Submitted to: Pest Management Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/16/2007
Publication Date: 7/30/2007
Citation: Li, A.Y., Chen, A.C., Miller, R., Davey, R.B., George, J.E. 2007. Acaricide resistance and synergism between permethrin and amitraz against susceptible and resistant strains of Boophilus microplus (Acari: Ixodidae). Pest Management Science. 63(9):882-889. Interpretive Summary: The control of the southern cattle tick, Boophilus microplus (Canestrini), in Mexico and many other countries relies on chemical acaricides. This pest has developed resistance to all major classes of acaricides in recent years. To better understand the resistance and develop resistance management strategies that benefit both Mexican ranchers and USDA’s Cattle Fever Tick Eradication Program (CFTEP), we conducted a study on various tick strains at the USDA-ARS, Cattle Fever Tick Research Laboratory at Moorefield and Knipling-Bushland Livestock Insects Research Laboratory in Kerrville, Texas. We used larval packet bioassay techniques to determine levels of resistance to permethrin and amitraz, and used a molecular technique to detect a sodium channel mutation that is involved in permethrin resistance. The tick strains that were evaluated in this study had various levels of resistance to both acaricides, and a sodium channel mutation was detected in all but one resistant tick strains. We found that mixtures of permethrin and amitraz had an enhanced toxicity to tick larvae. The finding of the synergistic effect between these two acaricides is significant. New acaricide formulations based on the results of this study may be developed to control ticks that are resistant to both acaricides.
Technical Abstract: To better understand the resistance and develop resistance management strategies that benefit both Mexican ranchers and USDA’s Cattle Fever Tick Eradication Program (CFTEP), the FAO original larval bioassay technique and a modified version were used to determine levels of resistance to permethrin and amitraz, respectively, in five strains of the southern cattle tick, Boophilus microplus (Canestrini). To examine mechanisms of resistance to permethrin, the frequency of a mutated sodium channel gene was determined using a PCR assay. The tick strains from Mexico and Brazil demonstrated 49- to over 667-fold resistance to permethrin, and up to 93-fold resistance to amitraz. While the San Roman strain from Mexico was the most permethrin-resistant strain, the Santa Luiza strain from Brazil was the most amitraz-resistant strain. A significant correlation was found between the permethrin resistance ratio and the allelic frequency of the sodium channel mutation. Significant synergism between permethrin and amitraz was found when one acaricide was tested in the presence of another. Synergism ratios ranged from 1.5 to 42.8 when amitraz was tested as a synergist for permethrin. Similar synergism ratios were obtained when permethrin was tested as a synergist for amitraz. Permethrin caused virtually no mortality in the San Roman strain, even at the highest concentration (30% [AI]). Adding 0.1% [AI] amitraz to permethrin led to a dramatic increase in larval mortality, even at very low concentrations of permethrin.