Cholinesterase Insensitivity: A Mechanism of Resistance in Mexican Strains of Boophilus microplus (Acari: Ixodidae) Against Coumaphos

Abstract

The acetylcholinesterases of three strains of larval Boophilus microplus (Canestrini) —one susceptible, the Escondido strain, and two resistant to coumaphos, the Tuxtla and Tuxpan strains, were studied. Initial studies showed that the total activity of acetylcholinesterase (AChE) of the susceptible strain was 4.5 and 7.6 times greater than that of the Tuxtla and Tuxpan strains, respectively. All three strains were more susceptible to chlorpyrifosoxon, the oxygen analog of chlorpyrifos, than to coroxon, the oxygen analog of coumaphos, in inhibition studies. The Tuxtla strain appeared to have at least two types of AChE, one susceptible to coroxon and the other resistant. Test results suggest that an altered AChE with decreased sensitivity to inhibition is an important defense mechanism used by this Mexican strain of B. microplus to develop resistance to coumaphos, and perhaps other anticholinesterase insecticides.