Orthopterans (primarily Anabrus simplex [Tettigoniidae] and Melanoplus grasshoppers [Acrididae]) show a striking attraction to P-addition plots on 25-year-old primary successional sites at Mount St. Helens, relative to N-addition or control plots. To attempt to determine effects of nitrogen (N) and phosphorus (P) on these insects’ growth and fitness, we conducted three sets of experiments in the Washington State University Vancouver (WSUV) greenhouse. We examined the growth response of adult and immature (nymph) grasshoppers of genus Melanoplus, when feeding on leaves in which we had manipulated K2HPO4 and KNO3 levels.
Three experiments were conducted for each life stage: Experiment 1 (NxP): a factorial manipulation of N and P; Experiment 2 (4P): four levels of P (control, low, medium, high); and Experiment 3 (Choice): animals composed their diet from shoots with supplemental N, P, or control. N and P levels were manipulated by soaking leaves of Hypochoeris , Plantago, and common grasses in water or solutions of 0.5%, 0.75%, and 1.75% P (4-P), or .02 M (NxP and Choice). Grasshoppers, in individual cages, were provided new leaf material every two days. Food ingested was estimated for each animal over the 14-18 days of each experiment. All analyses are of final mass, with initial mass as a random effect in mixed-effects ANOVA.
Several results pointed to positive effects of moderate P on growth and negative effects of high P on growth and survivorship, while the effect of NO3 was neutral or negative. In the NxP experiment, we found no effect of nutrient concentration on nymphal or adult growth. In the 4P experiment, we found negative effects of high- and medium-P treatments and a positive effect of low-P treatment on nymphs, indicating a higher optimal level of P than in available forage, but less than the higher levels we provided. For 4P adults, we found no treatment effect on females but a positive effect of medium P on males. In the Choice experiment, nymphs consumed more P- treated leaves, and grew much faster than on any of the 4-P experiment treatments. For Choice-experiment adults, there was a negative effect of N on growth of males but not females.
Our results suggest that grasshopper growth response to supplemental P potentially explains the attraction of orthopterans to P addition plots in primary succession; and contrast with those of other researchers who have focused on the effect of N or protein on growth.