The mountain pine beetle (Dendroctonus ponderosae Hopkins) is the most destructive insect that attacks lodgepole pine (Pinus contorta Dougl.), a species valued for multiple uses throughtout North America. The effective residual life of carbaryl, applied as a 2 percent suspension of Sevimol to the bark of lodgepole pine to prevent attack by mountain pine beetle, was evaluated near Dillon, Colorado. Trees (9,568) treated in 1982 under operational conditions were used to assess the efficacy of the treatment for one and two beetle flight periods after insecticide application. Estimated mortality of untreated trees was 0.91 percent compared with 0.074 percent for trees treated 16 months earlier with carbaryl. Residues of carbaryl were estimated at 359 ppm 16 months after application. When exposed to intense beetle pressure, bolts from trees treated 13 months earlier suffered fewer attacks and had shorter mean egg gallery length than did bolts from untreated trees; bolts from trees sprayed 3 months earlier suffered no attacks. Apparently a 2 percent suspension of carbaryl applied to the bole of lodgepole pine was effective in protecting lodgepole pine from mountain pine beetle for the flight period 3 months after application and even provided protection for a second flight period 16 months after treatment. I The results suggest that protection cost and I insecticide use could be reduced by 50 percent during a 4-year outbreak.