Tree-Ring Analysis of Outbreak Dynamics across an Insect’s Entire Range: The Pandora Moth System

Abstract

In montane forests of the western United States, pandora moth (Coloradia pandora Blake) defoliates local pines, primarily ponderosa pine (Pinus ponderosa Dougl. Ex Laws.) and Jeffrey pine (Pinus jeffreyi Grev. & Balf.). This defoliation stresses the tree, leaving a distinct outbreak signature in the tree-rings. The occurrence of pandora moth outbreaks has been recorded in ponderosa pine tree rings in Oregon as far back as 1500 years, however little is known of the outbreak history throughout the rest of the pandora moth range. To gain a better understanding of the spatiotemporal dynamics of pandora moth I have reconstructed outbreaks across the entire range of the insect using 121 tree-ring chronologies from the International Tree-Ring Databank (ITRDB) and 19 chronologies from sites sampled for this study using dendrochronological techniques. I then created a fine resolution habitat model for pandora moth and a gridded tree-ring network of non-host chronologies, which was used to validate statistically confirmed outbreak events across the entire range of the insect. Sites with confirmed outbreaks were then tested against the habitat model for accuracy of model parameters. ArcMap 10 was used to create an animated map of the spatial and temporal distribution of pandora moth across its range. One of my most notable findings is the record of outbreak events further north than previously documented, and the agreement between those northern sites with my habitat model. In my research I have demonstrated the use of dendrochronology to study outbreaks across an insect’s range. The methods I have used here can be applied to other range-wide analyses.