In caged plot experiments in the native grasslands of the Interior of the province of British Columbia, feeding during the whole nymphal life of the grasshopper Camnula pellucida reduced the yield of Poa pratensis by 5.1 pounds per acre for each grasshopper per square yard. For each unit of infestation of 1 young adult grasshopper per square yard, 1.0 pound per acre per day was destroyed. The corresponding figures for Amphitornus coloradus on Stipa comata were 3.5 and 0.57 pounds per acre. First-instar nymphs of the migratory grasshopper Melanoplus bilituratus failed to survive when confined to S. comata; nymphs in subsequent stages, that had previously fed on other foods, did not thrive on S. comata, and each unit of infestation of 1 per square yard finally reduced yields not more than 2.6 pounds per acre. The results suggest that, where a single chemical control treatment is desired, it should not be delayed past the third instar as the modal developmental stage.