2006 REU Student from Instituto Tecnólogico de Costa Rica, Costa Rica
Is phosphorus a limiting nutrient for nymphs of Melanoplus bivittatus (Orthroptera:Acrididae)?
Food can be highly variable in both availability and nutritional quality, especially for herbivores. Moreover, nutritional needs change depending upon physiological and biochemical requirements resulting from stress or developmental needs. Major nutrients for grasshoppers include N-based proteins/ free amino acids, and C-based carbohydrates/ sugars, as a source of energy, and from which lipids can be derived. While the effects of dietary nitrogen and carbohydrates on grasshoppers are well studied, the importance of phosphorus (P), a third major elemental category, remains unaddressed. Grasshopper performance studies using controlled diets and field-based estimates of herbivory on plants fertilized with nitrogen and phosphorus fertilizer were undertaken In the feeding study, fifth instar nymphs were fed artificial diets consisting of three different ratios of protein and carbohydrate, each crossed with three levels of phosphorus until final molt to the adult stage. In the field, leaf damage on the grass Andropogon gerardi and the forb Solidago missouriensis were estimated; grasshopper densities were measured in P- and N-fertilization plots. The interaction of P with the protein:carbohydrate diet treatment significantly affected the amount of diet consumed but not the amount of frass produced, suggesting a difference in diet assimilation. Growth rate (mass-increase/day) over the developmental stage increased with greater P consumption, possibly due to elevated metabolic rates or ARN transcription. High levels of P in the diet significantly delayed development. In the field, the interaction between phosphorus and nitrogen fertilization was significant with respect to foliar damage from insect herbivores (mostly grasshoppers). A higher grasshopper density (individuals/m2) was observed in all plots fertilized with nitrogen, and no effect of P-fertilization was detected. At the individual level, phosphorus can be limiting as expected. But, when scaling to the population level, other variables (i.e. food palatability) may change the behavior observed so that P-limitation is not readily observed.