A model for testing hypotheses of gypsy moth, Lymantria dispar L, population dynamics

A model for simulating long-term gypsy moth (Lymantria dispar L.) population dynamics in North America has been developed. Simulated ecological processes include larval dispersal, foliage consumption on different host trees, reproduction, and mortality due to natural enemies (virus, 4 guilds of parasitoids, and 2 guilds of predators). Population dynamics in several forest stands can be simulated taking into account the migration of gypsy moth and its natural enemies. The model fits well to the following observations and data: (1) quasi-periodic outbreaks at approximately 10-year intervals, (2) life-tables in different outbreak phases, (3) increased parasitism in artificially augmented gypsy moth populations, and (4) phase plots of population dynamics. Simulated survival curves are intermediate between those reported by J.S. Elkinton et al. and by J.R. Gould et al. It is shown that low-density gypsy moth populations can be stabilized by immigration from high-density areas without any density-dependent local processes. The model supports the following hypotheses about gypsy moth population dynamics: (1) density fluctuations of small mammal predators is the most probable synchronization factor in gypsy moth populations; (2) the outbreak frequency depends on the proportion of susceptible tree species and the density of small mammal predators; (3) bacterial insecticide can be applied less frequently for gypsy moth control than can a chemical insecticide of the same killing power because the bacterial insecticide spares invertebrate natural enemies; (4) the success of gypsy moth eradication programs depends on the initial gypsy moth population density and on the type of functional response of predators. An alternative form of population bimodality hypothesis is suggested, according to which there are two types of gypsy moth population dynamics: eruptive in susceptible stands and stable in resistant stands.