Effect of plant nutrient supplementation on grasshopper fitness

Louise Wynn

Insect herbivores require elemental nutrients such as nitrogen and phosphorus in proportions not available in plants. Recent research into insect herbivores’ nutrient requirements has turned toward the need for phosphorus at different life stages for males and females. Generalist grasshoppers of the species Melanoplus sanguinipes must compose an adequate diet from deficient leaves. We hypothesized that the grasshoppers’ needs would vary by life stage and sex, with final masses related to their nutrient intake. We conducted leaf-nutrient-enrichment experiments on grasshoppers in the greenhouse, using final mass and mortality as correlates of fitness. The first experiment (4P) was a no-choice experiment that compared the grasshoppers’ final mass on four levels of P supplementation; it included a Choice experiment that let grasshoppers compose their own diets from among N- and P-supplemented leaves, which established optimal nutrient intake amounts. These experiments addressed responses at different life stages to sub-optimal foods. The other experiment was a no-choice factorial N x P supplementation experiment addressing how varying amounts of N and P affect grasshopper growth. Our N and P supplementation greatly increased leaf N and P content and changed nutrient stoichiometric ratios. Grasshoppers’ growth and survival declined with high levels of leaf P, regardless of sex or life stage, and were highest when they were allowed to compose their own diets. N addition had no effect on nymphs, and had a positive effect on adults in the absence of P addition. Females ate more and gained more mass than males in every experiment. Our results show that grasshopper fitness is associated with the elemental nutrient content of the leaves they eat. When given leaves with concentrations of N or P too low or too high, grasshoppers’ final weights were lower than when allowed to choose their foods from leaves of varying nutrient concentrations; allowed to choose, the grasshoppers composed diets with nutrient concentrations closest to an ideal diet, with higher final weights. Our results support ecological stoichiometry and geometric framework research showing insect herbivores maintain homeostasis in spite of nutrient deficiencies or excesses and limits to nitrogen and phosphorus needs.

Effect of plant nutrient supplementation on grasshopper fitness

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