For control of larvae of Hypera postica. DDT dusts in dosages of 0.5, 1, 2, and 4 lbs. of the active ingredient per acre were applied with power equipment to replicated 1-acre plots in a heavily infested commercial field. Adequate to excellent control was obtained with as little as 1 lb. of technical DDT per acre, but 0.5 lb. did not prevent economic damage. Treated plots yielded 25-44% more hay than did untreated plots. The avg. DDT residue on the cured hay varied from 9 ppm. on 1948 first-crop hay treated with 0.5 lb. of DDT/acre to 42 ppm. on 1947 first-crop hay treated with 4 lbs. of DDT/acre. DDT is not recommended for use on hay alfalfa at this time; because the hazard to man arising from the feeding of DDT-treated forage has not been fully appraised.