Pesticide resistance is a genetic change in a parasite population that allows resistant parasites to survive treatment. Because it is genetically based, resistance to the same chemical group is passed on to the next generation.
There are a number of ways that resistance is expressed. For example, some pesticides have a mode of action that interferes with the way messages move along nerves to muscles, resulting in paralysis and eventual death of the parasite. Pests that have developed resistance have genetic changes in the message transporters that make the pesticide ineffective. Other pests develop resistance through genetic changes that increase the rate of detoxification or excretion of the pesticide. Some pests can also become behaviourally resistant so that they detect the pesticide and avoid treated areas.
Resistance is heritable, meaning that resistance is passed on from one generation to the next. Repeated use of the same chemical group can cause ongoing selection of the more resistant parasites, as susceptible types are killed. If the relative proportion of resistant lice in the population increases compared to susceptible lice, and the resistant lice come to dominate, then the effectiveness of the chemical to control the parasite will become compromised.
Importantly, resistance to different chemical groups occurs through different mutations within the pests. As a result, resistance to one chemical group does not usually equate to resistance to other chemical groups. However, resistance against an active within a chemical group, will confer resistance (though not necessarily to the same extent) to all other actives within the same chemical group. Avoid repetitive use of actives within the same chemical group to reduce the build-up of resistant individuals within populations.