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Use of acaricides for the control of Ixodes pacificus (Cooley & Kohls), the vector of
Lyme borreliosis in the western United States, can be a beneficial component in a program to reduce the morbidity of
Lyme borreliosis in California. Three commercially available acaricides, carbaryl, chlorpyrifos, and diazinon, were evaluated in laboratory bioassays for their effectiveness against I. pacificus adults. Based on bioassay results, chlorpyrifos and carbaryl were selected for field evaluations. Chlorpyrifos demonstrated the lowest LD50 in the laboratory and the best overall control in the field trials. Chlorpyrifos and carbaryl provided effective control with a residual effect on adult ticks up to 7 wk after a single treatment. A field application timed to coincide with the highest period of adult questing activity proved effective in the control of I. pacificus in a Sierran foothill habitat. Judicious use of either of the acaricides evaluated may help to reduce adult tick densities in a peridomestic environment and thereby reduce the risk of human exposure to
Lyme borreliosis and other tick-borne agents.