Pesticide & Insecticide Resistance

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

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

The formal 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 actually quantify.

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

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 extent of resistance to any of the common pediculicides.

The good news is that where 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.