Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/13/2006
Publication Date: 3/1/2007
Citation: Miller, R. J., Davey, R.B., George, J.E. 2007. First report of pemethrin-resistant Boophilus microplus (Acari: Ixodidae) collected within the United States. Journal of Medical Entomology. 44(2):308-315.
Interpretive Summary: The southern cattle tick, Boophilus microplus, is kept out of the United States by the successful execution of the Cattle Fever Tick Eradication Program administered by USDA, APHIS, VS. This program relies heavily on the human inspection of cattle within a quarantine zone along the Texas-Mexico border and the chemical treatment of tick-infested cattle. However, resistance to chemical pesticides has been developing in populations of Mexican southern cattle ticks for the past 25 years. This paper described the first characterization of a pyrethroid resistant population of southern cattle ticks collected within the United States. Experiments were preformed using several types of pesticides, chemical synergists, and gel electrophoresis to determine if metabolic enzyme activity was involved in resistance. It was found that enzymatic activity was not a major mechanism of resistance. However, a molecular genetics method (PCR) was used to determine that a mutation of the sodium channel gene was the major cause of resistance in this population. The results of this work benefit both the Cattle Fever Tick Eradication Program and cattle producers. Knowing the magnitude and mechanisms of pesticide resistance in southern cattle ticks present in the United States allows USDA APHIS VS personnel to choose the correct chemical needed to quickly and efficiently eradicate outbreak populations of ticks within the country. This is good for the producers for two reasons. 1) Cattle under quarantine can not be moved or sold; this is an economic burden on ranchers. 2) Quick and efficient eradication of southern cattle ticks from the United States keeps this damaging and disease caring pest away from the vast majority of the United States cattle industry. This saves money for producers and the American People. These results are good for science since it documents for the first time the occurrence, magnitude, and type of resistance found along the United States border as it is currently developing for the first time.
Technical Abstract: Boophilus microplus, collected in Hidalgo County, Texas, were determined to be resistant to permethrin. Discriminating dose (DD) tests at the LC99 and 2X the LC99 of susceptible ticks produced lower than expected mortalities for permethrin, but not for coumaphos or amitraz acaricides. Initial bioassay results confirmed the pyrethroid resistance detected in the DD assays. Two generations of selection with permethrin at a rate > 60%, increased the measured resistance ratios (RRs) from 9.5 (7.9-11.5) to 263 (217-320). Synergist studies did not implicate that metabolic enzymes were involved in permethrin resistance. Native gel electrophoresis verified that the CZS9 esterase was not involved in resistance to permethrin. PCR examination for the presence of a mutation of the sodium channel (Phe ' Ile amino acid substitution in the S6 trans-membrane segment of domain III), detected this mutation in the B&H population. The frequency of this mutation increased after selection with permethrin and concurrent increase in estimated RRs. The B&H population was eradicated from the United States by the USDA, APHIS, VS, Cattle Fever Tick Eradication Program through the use of the organophosphate acaricide coumaphos.