Ivermectin cattle pour-on: duration of a single late spring treatment against horn flies, Haematobia irritans (L.) (Diptera: Muscidae) in Missouri, USA.

Late spring treatment of cattle with a single dose of pour-on ivermectin (0.5 mg kg-1 body weight) resulted in reduced horn fly populations for approximately 6 weeks, with percentage efficacy exceeding 80% for at least 26 days post-treatment. Bioassay with house flies of dung from treated cattle demonstrated suppression of fly emergence for 11 days post-treatment. The subsequent increase in horn fly numbers on the treated herd was predicted by a developmental rate equation interpreted by the time of cessation of larval fly inhibition in field dung.