Exploring stable pattern formation in models of tussock moth populations

1. The western tussock moth (Orgyia vetusta) at the University of Californi a Bodega Marine Reserve (Sonoma County, California, USA) exhibits dense, lo calized populations in the midst of extensive habitats where variation in h ost 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 g enerated: (i) female tussock moths are wingless, producing a low effective dispersal distance for the hosts; and (ii) the tussock moth population is a ttacked by several species of widely dispersing wasp and fly parasitoids. 3. We consider a set of spatially explicit host-parasitoid models to examin e whether intrinsically generated patterns are possible within this system. These models include a spatially extended Nicholson-Bailey model to examin e general features of pattern formation in host-parasitoid systems, and two system-specific models, an individual-based simulation and a population-le vel analytic model to examine the details of this empirical system. 4. Both stable patterning and rapid extinction of the host population are i nitial-condition dependent outcomes of the general and specific models, imp lying that an intrinsically generated stable host pattern is feasible withi n the tussock moth system. 5. Stable patterning is enhanced by a large parasitoid-to-host dispersal ra tio, local host resource limitation, and increased parasitism at the host p atch's edge.