CONTROL OF LONE STAR TICKS ON CATTLE WITH IVERMECTIN

Pastured cattle were treated twice weekly with ivermectin either at 20 0 mu g/kg oral or 40 mu g/kg by injection. The numbers of lone star ti cks, Amblyomma americanum (L.), on the treated and the untreated group s within a single herd were observed for 4 wk. There was no significan t (P > 0.05) difference in the number of unengorged, small (4-6 mm), a nd medium (6-8 mm) female lone star ticks on the untreated control com pared with those treated orally. However, cattle receiving the oral tr eatment had significantly (P < 0.05) fewer large (8 mm) females than t he untreated cattle. Significantly (P < 0.05) fewer small, medium, and large female ticks were found on the injected cattle compared with th e untreated controls. Interestingly, there were significantly (P < 0.0 5) more unengorged females on the animals treated by injection than on those treated orally or left untreated. Animals treated orally had 4- 6 ppb ivermectin in their blood serum whereas those treated by injecti on had 13-15 ppb.