Mountain Pine Beetles Use Volatile Cues to Locate Host Limber Pine and Avoid Non-Host Great Basin Bristlecone Pine

Abstract

The tree-killing mountain pine beetle (Dendroctonus ponderosae Hopkins) is an important disturbance agent of western North American forests and recent outbreaks have affected tens of millions of hectares of trees. Most western North American pines (Pinus spp.) are hosts and are successfully attacked by mountain pine beetles whereas a handful of pine species are not suitable hosts and are rarely attacked. How pioneering females locate host trees is not well understood, with prevailing theory involving random landings and/or visual cues. Here we show that female mountain pine beetles orient toward volatile organic compounds (VOCs) from host limber pine (Pinus flexilis James) and away from VOCs of non-host Great Basin bristlecone pine (Pinus longaeva Bailey) in a Y-tube olfactometer. When presented with VOCs of both trees, females overwhelmingly choose limber pine over Great Basin bristlecone pine. Analysis of VOCs collected from co-occurring limber and Great Basin bristlecone pine trees revealed only a few quantitative differences. Noticeable differences included the monoterpenes 3-carene and D-limonene which were produced in greater amounts by host limber pine. We found no evidence that 3-carene is important for beetles when selecting trees, it was not attractive alone and its addition to Great Basin bristlecone pine VOCs did not alter female selection. However, addition of D-limonene to Great Basin bristlecone pine VOCs disrupted the ability of beetles to distinguish between tree species. When presented alone, D-limonene did not affect behavior, suggesting that the response is mediated by multiple compounds. A better understanding of host selection by mountain pine beetles could improve strategies for managing this important forest insect. Moreover, elucidating how Great Basin bristlecone pine escapes attack by mountain pine beetles could provide insight into mechanisms underlying the incredible longevity of this tree species.

Fig 1. Photographs of limber pine (Pinus flexilis) and Great Basin bristlecone pine (Pinus longaeva) forests (a) on Cave Mountain in east-central Nevada, and (b) in the Spring Mountains in southern Nevada.

These tree species co-occur in nearly equal abundance on and near the top of Cave Mountain (a), the dead trees (gray trees) are mountain pine beetle-killed limber pine. In the Spring Mountains (b), these tree species co-occur below dashed line, note many dead limber pine trees (gray trees), but a nearly pure stand of un-attacked bristlecone pine occurs above the dashed line.

Fig 2. Total volatiles emitted (ng per hour per gram, n = 15) by limber pine (Pinus flexilis) and Great Basin bristlecone pine (Pinus longaeva) at Cave Mountain, Nevada.

These tree species co-occur in nearly equal abundance at this site and many limber pines have been killed by mountain pine beetles whereas bristlecone pines have not been attacked. Note different scale for α-pinene.

Fig 3. Relative composition of volatiles emitted by co-occurring limber pine (Pinus flexilis) and Great Basin bristlecone pine (Pinus longaeva) at Cave Mountain, Nevada.

Compounds in the legend are listed from most abundant (top) to least abundant (bottom) emitted by Great Basin Bristlecone pine.

Fig 4. Behavioral responses of walking female mountain pine beetles (Dendroctonus ponderosae) to different odor sources in a Y-tube olfactometer.

We used chi-square statistical tests for comparison between the numbers for each choice. NR = no response after 10 minutes. Significant results shown in bold. N = total number of trials (including no responses).