Mountain pine beetle host-range expansion threatens the boreal forest

Abstract

The current epidemic of the mountain pine beetle (MPB), an indigenous pest of western North American pine, has resulted in significant losses of lodgepole pine. The leading edge has reached Alberta where forest composition shifts from lodgepole to jack pine through a hybrid zone. The susceptibility of jack pine to MPB is a major concern, but there has been no evidence of host-range expansion, in part due to the difficulty in distinguishing the parentals and their hybrids. We tested the utility of a panel of microsatellite loci optimized for both species to classify lodgepole pine, jack pine and their hybrids using simulated data. We were able to accurately classify simulated individuals, and hence applied these markers to identify the ancestry of attacked trees. Here we show for the first time successful MPB attack in natural jack pine stands at the leading edge of the epidemic. This once unsuitable habitat is now a novel environment for MPB to exploit, a potential risk which could be exacerbated by further climate change. The consequences of host-range expansion for the vast boreal ecosystem could be significant.

Fig. 1

Sampling locations for lodgepole pine, jack pine and hybrids across western Canada analysed at 12 microsatellite loci. MPB attack data from 1958 to 2009 for British Columbia and from 1975 to 2009 for Alberta are indicated in red (Thandi & Taylor unpublished data), and approximate pine volume is shaded in green, where maximum densities are over 500 m³/ha (Yemshanov & McKenney unpublished data). Top-right inset of North America illustrates the location of the Ontario/Minnesota samples. Range distributions for jack pine and lodgepole pine were obtained from USGS (http://esp.cr.usgs.gov/data/atlas/little/, accessed 29 July 2010) and are based on Little (1971).

Fig. 3

Work-flow for generating genotypes for simulations to assess the capability of 12 microsatellite loci to resolve the species identify of jack pine, lodgepole pine and their hybrids.

Fig. 2

Images from putative jack pine attacked by MPB in north-central Alberta. Left panel shows the red needles indicating tree die-off typical of trees one-year after MPB infection. Right panel shows a set of well-developed larval galleries, including a pupal chamber, indicating successful completion of larval development. Adult beetles from the parental generation (bottom left) occupy a different gallery. Images courtesy of James D. Weber, Canadian Forest Services.

Fig. 4

Ancestry plots from simulated lodgepole pine (tan), jack pine (blue), and various hybrid crosses (red for Newhybrids) generated in Newhybrids (top) and Structure (bottom).

Fig. 5

Ancestry plots generated in Newhybrids (top) and Structure (bottom) for 678 trees sampled across Alberta, British Columbia, Ontario, Minnesota, and Saskatchewan illustrating lodgepole pine (tan), jack pine (blue) and hybrid (red for Newhybrids) ancestry.

Fig. 6

Proportion of lodgepole pine (tan), jack pine (blue), and hybrids (red) at 24 locations in western Canada. Assignment to species categories was based on results from Newhybrids and Structure. Range distributions for jack pine and lodgepole pine were obtained from USGS (http://esp.cr.usgs.gov/data/atlas/little/, accessed 29 July 2010) and are based on Little (1971).