Verbenone and green-leaf volatiles reduce whitebark pine mortality in a northern range-expanding mountain pine beetle outbreak

Abstract

This is the first study testing the effectiveness of semiochemical treatments to protect individual trees from a range-expanding mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins, 1902) attack into newly exposed host populations of endangered whitebark pine (Pinus albicaulis Engelm.). We investigated the effectiveness of a combination of verbenone and green-leaf volatiles (GLV) to protect rare and valuable disease-resistant trees during a MPB epidemic from 2015 to 2018 in Jasper National Park, Canada. Treatments reduced the proportion of trees attacked by MPB for all diameter classes, across all stands, from 46% to 60%. We also evaluated the effect of the exotic disease white pine blister rust (caused by the fungus Cronartium ribicola J.C. Fisch.) — the species’ other main regional threat. MPB were less likely to attack large rust-infected trees than healthy trees, emphasizing the value of the semiochemical treatment. Protecting large cone-bearing disease-resistant whitebark pine trees is fundamental to whitebark pine recovery. Maintaining reproductive trees on the landscape increases the frequency and diversity of rust-resistant genotypes more effectively than just planting seedlings to replace MPB-killed trees, because this slow-growing species takes over 80 years to reproduce. Our study confirmed that protecting large rust-resistant trees with verbenone and GLV is a proactive and effective treatment against MPB for whitebark pine in naïve populations.