Evaluation of injectable abamectin to control natural infestations of Haematobia irritans (Diptera : Muscidae) in cattle

The effect of 200 mu g/kg of body weight of injectable 1% abamectin to cont rol Hematobia irritans (L.) was evaluated in heifers treated every 45 or 63 d versus control heifers. Fly numbers were estimated every 7 d with additi onal counts on days 1, 4, and 10 after treatment. Samples of feces were obt ained up to 28 d after treatment from treated and control heifers to cultur e H. irritans in the laboratory. The a irritans population reduction for 14 d after treatment ranged from 51% (3rd treatment of heifers treated every 45 d) to >73% in the Ist treatments of both groups of heifers and the 2nd t reatment of the group treated every 63 d. Medication every 45 d did not pro vide a better control than treatment every 63 d. The abamectin arrested the immature stages for 7 d after the Ist treatment; this period increased to 28 d after the 3rd treatment but its effect on heifer infestation was low, probably because of ny immigration. Abamectin may help maintain H, irritans populations below the threshold of economic damage.