CHANGES IN WESTERN WHEATGRASS FOLIAGE QUALITY FOLLOWING DEFOLIATION -CONSEQUENCES FOR A GRAMINIVOROUS GRASSHOPPER

We determined the effects of defoliation by a graminivorous grasshoppe r on the foliage quality of the C3 plant, western wheatgrass (Pascopyr um smithii [Rydb] A. Love). Additionally, we determined the effects of this defoliation upon the subsequent feeding of the graminivorous gra sshopper Phoetaliotes nebrascensis Thomas (Orthoptera: Acrididae). In field and greenhouse studies, graminivorous grasshopper herbivory alte red the quality of remaining western wheatgrass foilage. In the greenh ouse, severe (50% foliage removal) grasshopper grazing (638 grasshoppe rs/m2 for 72 h) resulted in decreased foliar nitrogen (- 12%), carbohy drate (- 11%) and water (- 2.5%) concentrations, and increased phenoli c concentrations ( + 43%). These changes were associated with decrease d adult female grasshopper mass gain, consumption rate, approximate di gestibility, and food conversion efficiencies. In the field, moderate (14% foliage removal) grasshopper grazing (20 grasshoppers/m2 for 20 d ays) led to a 10% reduction in foliar nitrogen concentrations. Foliage quality changes in the field were not associated with any reductions in grasshopper mass gain, consumption rates, food digestibility, or co nversion efficiencies. The results presented here are consistent with the hypothesis that defoliation leads to a reallocation of carbon and nitrogen compounds within the plant such that foliage quality for P. n ebrascensis is reduced.