Bark Beetle Flight in Relation to Lodgepole Pine Host Chemistry: Another Case for Coevolution?

Mountain pine beetle (MPB) has devastated western North American forests from New Mexico to northern Canada over the past decade. Each year, female MPB initiate a new colonization by flying to a suitable host tree from late July to early August. The brief flight period occurs after the threat of spring freezing temperatures have subsided and yet early enough to provide suitable temperatures for oviposition. Water stress has been indicated as a factor that also influences beetle flight and host selection. Here we propose a third factor, host chemistry, for determining the timing of beetle flight. Previous research has shown that the host chemistry for lodgepole pine differs with altitude, temperature, humidity, and precipitation. Our research demonstrates the relationship between changes in the host chemistry across the growing season and at different plot densities and the MPB flight period. During the summers of 2008 and 2009, 24 healthy green trees were selected within 4 different density forest plots within the Colorado State Forest Park, Jackson County, CO. Bi-weekly, foliage was collected and terpene chemistry was analyzed by headspace solid phase microextraction and gas chromatography mass spectrometry. Of the 72 host compounds we tracked, alpha-pinene, myrcene, and 3-carene are known to attract MPB. Others are known deterrents (e.g. limonene) that have been shown to correspond with beetle flight. We show that MPB flight behavior corresponds with seasonal changes in host chemistry. Differences in host chemistry across a forest plot density gradient are also discussed. Alpha-pinene is shown to have the highest quantities in June and decrease by late July. The alpha-pinene pattern repeats regardless of plot density. Limonene and myrcene had the highest quantities in the lowest density plot, decreasing as plot density increased. In July, 3-carene quantities were the highest and then dropped by late July. Beta-pinene stayed relatively consistent across the season and across a gradient in plot density. An unknown compound, close in molecular weight to limonene, was found mostly in uncolonized trees. It had the highest quantities in the 2nd densest plot with no distinguishable seasonal pattern in quantity shifts. Our findings have implications for managing seasonal plot density characteristics as determined by shifts in host chemistry.