Biological barriers to forest pest invasions: A novel host tree slows mountain pine beetle range expansion

TL;DR

This study reveals that the slower eastward expansion of mountain pine beetle into jack pine forests is mainly due to the beetle's difficulty in locating and attacking smaller, chemically less attractive jack pines, rather than reproductive issues.

Contribution

It identifies specific phenotypic traits of jack pine that impede beetle host-finding and attack success, providing new insights into factors limiting pest range expansion.

Findings

Beetle's difficulty in host-finding and attacking jack pine is the main cause of range expansion slowdown.

Jack pine's smaller size, thinner phloem, and lower monoterpene levels weaken chemical cues.

The risk of further eastward spread is reduced but remains uncertain due to ecological complexities.

Abstract

Following widespread outbreaks across western North America, mountain pine beetle recently expanded its range from British Columbia into Alberta. However, mountain pine beetle's eastward expansion across Canada has stalled unexpectedly, defying predictions of rapid spread through jack pine, a novel host tree. This study investigates the underlying causes of this deceleration using an integrative approach combining statistical modeling, simulations, and experimental data. We find that the slow spread is primarily due to mountain pine beetle's difficulty in finding and successfully attacking jack pine trees, rather than issues with reproduction or larval development. The underlying mechanism impeding beetle range expansion has been hypothesized to be lower pine volumes in eastern forests, which are primarily a consequence of lower stem density. However, our analysis suggests that jack pine's phenotype itself is the primary impediment. We propose that jack pine's smaller size, thinner phloem, and lower monoterpene concentrations result in weaker chemical cues during the host-finding and mass-attack stages of MPB's life cycle, ultimately leading to fewer successful attacks. These findings suggest a reduced risk of further eastward spread, but should be interpreted cautiously due to enormous policy implications and the inherent limitations of ecological forecasting.