Evaluation of insecticide ear tags containing ethion for control of pyrethroid resistant Haematobia irritans (L.) on dairy cattle

A field study was conducted in central Argentina to evaluate the efficacy o f ear tags containing 36% ethion against pyrethroid resistant populations o f Haematobia irritans on grazing dairy cattle. The treated group consisted of 45 milking Holstein cows which received two tags per head and the contro l consisted of 22 dry cows from the same cohort. Treated and control groups were grazed on similar lucerne pastures separated for a minimum distance o f 800 m but they were not isolated from other cattle herds on the same or c ontiguous properties. In both groups, horn fly estimation were made weekly by examining cattle in the pastures with the aid of binoculars. The percent age reduction of fly numbers on treated cows was considered as efficacy of control provided by the tags. The ethion ear tags provided a range of 85-99 % reduction in horn fly numbers for 16 weeks. During this period, the weekl y mean number of H. irritans on the tagged cows ranged between 0.44 and 28. 26 compared with 143.5 and 239.1 in control animals. The ethion ear tags co uld be a useful tool for the control of H. irritans mainly in areas where p opulations of this insect have developed resistance to pyrethroid insectici des. (C) 2000 Elsevier Science B.V. All rights reserved.