Exploring stable pattern formation in models of tussock moth populations

1. The western tussock moth (Orgyia vetusta) at the University of California Bodega Marine Reserve (Sonoma County, California, USA) exhibits dense, localized populations in the midst of extensive habitats where variation in host plant quality or predator abundance is unable to explain the restricted extent of the outbreaks. 2. Two primary features suggest that the host patterning is intrinsically generated: (i) female tussock moths are wingless, producing a low effective dispersal distance for the hosts; and (ii) the tussock moth population is attacked by several species of widely dispersing wasp and fly parasitoids. 3. We consider a set of spatially explicit host-parasitoid models to examine whether intrinsically generated patterns are possible within this system. These models include a spatially extended Nicholson-Bailey model to examine general features of pattern formation in host-parasitoid systems, and two system-specific models, an individual-based simulation and a population-level analytic model to examine the details of this empirical system. 4. Both stable patterning and rapid extinction of the host population are initial-condition dependent outcomes of the general and specific models, implying that an intrinsically generated stable host pattern is feasible within the tussock moth system. 5. Stable patterning is enhanced by a large parasitoid-to-host dispersal ratio, local host resource limitation, and increased parasitism at the host patch's edge.