Modeling nitrogen flux by larval insect herbivores from a temperate hardwood forest.

Herbivorous insects flux considerable amounts of nitrogen from the forest canopy to the soil in the form of frass. The amount of nitrogen fluxed varies depending on the characteristics of the herbivores, their food resources, and their physical environment. We used concepts from metabolic ecology and ecological stoichiometry to develop a general model of individual nitrogen flux via frass fall for moth and sawfly larvae from a temperate hardwood forest in northern Wisconsin, USA. We found that individual nitrogen flux (Q(N), mg N/day) was related to larval body mass (M(B), mg dry), short-term variation in environmental temperature (T, K), and larval nitrogen concentration (N(B), proportion dry mass) as Q(N) = e(25.75) M(B)(0.77) e(-0.83/kT) N(B)(-1.56), where k is Boltzmann's constant (8.62 x 10(-5) eV/K). We also found that larval nitrogen flux did not vary with the nitrogen concentration of food, and suggest that this was due to compensatory feeding by larvae living on low-quality leaves. With further work, models of individual N flux could be used to scale individual fluxes to population and community levels, and thus link the characteristics of insect herbivore communities with the flow of nitrogen through forested ecosystems.