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Modeling the population dynamics of jack pine budworm Choristoneura pinus in light of accelerating climate change

Project Abstract: 
Deforestation is a global problem that can result in erosion, desertification, climatic changes, and displacement or extinction of plant and animal species. Insect outbreaks play a major role in forest destruction, particularly in boreal forests, a role that is expected to increase in importance due to global climate change. Healthy boreal forests act as carbon sinks, but insect defoliation can turn forests into carbon sources during severe outbreaks. Over the next 50-100 years, mitigation of the effects of insect outbreaks will therefore be a major issue in forest management. Effective intervention requires an in-depth understanding of the factors regulating outbreaks. I propose to combine mathematical modeling and field experiments to identify the mechanisms driving the cyclical dynamics of the jack pine budworm (Choristoneura pinus), a defoliating insect native to Canada and the northern United States. By quantifying the effects of weather on key features of budworm biology and combining population models with down-scaled climate models, I will predict how climate change will affect budworm population dynamics in order to guide efforts aimed at mitigating the effects of outbreaks. The jack pine budworm (JPBW) feeds primarily on jack pine (<em>Pinus banksiana</em>), a major component of North American boreal forests. Interactions between the JPBW, its food source, and its parasitoids drive long-period cycles in JPBW abundance, leading to extensive defoliation and tree death during severe JPBW outbreaks. Parasitoids are suspected to play a key role in the collapse of outbreaks. Jack pine demography is influential because trees older than ~45 years are much more susceptible to damage by JPBW. The resulting dead stands provide fuel for forest fires, which release jack pine seeds from their serotinous cones and permit the rapid growth of new trees. The periodicity of insect outbreaks and the frequency of wildfires are therefore strongly interdependent. Intuiting the effects of climate change and wildfires on JPBW population dynamics requires a mechanistic model. For example, climate change resulting in a drier, warmer habitat might encourage fires even when only a small fraction of trees are dead, leading to more frequent but less severe JPBW outbreaks. Alternatively, environmental effects of climate change might cause fires to be more widespread and destructive, leading to more severe JPBW outbreaks that occur less frequently. I will determine which potential outcome is more likely by constructing a model of JPBW outbreaks that includes fire frequency, and by estimating the parameters of the model using experimental and literature data.
Years Active: 
Experimental Field Methods: To predict the effects of climate change, in my fieldwork I will determine whether and how parasitoid attack rates vary with JPBW density, and whether these rates are affected by tree quality. My first year of data collection strongly suggests that density-dependent parasitoid attacks can terminate outbreaks. From May to July 2012 I collected JPBW weekly in each of three populations with different JPBW densities, such that the sites were at least 10 km apart. At each site I collected 50 insects from 10 different trees at roughly 10m intervals, and I reared these insects until pupation or death. I measured tree diameter at breast height (DBH), a proxy for quality, and I noted other factors such as forest composition. Rates of parasitism by the three most common parasitoids&nbsp;caused 50-95% mortality across sites, while unexplained mortality was less than 15%.&nbsp; In combination with density-independent mortality,&nbsp;the result was the three study populations declined by 98%, 95%, and 99% respectively over the course of the larval period. Density data were collected early in the season while parasitization rates were collected at the height of the outbreak. AIC analysis showed that parasitoid responses were strongly density-dependent (AIC difference &gt;30 between model with density-dependence and model without), and suggested that tree quality may have an effect as well (second-best model included DBH). As these populations have collapsed, in 2013 I will move my field sites to central Michigan, where a new outbreak has been reported. This population will take roughly 3 years to collapse and will provide sufficient data for my project. I will continue to collect larvae from multiple sites as described above and I will also carry out experiments to determine parasitoid attack rates at low densities, by deploying larvae from JPBW outbreaks in non-outbreaking jack pine stands.