first organophosphate-resistant strain of tropical cattle ticks 

 identified in Mexico. The Tuxpan strain, which had been in 

 the laboratory in Mexico for 20 generations, had undergone 

 extensive selection for resistance against coumaphos. 

 Therefore, when the Tuxpan strain was received at Mission, 

 research workers tested it against the flowable formulation of 

 coumaphos by treating Tuxpan strain-infested cattle in a 

 dipping vat. Groups of cattle infested with larval, nymphal, 

 and adult ticks of the Tuxpan strain were dipped in either 

 0.125, .02, or 0.3 percent coumaphos. For comparison, cattle 

 infested with a susceptible strain of tropical cattle ticks were 

 dipped in'O.1 25 percent coumaphos. Greater than 99 percent 

 control of the Tuxpan ticks was achieved with all three 

 concentrations of coumaphos, and the level of control with 

 the 0.125 percent treatment against the susceptible strain was 

 also 99 percent. It was concluded that in the laboratory in 

 Mexico, continuous selection pressure had not been 

 maintained against the Tuxpan strain each time it was cycled; 

 and, although some resistant genes were present, their 

 frequency in the laboratory population had diminished. Efforts 

 to obtain a more homogeneous colony of Tuxpan ticks is 

 underway and further evaluations of this strain will be 

 postponed until that goal is achieved. 



Tests of several acaricides, which may be alternatives to 

 presently available organophosphate acaricides, are 



scheduled for evaluation in the near future; and next year 

 information should be available on the results of this activity. 

 Research to determine the biochemical mechanisms of 

 acaricide resistance is focusing on metabolic factors, altered 

 permeability of the tick cuticle, and insensitive 

 cholinesterases as explanations for the altered behavior of 

 the resistant ticks. Preliminary evidence indicates that the 

 ticks in the Tuxtia strain have an altered cholinesterase as 

 well as some enhanced ability metabolize coumaphos. 



All research at the Cattle Fever Tick Research Laboratory is 

 not restricted to the problem of acaricide resistance. A 

 considerable effort is being made to further define the factors 

 that limit the survival and rates of growth of populations of 

 both B. Microplus and B. annulatus. The precise role of 

 white-tailed deer in the maintenance and distribution of 

 Boophilus annulatus is another aspect of the ecology of cattle 

 fever ticks that is currently receiving attention. Funding for a 3 

 year pilot test of sterile hybrid Boophilus eradication 

 technology is anticipated for fiscal year 1987. The study will 

 be performed on an approximately 160-acre facility on the 

 island of St. Croix, and will provide an opportunity to test the 

 feasibility of employing this technique as a component of an 

 eradication effort. 



