Mountain pine beetle host selection behavior confirms high resistance in Great Basin bristlecone pine

Abstract

Over the last two decades, mountain pine beetle (Dendroctonus ponderosae) populations reached epidemic levels across much of western North America, including high elevations where cool temperatures previously limited mountain pine beetle persistence. Many high-elevation pine species are susceptible hosts and experienced high levels of mortality in recent outbreaks, but co-occurring Great Basin bristlecone pines (Pinus longaeva) were not attacked. Using no-choice attack box experiments, we compared Great Basin bristlecone pine resistance to mountain pine beetle with that of limber pine (P. flexilis), a well-documented mountain pine beetle host. We confined sets of mountain pine beetles onto 36 pairs of living Great Basin bristlecone and limber pines and recorded beetle status after 48 h. To test the role of induced defenses in Great Basin bristlecone pine resistance, we then repeated the tests on 20 paired sections of Great Basin bristlecone and limber pines that had been recently cut, thereby removing their capacity for induced defensive reactions to an attack. In tests on cut trees, we also investigated the potential for population-level differences in mountain pine beetle host selection behavior by testing beetles from two separate geographic regions. Beetles placed on Great Basin bristlecone pine rarely initiated attacks relative to those placed on limber pine in both studies, regardless of the beetle population source. Our results indicate that Great Basin bristlecone pine has a high level of resistance to mountain pine beetle due at least in part to stimuli that repel pioneering attackers from initiating attacks, even when induced defenses are compromised.