Most forest insect populations are regulated by a hierarchy of natural enemies (see figure). When densities are very low, generalist vertebrate predators with sigmoid functional responses often have the greatest impact on individual survival and, under certain conditions, may regulate the pest population at very sparse densities. Vertebrates may be most effective at regulating pest species whose larvae are camouflaged and exposed (birds) or that pupate on the forest floor (small rodents). Generalist arthropod predators and parasitoids may also be capable of regulating pest populations at sparse densities by switching from one prey species to another and aggregating in areas where prey are abundant. However, regulation by generalist enemies is tenuous because there will usually be a threshold density, above which the pest population can escape from the controlling influence of these enemies.

Once the pest has escaped from regulation at sparse densities by generalists, populations can grow rapidly towards outbreak levels. This rapid increase in the food supply for specialist natural enemies, especially insect parasitoids, may lead to buildup of their populations and the eventual suppression of the pest by their numerical responses (see figure). Because of time delays in the numerical response of natural enemies, this suppression of the pest population will normally go through a typical "boom and bust" cycle (4^th principle). Some species, which for one reason or another are not strongly affected by generalist predators (e.g., brightly colored, distasteful, or hairy species), may exhibit continuous cycles due to their interaction with specialist parasitoids.

Should the pest escape from the controlling influence of its generalist and specialist enemies and reach very high densities, then pathogen epizootics may be initialed (see figure), and these can suppress the pest population to very low densities where predators and parasitoids can once again exert their controlling influence. Of course, if all these natural enemies fail to terminate the outbreak, starvation will eventually cause the collapse of the pest population.

© 1998 Alan A. Berryman