Origin of an insect outbreak: escape in space or time from natural enemies?

Initiation of insect outbreaks is poorly understood, and may involve sporad ic events that temporarily release insect populations from predation or par asitism. While studying a declining outbreak of the western tussock moth (O rgyia vetusta) on bush lupine (Lupinus arboreus), we witnessed the onset of a new tussock moth outbreak, separated by 1,000 m in space and 2 months in phenological timing from the original population. This new population unde rwent explosive growth for 2 years and then collapsed because of a massive die-off of lupines. We tested whether during its growth phase, this new out break benefited by escaping in either space or time from the natural enemie s attacking the original population. In experimental populations on single hushes, we compared predation and parasitism at the sites of the new and th e old outbreak. At the site of the old outbreak, we compared predation and parasitism early and late in the season. Parasitism was significantly lower and population growth significantly higher at the new outbreak site than t he old one. Neither seasonal timing, predator exclusion, nor their interact ion significantly affected survival at either site. Thus the new outbreak a ppeared to escape in space from parasitism. These results corroborate our p revious experimental findings, which suggest that as predicted by theory, t he interaction between the tussock moth and its parasitoids can produce lar ge-scale spatial patterning in population densities.