Dynamics of Haematobia irritans irritans (Diptera : Muscidae) infestation on Nelore cattle in the Pantanal, Brazil

From June 1993 to May 1995, horn fly counts were conducted twice a month on untreated Nelore cattle raised extensively in the Pantanal. Horn fly popul ation showed a bimodal fluctuation and peaks were observed every year after the beginning (November/December) and at the end (May/June) of the rainy s eason, which coincided with mid-late spring and mid-late fall. respectively . Horn flies were present on cattle throughout the year in at least 64% of the animals. Mean horn fly numbers on animals did not exceed 85 flies/cow d uring peaks and were under 35 flies/cow in most of the remaining periods. T he highest infestations (population peaks) were short and dropped suddenly within two weeks. Less than 15% of the animals in both herds could be consi dered as "fly-susceptible'' -showing consistently higher infestations, or " fly-resistant" -showing consistently lower infestations.