Pesticide Resistance 

- One reason pesticides stop working -

Summary: Pesticides sometimes fail to control a target pest either because of mistakes made during application, or a phenomenon called pesticide resistance. Pesticide resistance is not as common as many people believe, and is often reversible.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

What is pesticide resistance?

Sometimes a pesticide (insecticide, herbicide, fungicide) does not work as expected and fails to control the target pest. This is called control failure and can be caused by a number of things but often it can be traced to errors made during application. In the case of head lice treatments, for example, these errors usually result from not following package instructions carefully (see How to Control Head Lice).

Less often, the control failure is caused by something called pesticide resistance. If a pest population for some reason becomes less susceptible to the pesticide being used we often label that pest population pesticide resistant.

human body or head louse

human louse about 2-3 mm (1/8") long

The scientific definition of resistance is "a genetic change in the target pest population that causes it to be less susceptible to a pesticide than it would have been prior to exposure to the active ingredient". Genetic changes, however, can be difficult to accurately measure.

To determine if pesticide resistance is at work we can compare the suspect population to a "wild" population of the same species in terms of their susceptibility to the pesticide in question. Wild populations are defined as those that have not been exposed to the pesticide. If differences between the test and wild populations in can be demonstrated by something called a dose-mortality study, pesticide resistance may be inferred in the test population.

Populations become resistant to a particular pesticide after repeated and prolonged exposure. The individuals that survive an application may produce offspring that share the "resistance genes" and then pass them on to their offspring.

Small, isolated and intensively treated populations are the most likely to become resistant. In the case of head lice not enough of these studies have been done to be able to determine the full extent of resistance to any of the common lice insecticide. However, there have been instances of confirmed resistance to permethrin, the active ingredient in Nix (tm) and other lice control products but only in certain geographic regions.

Except in some intensively treated agricultural crops, however, pesticide resistance is still fairly rare and it has not been found to be widespread in human lice. Therefore, be cautious when trying to figure out what went wrong if a head lice treatment fails because more often then not the cause is some type of misapplication or incomplete nit-picking.

The good news is that when resistance is found the cause is usually intensive overuse of an active ingredient (pesticide) and this can be reversed. When use of the active ingredient is stopped, resistance will diminish and the population will return to the wild, susceptible state.

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Insecticides for Home Use

Pesticide Types

Pesticide References

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