Influence of selective treatment on Haematobia irritans infestation of untreated cattle

To reduce the use of insecticide treatments against Haematobia irritans we evaluated the impact of treating 15% of the bovines, with the greatest number of flies including bulls, with 40% diazinon ear tags, on the infestation of untreated cows. Horn fly susceptibility to diazinon was measured before and after treatment, and peaks of infestation were recorded. Three groups of Bradford bovines were evaluated: Group 1 (control untreated), Group 2 (15% treated) and Group 3 (control 100% treated). Weekly counts of horn flies were performed on the same animals for 78 days. Two peaks of infestation were recorded, and a higher number of horn flies occurred in the untreated control group than in the untreated cows of the selectively treated group throughout the entire period of the study, except for a single week. The horn fly field population was significantly more susceptible to diazinon than the reference susceptible strain both before and after insecticide treatment. In conclusion, treatment of 15% of the most infested animals from a herd, with 40% diazinon ear tags, quickly reduced horn fly infestations of the entire herd and may be a practical approach for horn fly control, reducing costs and chemical use.