Attraction of an eruptive herbivore, mountain pine beetle, to lodgepole pine vs. interior hybrid spruce as a function of insect population density and host availability

An outbreak of mountain pine beetle in British Columbia, Canada, has caused severe losses of lodgepole pine within 14 million hectares of forest. Lodgepole pine is the preferred host species of mountain pine beetle in British Columbia. However, there has been a recent increase in the occurrence of mountain pine beetle attack on interior hybrid spruce in areas of the province that have experienced the most severe lodgepole pine mortality. In this study we test the hypothesis that host specificity declines with increasing insect population pressure and decreasing host availability. We examined the attraction of mountain pine beetle to pine vs. spruce logs within central British Columbia as a function of within-stand insect population pressure, and pine vs. spruce availability in the immediate local habitat. The importance of these factors on mountain pine beetle host selection were determined under both simulated “pioneering” and mass-attack conditions. Our results indicate that pioneering mountain pine beetle are highly discriminating towards the preferred host, lodgepole pine, and are not influenced by the availability of spruce within a stand. However, at higher population densities (i.e., during mass-attack) mountain pine beetle displayed evidence of preferring the species of host that was most prevalent within a stand. Moreover, at higher population densities the insects appear to shift their preference toward pine hosts, suggesting that host specialization may actually be enhanced at high densities. Increasing host fidelity at high population densities may contribute to positive feedback in outbreak population dynamics.