DURATION OF EFFICACY OF DIAZINON EAR TAGS AND IVERMECTIN POUR-ON FOR CONTROL OF HORN FLY (DIPTERA, MUSCIDAE)

A 2-yr field study was conducted to develop a model to predict the dur ation of horn fly Haematobia irritans irritans (L.), control measures in nonisolated cattle herds. Piecewise regression was used to model th e duration of diazinon-impregnated ear tags and it indicated that thes e provided >90% reduction in horn fly numbers for 50 d. Control rapidl y declined thereafter A logistic model was used to estimate the durati on of efficacy of pour-on ivermectin. Ivermectin reduced adult numbers >90% for 8-16 d and >50% for 18-26 d. Ivermectin treatment reduced la rval survival in dung >90% for 5-15 d and >50% for 19-24 d. Ivermectin treatment of animals already bearing diazinon-impregnated ear tags di d not increase the level of control, but additively increased the dura tion of control. Average daily gains were not affected by treatment, p ossibly because horn fly populations were relatively low during the st udy.