Mode of inheritance of amitraz resistance in a Brazilian strain of the southern cattle tick, Boophilus microplus (Acari: Ixodidae)

The southern cattle tick, Boophilus microplus (Canestrini), has developed resistance to amitraz in several countries in recent years. A study was conducted at the USDA Cattle Fever Tick Research Laboratory in Texas to investigate the mode of inheritance of amitraz resistance with cross-mating experiments. The Munoz strain, a laboratory reared acaricide-susceptible reference strain, was used as the susceptible parent and the Santa Luiza strain, originating in Brazil, was used as the resistant parent. A modified Food and Agriculture Organization Larval Packet Test was used to measure the levels of susceptibility of larvae of the parental strains, F-1, backcross, F-2, and F-3 generations. Results of reciprocal crossing experiments suggested that amitraz resistance was inherited as an incomplete recessive trait. There was a strong maternal effect on larval progeny's susceptibility to amitraz in both the F-1 and the subsequent generations. The values of the degree of dominance were estimated at -0.156 and -0.500 for the F-1 larvae with resistant and susceptible female parents, respectively. Results of bioassays on larval progeny of the F-1 backcrossed with the resistant parent strain and that of the F-2 generations suggested that more than one gene was responsible for amitraz resistance in the Santa Luiza strain. Comparisons of biological parameters (engorged female weight, egg mass weight, and female-to-egg weight conversion efficiency index) indicated significant differences between different genotypes. The differences appeared to be heritable, but not related to amitraz resistance. Results from this study may have significant implications for the management of amitraz resistance.