The dynamics of outbreaks: Further simulation experiments with the western tent caterpillar

Summary A stochastic model of western tent-caterpillar populations on southern Vancouver Island was used to assess conditions leading to outbreaks. Three qualitatively different populations-declining, minimal, and recovering-were exposed to six-year climatic sequences incorporating various combinations of good, bad, and mediocre spring weather. Starting populations were either distributed randomly or concentrated in and around small-, medium-, or large refuges. The results showed that virtually any type of population could temporarily increase in numbers during two highly favorable springs. Only a very few vigorous populations, however, could sustain the momentum required to surge to outbreak proportions. No low-quality population could do so. And even vigorous populations could not achieve outbreak status unless they were initially concentrated in a sufficiently large refuge that was close enough to potentially suitable habitats to permit rapid immigration into such places as soon as their local climates improved. The concept of “climatic release” of insect populations is discussed in the light of these findings. The results demonstrate that climatic release is inextricably linked with a population's qualitative profile and spatial distribution. Order of authorship determined by chance.