INTAKE AND EFFICACY OF METHOPRENE AND STIROFOS MINERAL BLOCKS FOR CONTROL OF HORN FLIES (DIPTERA, MUSCIDAE) ON PASTURED BEEF-CATTLE

Pastured herds of Shorthorn beef cattle, Bos taurus L., in western Min nesota were treated with different insecticidal mineral blocks during the summers of 1990 and 1991. Treatments were noninsecticidal blocks ( MoorMan's 646-B Special Range Mineral Block A), 0.02% methoprene block s (MoorMan's 650-B), and 0.5% stirofos blocks (Sweet-lix). Mineral int ake was monitored once or twice per wk. To evaluate efficacy, identifi ed animals from the three herds were commingled for 18 h in one pastur e to allow naturally occurring horn flies, Haematobia irritans irritan s (L.), to oviposit in the animals' dung pats. Just before adults emer ged, horn fly puparia were extracted to assess the effects of the inse cticidal minerals on the developing flies. Intake of all three blocks varied substantially among weeks but most often exceeded prescribed ra tes. Methoprene blocks consistently increased pupal mortality, but off setting increases in pupal densities negated effectiveness. Stirofos b locks did not increase larval mortality. Neither insecticidal block ef fectively prevented within-pasture production of horn flies under fiel d conditions.