Cyclic epidemics, population crashes, and irregular eruptions in simulated populations of the mountain pine beetle, Dendroctonus ponderosae

The native Mountain Pine Beetle infests numerous native pine species in North America and can cause extensive mortality when populations enter an epidemic state. We used an agent based cellular model of coupled beetle and host tree populations to investigate the effects on long-term population dynamics of modifying three model components, representing factors that land managers have varying degrees of control over: number of host trees, health of host trees, and number of surviving beetle offspring. Within the parameter space, various behavior types emerged in the simulations: population crashes, regular endemic/epidemic cycles, and sporadic cycles. The largest, recurring epidemics occurred in simulations with dense populations of mostly vigorous trees and moderately high beetle offspring production. The fewest epidemics occurred with low beetle reproduction, and low tree population density. With all other factors held constant, reducing the tree population below a threshold reduced the proportion of cells experiencing beetle population epidemics. These results are consistent with field observations of reduced tree losses to beetle epidemics in thinned forest stands. The long-term simulations used in this study provide novel insights not captured by single-epidemic simulations, such as the fact that it was very difficult to maintain endemic populations for long periods, and that epidemics tended to be more erratic at higher tree densities.