Unexpected spatial patterns in an insect outbreak match a predator diffusion model

An outbreak of the western tussock moth (Orgyia vetusta) with a sharp spatial boundary has been hypothesized to be the result of an interaction with mobile parasitoids. We report here a striking correspondence between the qualitative prediction of a general model for a mobile predator and a sessile prey and initial observations of the outbreak of western tussock moth. The model makes the counter-intuitive prediction that the prey species will have its highest population density at the edge of an outbreak, which corresponds to observations made of the western tussock moth. This is a striking pattern that should be looked for in other population systems.