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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 accurately quantify.

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 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 pediculicides. 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 mis-application 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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