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.