Submitted to: Pesticide Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 3, 2004
Publication Date: March 1, 2005
Citation: Li, Andrew Y., Pruett, John H., Davey, Ronald B., George, John E. 2005. Toxicological and biochemical characterization of coumaphos resistance in the San Roman strain of Boophilus microplus (Acari:Ixodidae). Pesticide Biochemistry and Physiology. 81:145-153. Interpretive Summary: The organophosphate acaricide coumaphos is the only registered acaricide that can be used in dipping vats at USDA cattle importation facilities along the U.S.-Mexican border to eliminate the southern cattle tick, Boophilus microplus (Canestrini), from cattle that are for importation from Mexico to the U.S. The southern cattle tick in Mexico has developed resistance to almost all major classes of acaricides in recent years. Resistance to acaricides, particularly to coumaphos, poses a major threat to the continued success of the USDA Cattle Fever Tick Eradication Program (CFTEP) that protects the U.S. cattle industry from the deadly tick-borne cattle fever disease. The research at USDA, ARS, Knipling-Bushland Livestock Insects Research Laboratory has focused on elucidation of tick resistance mechanisms and development of resistance detection and mitigation technology. The objective of this study was to determine the mechanisms of resistance to coumaphos in a resistant tick strain collected from Mexico. Bioassays with synergist and biochemical assays with acetylcholinesterase (AChE) revealed the existence of a resistant mechanism that involves both an oxidative metabolic detoxification mechanism and an insensitive AChE in the resistant tick strain. The results from this study set the foundation for further isolation and characterization of the specific oxidative enzymes that contribute to coumaphos resistance in B. microplus.
Technical Abstract: The San Roman strain of the southern cattle tick, Boophilus microplus collected from Mexico had a high-level resistance to the organophosphate acaricide coumaphos. Larval packet bioassays of coumaphos with and without piperonyl butoxide (PBO), an inhibitor of cytP450s, on larvae of this resistant strain and a susceptible reference strain resulted in reduced toxicity of coumaphos in the susceptible strain and enhanced toxicity in the resistant strain. The results indicated the existence of an oxidative detoxification mechanism in the resistant strain. Bioassays using coroxon with and without PBO resulted in differential synergism of coroxon toxicity in those tick strains. The synergism ratio of PBO on coroxon in the resistant strain was 4.5 times of that in the susceptible strain. The results suggested that the activity of the cytP450-based metabolic detoxification was enhanced in the resistant ticks. Comparisons of AChE activity and enzyme inhibition kinetics by coroxon in both susceptible and resistant strains revealed that the resistant San Roman strain had an insensitive AChE, with a reduced phosphorylation rate, resulting in a reduced bimolecular reaction constant. Both insensitive AChE and an enhanced activity of a cytP450-based metabolic detoxification mechanism contributed to coumaphos resistance in the San Roman strain of B. microplus.