Cyclic outbreaks of forest insects: A two-dimensional individual-based model

Abstract

An individual-based model is presented which shows that cyclic insects outbreaks in the forest (here presented as a certain type of the dynamics of total numbers of insects in the forest) can be a result of the joint action of four factors: (1) spatial structure of the system — resources, green tree biomass, used by larval insects are distributed in a two-dimensional space, (2) properties of trees — their limited resistance to feeding by insects, and regeneration ability after some time of relaxation, when they cannot sustain a local insect population, (3) insect properties — the most important of which is the ability of adult individuals to move between spatially distributed resources and to lay eggs on neighbouring trees, and (4) the type of dynamics of local insect population exploiting resources of a tree, which depends on individual variability of insects and their progeny production. Insects disperse from the original tree and occupy successive not infested trees over the forest, omitting the trees with exploited resources that need time for regeneration. The cyclic outbreak dynamics will take place when insects have opportunity to return to the places where there are trees with regenerated resources. In addition to the cyclic outbreak, we will observe permanent outbreaks (small fluctuations of insect, combined with a similar dynamics of tree biomass) or pulse outbreaks (an increase in the number of insects, followed by their extinction and leading to a temporary destruction of tree biomass). The type of the simulation output will depend on the values of the model parameters. Intermediate resistance of trees to insect feeding and their not very short regeneration time will lead to cyclic outbreaks. Additionally great number of dispersing insects will promote cyclic outbreaks. High degree of individual variability, high mortality of adult insects or small progeny production will produce favourable conditions for permanent outbreaks. Pulse outbreaks can easily occur when insect individual variability is small or progeny production is high.