Importance of grasshopper defoliation period on southwestern blue grama-dominated rangeland.

Abstract

Most economic assessments of grasshopper damage are based on how much plant tissue insects consume or destroy without considering factors that influence the ability of individual plants and communities to respond to damage. Properly grazed perennial warm-season grasses, such as blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Griffiths], can withstand considerable defoliation. We investigated the effects of heavy defoliation on blue grama rangeland by caging bigheaded grasshoppers [Aulocara elliotti (Thomas)] during early, mid, and late growing seasons for 2 years in southwestern New Mexico. Peak standing crop (PSC) of blue grama defoliated in the early-season was the same as that in cages protected from defoliation both years. However, peak standing crop of blue grama was reduced in cages defoliated during the mid and late growing seasons in both years. The importance of midseason feeding was compounded by significant changes in relative proportions of various herbage categories in the standing crop. Forbs and sedges made up a larger percentage of the total forage production at PSC after mid-season defoliation during both years. On rangelands where blue grama is dominant, even very high densities of early-season grasshoppers may not influence herbage production. Substantial declines in grasshopper densities observed before summer rains during both years should influence management decisions. Unless early-season forb production is an important part of a ranch management plan, the damage potential of early- and mid-season grasshopper species may be lower on southwestern rangelands where mid- to late-summer precipitation patterns occur than reported in the literature for other areas of the western United States due to later maturation of warm-season grasses. In most years, depending upon precipitation patterns, there may be adequate growth following heavy early-season herbivory to feed both livestock and grasshoppers.