Varying levels of resistance to pyrethroid cattle ear devices have been found in northern California.

The authors are grateful to the following, who assisted in the field tests: cattlemen in the various counties for use of their cattle and facilities; N. Martin, Staff Research Associate, UC Sierra Foothill Range Field Station; C. Paddock, R. Suther and C. Roudebush, Laboratory Assistants, Extension Veterinary Medicine, UC Davis. The authors also wish to acknowledge the assistance of G.P. Georghiou, Professor of Entomology, UC Riverside, and S.E. Kuntz, Research Leader, USDA Livestock Insects Laboratory, Kerrville, TX.