Acaricide resistance is a genetic change (mutation) in individual ticks within a parasite population that allows resistant ticks 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, gene mutations that block the active have been shown to be responsible for most resistance to synthetic pyrethroids (SPs) in ticks and it is likely the main mechanism for resistance to amitraz. In contrast, detoxification mechanisms in ticks have been shown to be important for organophosphates (OPs) and to a lesser extent for SPs.
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 ticks in the population increases compared to susceptible ticks, and the resistant ticks 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 ticks. 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.