Local Food Depletion and the Foraging Behavior of a Specialist Grasshopper, Hesperotettix Viridis

Foraging behavior of the grasshopper Hesperotettix viridis was studied in a New Mexico grassland where it fed almost exclusively on the shrub Gutierrezia microcephala (Compositae). During its annual 4—mo feeding period, H. viridis progressively reduced the quality and quantity of edible host plant tissues. By midsummer, when most grasshoppers reached maturity, plants attacked by grasshoppers had significantly lower shoot water content, nitrogen content, and edible biomass per stem than plants experimentally protected from feeding damage. The timing and intensity of defoliation often varied strongly among nearby plants, but by the end of the summer all plants were seriously damaged. Using mark—recapture experiments, I examined how foraging of H. viridis was affected by plant spacing, grasshopper density, and the intensity of prior feeding damage to individual plants. Grasshoppers fed in short bouts and sampled different plants continuously; most animals found on a plant abandoned it within a day. The tenure of grasshoppers on plants tended to be shorter for plants growing in dense stands. Tenure was not affected by current grasshopper density on a plant, but did vary with a plant's history of feeding damage. Grasshoppers remained longer on plants that had been protected from herbivory than on plants that had suffered average or artificially enhanced levels of feeding damage. H. virdis nymphs grew more rapidly when reared on the protected plants. Animals confined for 7 d on high— vs. low—quality plants differed in their subsequent foraging behavior, suggesting that grasshoppers use information related to their recent feeding history to assess the quality of particular plants. Tenure of H. viridis was shorter on plants sprayed with a chemical deterrent (nicotine sulfate). However, grasshoppers preferred protected plants, even when sprayed with nicotine, relative to depleted plants exposed to natural herbivory. Gradual, spatially variable food depletion thus appears to be of key importance in the feeding biology of H. viridis. Selective foraging of grasshoppers for undamaged plants in turn tends to equalize the cumulative amount of feeding damage per plant. Extensive food depletion may reduce the amount of discrimination by herbivores against plants containing chemical deterrents.