Sex Ratios and Mating Status of the Horn Fly on Pastured Cattle as a Function of Repellent-Oil Treatments.

Haematobia irritans (L.) (Diptera: Muscidae) were netted from pastured cattle in California over two summers, with the goal of documenting on-host sex ratios and mating success as they might vary with repellent applications. Sex ratios were calculated from 100 adults per herd per date, and mating status of females (n = 30 per herd per date) was assessed by dissection of spermathecae. Flies were collected before, during, and after 2-wk treatment periods with each of two repellent-oil mixtures: a 15% mixture of fatty acids (C8, C9, C10) in mineral oil or a 2% solution of geraniol in mineral oil. During the treatment period, flies were collected 3-4 d after the prior treatment and immediately before the next treatment. A separate herd used in the second year was untreated. Overall, the proportion female was 0.65 ± 0.01 (mean ± SE) (range 0.28-0.81). Sex ratios were not altered over time by treatments with either repellent. Overall, proportion of females mated was 0.744 ± 0.018 (range 0.40-1.00). Mating status was significantly affected by the two repellent treatments, which did not differ from each other. For treatment herds, proportion mated before treatment was 0.789 ± 0.039, dropping significantly to 0.659 ± 0.038 during the treatment period, and rebounding to 0.750 ± 0.035 after treatments ceased. Treatments probably killed or repelled the existing heterogeneous fly population, and during treatment relatively more flies were unmated recent arrivals.