Mobile parasitoids may restrict the spatial spread of an insect outbreak

1. The western tussock moth Orgyia vetusta, a defoliator with flightless adult females, forms localized outbreaks within larger areas of its habitat. These outbreaks may persist for several (> 10) years with little tendency to spread. 2. According to reaction-diffusion models, spatial patterns in population density may arise from the interaction between a mobile predator and a less-mobile prey. This occurs because predators 'spill over' around the edges of prey populations, creating zones in which ratios of predators to prey are elevated. 3. To test this 'predator diffusion' hypothesis for tussock moths, we placed their eggs and larvae along a 500-m transect leading away from the edge of a tussock moth outbreak, through suitable but sparsely occupied habitat. We measured attack rates by a wasp egg parasitoid and four species of tachinid fly larval-to-pupal parasitoids on these experimental immature moths. 4. As predicted, rates of parasitism were elevated in the zone surrounding the outbreak. For some parasitoid species, the relationship between parasitism and distance from the outbreak was linear; for others it was parabolic. 5. These results are compatible with several explanations, including the predator diffusion hypothesis. Whatever the mechanism, the results suggest that parasitism restricts the spatial distribution and therefore the global abundance of the host insect in this system.