Submitted to: Experimental and Applied Acarology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2005
Publication Date: 11/1/2005
Citation: Li, A.Y., Davey, R.B., Miller, R., George, J.E. 2005. Mode of inheritance of amitraz resistance in a Brazilian strain of the southern cattle tick, Boophilus microplus (Acari: Ixodidae). Experimental and Applied Acarology. 37:183-198. Interpretive Summary: Resistance to various acaricides, including amitraz, in the cattle fever tick Boophilus microplus has made the control of this ectoparasite of cattle and a vector of the cattle fever disease increasingly difficult in many countries, including Mexico and the United States. The emergence of resistance to amitraz and other acaricides in Mexican tick populations in recent years poses a major threat to the USDA’s Cattle Fever Tick Eradication Program. Compared to a highly resistant tick strain originated in Brazil, tick populations in Mexico demonstrated relatively low levels of resistance to amitraz. Knowledge on the mode of inheritance of amitraz resistance in B. microplus would enhance our understanding of dynamics of amitraz resistance development in Mexico and help formulation of new resistance mitigation strategies. The Brazilian tick strain offered a unique opportunity to study amitraz resistance in B. microplus, and what we found in this highly resistant Brazilian tick strain may also apply to ticks in Mexico. Therefore, a study was conducted at USDA ARS Knipling-Bushland Livestock Insects Research Laboratory in Texas to examine the genetic basis of amitraz resistance in this Brazilian tick strain. Data collected from this study indicated that resistance to amitraz was inherited as an incomplete recessive trait and more than one gene were involved.
Technical Abstract: 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. A modified Food and Agriculture Organization Larval Packet Test was used to measure the levels of susceptibility of larvae of the parental strain, F1, backcross, F2, and F3 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 F1 and the subsequent generations. The values of the degree of dominance were estimated at – 0.156 and – 0.500 for the F1 larvae with resistant and susceptible female parents, respectively. Results of bioassays on larval progeny of the F1 backcrossed with the resistant parent strain and that of the F2 generations suggested that more than one gene was responsible for amitraz resistance in the resistant 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.