Researchers at the University of Glasgow have identified the genetic basis for at least one form of pesticide resistance in the cattle tick Rhipicephalus microplus, an important parasite of cattle in the tropics and subtropics. Populations of these ticks have developed resistance to Amitraz, a widely used acaricide for controlling the pests, which cause anaemia, reduced rate of growth and death.

According to an article from Phys.org, about 80 percent of cattle around the world, mostly in the tropics and sub-tropics, are exposed to the cattle tick, with a global cost of tick-borne diseases and control measures estimated to be more than $6 billion annually.

Resistance to acaricides is found in about 20 percent of Australian tick populations and more than 50 percent of Mexican ticks.

In their report, published in the Proceedings of the National Academy of Sciences, the researchers describe in detail the evolution of amitraz resistance in replicated populations of ticks in the field, using divergent selection pressures with amitraz.

They also demonstrate a close association between resistance to amitraz and a specific allele of the β-adrenergic octopamine receptor gene, which they propose confers resistance to amitraz.

The researchers established six populations of R. microplus ticks in similar paddocks, using strains of ticks known to be susceptible and resistant to amitraz and synthetic pyrethroids. They managed each population using one of three acaricide treatment regimes: always amitraz, always spinosad, or rotation between amitraz and spinosad.

Using DNA testing, they found that treatment witn amitraz increased the frequency of a particular gene mutation while increasing the prevalence of amitraz-resistance. They conclude that polymorphisms in the RmβAOR gene are likely to confer resistance to amitraz.

This research could lead to new genetic tests for resistance to assist farmers in making tick-control decisions. Better understanding of the mode of resistance also could enable empirical studies on field and laboratory populations of ticks to test the effectiveness of resistance management strategies, according to the Phys.org article.