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 duration of horn fly Haematobia irritans irritans (L.), control measures in nonisolated cattle herds. Piecewise regression was used to model the duration of diazinon-impregnated ear tags and it indicated that these provided >90% reduction in horn fly numbers for 50 d. Control rapidly declined thereafter A logistic model was used to estimate the duration of efficacy of pour-on ivermectin. Ivermectin reduced adult numbers >90% for 8-16 d and >50% for 18-26 d. Ivermectin treatment reduced larval survival in dung >90% for 5-15 d and >50% for 19-24 d. Ivermectin treatment of animals already bearing diazinon-impregnated ear tags did not increase the level of control, but additively increased the duration of control. Average daily gains were not affected by treatment, possibly because horn fly populations were relatively low during the study.