Acaricide resistance is an inevitable consequence of using acaricides. The more often an acaricide is used, the greater the number of ticks exposed, and the quicker resistance is likely to develop.
There are several strategies to manage resistance, but by far the most effective measure is to reduce the number of times that acaricides are applied.
Strategies for delaying the emergence of acaricide resistance include:
These strategies are discussed in more detail below.
When acaricides are applied to a mob of cattle, not all ticks in the population associated with those cattle are exposed to the acaricide. Ticks that are living in the pasture, either as females that have dropped from the host and waiting to lay eggs, as eggs, or as larvae waiting to attach to a host, are not exposed. The non-exposed part of the population is said to be ‘in refugia’. The higher the proportion of a population that is in refugia at the time of an acaricide application, the lower the selection for acaricide resistance. It is possible to increase the refugia at the time of a treatment by only treating the most heavily infested cattle. Of course, this will not be an acceptable option if tick-borne diseases are a serious concern. The proportion of ticks in refugia is generally lowest at the very beginning of the tick season, in those areas in which the seasonality of ticks is pronounced. A higher proportion of the total tick population will be on cattle just after the ‘spring rise’ of ticks than at the end of the season. This is precisely why ‘strategic’ tick control programs concentrate treatments during spring and early summer – there is a greater effect on the population for a smaller number of treatments.
Under some circumstances there might be a benefit from rotation or alternation between acaricide chemical groups. There is no benefit at all to rotations that involve different brands or different actives from within the same chemical group. The infrastructure and available products will usually determine whether a rotation strategy is logistically possible. Clearly, when there is already evidence of resistance to the acaricide in use it makes no sense to continue using it and rotation onto another group of acaricides is advisable. In some cases it might be possible to return to the acaricide against which resistance has been noted, but for most acaricides it does not seem to be an option.
For example, experiments have shown the potential reversion of cattle tick susceptibility to amitraz following a three year rotation to a different chemical group. Conversely reversion to synthetic pyrethroid (SP) and organophosphate (OP) susceptibility appears to be more difficult to achieve, possibly due to their different modes of inheritance.
Whether combination products are a good idea for managing resistance is currently the subject of debate among scientists and policy makers. Theoretically, acaricide products that contain combinations of acaricides from different chemical groups to target the same parasite are a good idea if the chemical groups are new, and there is little or no resistance in the target populations. However, in practice, combinations are more likely to be introduced when acaricide resistance is already widespread. In such circumstances, combination chemicals are working as single actives and using them together is probably contributing to the acceleration of resistance against several acaricides at once.
Acaricides should always be used exactly as specified on the product label. Wide-spread under-dosing of a chemical (e.g. under-estimating the weight of animals being treated, poor application technique, uncalibrated dosing) has often been blamed for the rapid emergence of resistance. Read the product label to ensure the product isn’t unintentionally exposing non-target parasites to chemicals (e.g. products to treat ticks can also affect worms or lice).