dominant trait. When a resistant and a susceptible individual mate with each other, their progeny has an intermediate level of resistance (more resistant than the susceptible parent, but not as resistant as the resistant parent).

Adaptation to pesticides usually decreases relative fitness of organisms in the absence of pesticides. Resistant individuals often have reduced reproductive output, life expectancy, mobility, etc. Therefore, relatively few of them persist in a population that is not exposed to a particular insecticide to which they have developed resistance.

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.