Persistent, localized outbreaks in the western tussock moth Orgyia vetusta: The roles of resource quality, predation and poor dispersal

1. On stands of the abundant perennial lupins Lupinus arboreus and L. chamissonis on the California coast, the western tussock moth Orgyia vetusta Bdv. occurs as only a few localized populations, which are locally dense enough to defoliate the host plants and yet do not appear to spread. 2. To determine what limits the spatial distribution of this insect, and therefore its global abundance, experiments were performed at three sites, examining the roles of host-plant quality, predator abundance, rates of predation and dispersal. 3. At no site did host quality, predator abundance or rates of predation vary spatially in a way that would explain the location of the outbreak. However, adult female tussock moths are flightless, and rates of long-distance aerial dispersal ('ballooning') by first-instar larvae were very low at all sites. Poor dispersal is the only consistent explanation for the limited distribution of the tussock moth.