Detecting resistance to organophosphates and carbamates in the cattle tickBoophilus microplus, with a propoxur-based biochemical test

Rapid and sensitive detection of resistance to insecticides in arthropods i s needed. In the cattle tick, Boophilus microplus, resistance to a variety of acaricides is widespread. The most commonly used assay for resistance, t he larval packet test, takes at least two, but generally six weeks for a on e-host tick like B. microplus to complete and may take up to three months t o complete for three-host ticks. Here we describe a test for resistance to organophosphate acaricides that can be used on larvae and adult ticks which takes less than 24 hours. The test measures the difference in acetylcholin esterase (AChE) activity in homogenates of ticks in the presence and absenc e of propoxur, a carbamate acaricide. We found clear discrimination of orga nophosphate-susceptible and organophosphate-resistant adults with 100 mu M propoxur. AChE from susceptible ticks had almost no activity at this concen tration of propoxur whereas AChE from resistant ticks had 67% of its potent ial activity. AChE from heterozygote ticks could also be distinguished from AChE from homozygous-susceptible and homozygous-resistant ticks. This is t he first biochemical test for resistance to an acaricide. Rapid, sensitive tests like ours will allow resistance to organophosphates to be detected so on after it develops in the field, thus, the spread of resistance might be slowed and the useful life of acaricides extended.