Abstract The test was done on epidemic larch casebearer populations in young western larch stands near LaGrande, Oregon. Orthene was applied to needle mining larvae on Sept 9, 12, and 13 by a helicoptermounted conventional boom and nozzle system with 43 tee-Jet nozzles and no. 8002 flat fan spray tips. A mixture of 90% water and 10% ethylene glycol served as carrier. The formulation was sprayed at the rate of 1 gal/acre at 60 mph and 40 psi. Three plots were sprayed and 1 was left untreated in each of 3 blocks, but sequence of plot treatment within each block was randomly assigned. Plots were 50 acres in size and ranged from 4000 to 4800 ft elevation. C. laricella larvae were counted on 1 upper crown sample, 2 midcrown samples, and 1 lower crown sample collected from 15 trees in each of the 12 plots the day before spraying and 6 weeks after spraying. Data were transformed to number of larvae/100 fascicles of western larch needles.