Response of Needle-and-Thread and Western Wheatgrass to Defoliation by Grasshoppers

Abstract

Field and greenhouse studies were conducted to. measure changes in plant growth resulting from grasshopper defoliation. All data indicated that as grasshopper grazing intensity on needleand-thread grass increased, total root weight decreased. A greenhouse study with western wheatgrass showed that heavy grazing (80% removal of top growth) for a 16-day period reduced top growth 82%, root growth 85%, crown growth 81%, rhizome growth 180%, and depth of root penetration 4%. Field obserrations indicated that most grasshopper defoliation of needle-andthread grass and western wheatgrass occurs after seasonal growth has been completed. Most plants that benefit mankind also provide food and shelter for many species of insects. Grasshoppers have long been considered the major pests inhabiting rangeland throughout the western United States and Canada, and since they feed on most forage plants, especially grasses, they have the potential to increase to outbreak numbers in any year and at many locations (Hewitt 1977). Species distribution, biology, food and habitat preferences, and economic importance have been determined for most grasshopper species inhabiting western rangeland. However, very little is known about the effects of insect defoliation. The effects of clipping or grazing plants with livestock have been studied for several plant species, but the effects of grasshopper grazing have never been adequately documented. Grasshoppers lower the production of rangeland forage bydefohating plants, part of which is consumed and part falls to the ground as litter. The effect of this grazing or defoliation upon plant development depends upon the intensity, frequency, selectivity, and season of use. In this study, we measured changes in plant growth resulting from grasshopper defoliation. The development, vigor, and survival of needle-and-thread grass (Stipa comata Trin. & Rupr.) and western wheatgrass (Agropyron smithii Rydb.) were measured under different grasshopper density infestations in both the greenhouse and the field.