We conducted a laboratory study to evaluate the effects of body mass, environmental temperature, and food quality on phosphorus (P) efflux by caterpillars of the whitemarked tussock moth, Orygia leucostigma, J. E. Smith (Lepidoptera: Lymantriidae). We found that individual phosphorus efflux rate (Q the rate at which excreted and unassimilated P was egested in frass, mgP/day) was related to larval mass (M, mg dry) and environmental temperature (T,K) as Q = e^14.69 M^1.00e^-0.54/kT, where K is Boltzmann's constant (8.62 × 10^-5 eV/K, 1 eV = 1.60 × 10-19J). We also found that P efflux was not related to food phosphorous concentration, and suggest that this result was due to compensatory feeding by larvae eating low quality leaves. The P efflux model resulting from this analysis was simple and powerful. Thus, it appears that this type of model can be used to scale P flux from individual larvae to the population level and link species of insect herbivores to ecosystem processes.