Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2003
Publication Date: 3/1/2004
Citation: LI, A.Y., DAVEY, R.B., MILLER, R., GEORGE, J.E. DETECTION AND CHARACTERIZATION OF AMITRAZ RESISTANCE IN THE SOUTHERN CATTLE TICK, BOOPHILUS MICROPLUS (ACARI: IXODIDAE). JOURNAL OF MEDICAL ENTOMOLOGY. 2004. v. 41. p. 193-200. Interpretive Summary: Resistance of the southern cattle tick in Mexico to coumaphos, the only acaricide approved for use in the dipping vats at the USDA's cattle importing facility, poses a major threat to the continued success of Cattle Fever Tick Eradication Program (CFTEP). Amitraz, a formamidine acaricide, has the potential to be used as an alternative to coumaphos based on its high efficacy in controlling ticks. Due to the increasing use of amitraz in Mexico in recent years, we suspect the existence of ticks resistant to amitraz in Mexico. A study was conducted to determine amitraz susceptibility in 15 strains of the cattle tick collected from four major cattle-producing areas in Mexico. Low-order (1.68- to 4.58 fold) resistance was detected in 12 of those Mexican strains. Our laboratory using an amitraz-resistant tick strain originated in Brazil, also demonstrated the potential of the southern cattle tick to develop high level (up to 154-fold) of resistance to amitraz. Results of synergist bioassays indicate that metabolic enzymes only play a minor role in amitraz-resistance. It is likely that a target site mutation is the major mechanism of resistance to amitraz. The potential for the cattle tick to develop a high level of amitraz resistance has serious implications on the strategies the USDA may adopt in the future to prevent B. microplus from re-entering the U.S. from Mexico through cattle importation.
Technical Abstract: Amitraz, a formamidine acaricide, plays an important role in the control of the southern cattle tick, Boophilus microplus (Canestrini), and other tick species that infest cattle, dogs and wild animals. Although resistance to amitraz in B. microplus was previously reported in several countries the actual measurement of the level of amitraz resistance in ticks has been difficult to determine due to the lack of a proper bioassay technique. We conducted a survey, using a newly reported technique which was a modification of the standard FAO larval packet test, to measure the levels of resistance to amitraz in 15 strains of B. microplus from four major cattle-producing states in Mexico. Low-order resistance (1.68- to 4.58-fold) was detected in 12 of those strains. Our laboratory selection using amitraz on larvae of the Santa Luiza strain, which originated from Brazil, achieved a 153.93-fold resistance at F6, indicating the potential for high resistance to this acaricide in B. microplus. Both triphenylphosphate (TPP) and piperonyl butoxide (PBO) significantly synergized amitraz toxicity in both resistant and susceptible tick strains. Diethyl maleate (DEM) synergized amitraz toxicity in one resistant strain, but had no effect on the susceptible strain while having minor antagonistic effects on two other resistant strains. Target site insensitivity, instead of metabolic detoxification mechanisms, might be responsible for amitraz resistance observed in the Santa Luiza strain, and possibly in other amitraz resistant B. microplus ticks from Mexico. The Santa Luiza strain also demonstrated high resistance to pyrethroids and moderate resistance to OPs. Multiple resistance shown in this strain and other B. microplus strains from Mexico poses a significant challenge to the management of B. microplus resistance to acaricides in Mexico.