PS 58-137
Modeling the population dynamics of jack pine budworm Choristoneura pinus in light of accelerating climate change
Deforestation is a global problem that can cause desertification, climatic changes, and displacement or extinction of plant and animal species. Insect outbreaks play a major role in forest destruction, as insect defoliation can turn boreal forests from carbon sinks into carbon sources. Effective intervention requires an understanding of the factors regulating outbreaks. We are combining mathematical modeling and field experiments to identify the mechanisms driving the cyclical dynamics of the jack pine budworm (Choristoneura pinus). By quantifying the effects of weather on key features of budworm biology and combining population models with down-scaled climate models, we will predict how climate change will affect budworm outbreaks. We collected data in jack pine budworm populations of variable densities at three sites in Wisconsin in 2012 and seven sites in Michigan in 2013. We recorded budworm density, rates of parasitization, measures of tree quality, temperature, precipitation, and forest composition. To isolate the effects of host-tree quality, we experimentally excluded parasitoids from larval budworms. We covered budworm-infested branches of 45 jack pine trees with cloth bags for 1-6 weeks and compared rates of parasitization and survival between insects on the protected and exposed branches.
Results/Conclusions
Our analyses show a positive correlation between budworm survival and density of pollen cones, but contrary to previous research, we found a parabolic relationship between survival and tree age, with decreased survival on both young and overmature trees. We also confirmed that, as with many defoliating insects, rates of parasitization are density-dependent and are likely an important driver of budworm outbreak collapse. Parasitoids caused up to 95% mortality in collapsing budworm populations in Wisconsin, while outbreak-level Michigan populations suffered 30-75% mortality from parasitism. We used our experimental parasitoid exclusion data to fit nonlinear models of budworm survival, using maximum likelihood to choose between models. The model including the parasitoid exclusion treatment explains the data better than the null model (ΔAIC=89). We expanded this within-season model to construct a preliminary population model including equations for the change in budworm, jack pine, and parasitoid population densities, and allowing for forest age structure. The model produces complex long-period, large-amplitude fluctuations that mimic long-term budworm population dynamics, but are strikingly different from the dynamics of host-parasitoid interactions alone. The model combines high-performance computing with field experiments, giving us insight into the complex interactions between defoliating insect outbreaks, parasitoid attacks, and host plant quality.