Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 15, 2008
Publication Date: September 7, 2008
Citation: Miller, R., Li, A.Y., Tijerina, M.A., Davey, R.B., George, J.E. 2008. Differential response to diazinon and coumaphos in a strain of Boophilus microplus (Acari: Ixodidae) collected in Mexico. Journal of Medical Entomology. 45(5):905-911. Interpretive Summary: Organophosphate (OP) resistance has been developing in Mexico since the 1980s and is a threat to the integrity of the Cattle Fever Tick Eradication Program (CFTEP) because the CFTEP relies on the OP pesticide, coumaphos, for the eradication of tick outbreaks discovered in Texas. We describe a strain of ticks collected close to Texas in Nuevo Leon, Mexico, that is resistant to two types of OP pesticides, coumaphos and diazinon, not previously reported on in the literature. Additionally, we show that the use of an acetylcholinesterase (AChE) inhibition assay will rapidly detect resistance to both of these OP pesticides. This study informs USDA, APHIS inspection personnel working in the CFTEP the potential OP resistance patterns cattle fever ticks are capable of developing. This allows them to plan in response to the potential risks associated with the use of OP pesticides to eradicate ticks on cattle. Also, the potential for a rapid OP resistance diagnostic test is validated in this study, which, if further developed, will allow for rational decisions on the most efficient means to eradicate cattle fever tick outbreaks within the United States.
Technical Abstract: Boophilus microplus, collected from Nuevo Leon, Mexico were found to be highly resistant to diazinon but not highly resistant to coumaphos, suggesting that different mechanisms of resistance were present in these ticks than other Mexican organophosphate (OP)-resistant ticks reported previously. When exposed to coumaphos and PBO or TPP, the LC50 estimate was reduced by 3.5 and 6.3 fold, respectively, suggesting that mono-oxygenases and/or esterases were involved in resistance to coumaphos. Additionally, it was determined that this strain had an AChE that was insensitive to the active form of coumaphos, coroxon taking at least 24 minutes longer to reach 50% reduction in AChE activity when compared to the susceptible strain. When exposed to diazinon, none of the synergists tested significantly lowered the LC50. However, it was determined that it took 6 times longer to reach 60% inhibition of AChE in the resistant strain compared to the susceptible strain when exposed to the active form of diazinon, diazoxon. Insensitive AChE seems to be very common in OP resistant B. microplus. The potential benefits for the development of a field-portable AChE inhibition assay kit are discussed.