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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 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.
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