Responses of Haematobia irritans to repellents applied to host cattle

Herds of beef cattle were housed in specific pastures, and their horn fly numbers were recorded during 3 summers in southern California and in eastern North Carolina. Each herd over time was sequentially untreated for at least 2 weeks, treated twice per week with one of two designated repellents (geraniol or a mixture of straight-chain fatty acids) for at least two weeks, and then untreated for at least two weeks. Control herds were totally untreated. Horn flies were counted visually on cattle twice weekly. Cattle fly defensive behaviors also were monitored to assist in evaluating irritation. Adult flies were collected and evaluated in several ways. First, heads were removed and processed for pterin, which reflects age. This showed us if average fly age (or survival) was impacted by the treatments. Second, designated flies were tested using a biochemical test to quantify hemoglobin (a measure of blood meal size). This showed us if flies were feeding normally. Third, females were dissected, crushing spermathecae to assess mating condition (virgin or mated) and examining fecundity (number of active follicles and stage of development per fly). In aggregate the data provide a uniquely intensive examination of horn fly population biology and how those populations respond to repellents which may or may not kill them. The repellents distinctly reduced horn fly numbers on cattle for 1-3 days post-treatment, but the level of  effectiveness varied with different trials and locations. Flies hit directly with the fatty acids were killed, while flies hit by geraniol recovered. Consecutive treatments suppressed fly numbers over time. In California embedded controls in a larger group of treated cattle allowed an examination of fly movement following spraying.