Verbenone decreases whitebark pine mortality throughout a mountain pine beetle outbreak

Abstract

Mountain pine beetles (Dendroctonus ponderosae Hopkins) have been killing pines on millions of ha throughout the western United States since 2000. One species being affected is whitebark pine (Pinus albicaulis Engelm.), a five-needle pine already experiencing a number of other threats. Whitebark pine is a keystone species providing a variety of values including watershed protection and food and habitat for wildlife. An increasingly used method of protecting pines from mountain pine beetles involves the antiaggregation pheromone verbenone, but no studies have evaluated the ability of verbenone applied annually to protect whitebark pines throughout mountain pine beetle outbreaks. We applied verbenone pouches annually for 7 years until an outbreak ended. Probabilities that whitebark pines survived through the end of the outbreak were 0.34 ± 0.15 for control trees and 0.68 ± 0.17 for trees treated with verbenone once per year. Evidence from a second verbenone treatment that was discontinued before the end of the outbreak suggested that applying verbenone twice, as opposed to once, per year may more effectively protect trees. Increased survival did not appear to vary with tree size (i.e., dbh). We believe increased survival of the magnitude we observed could reduce risks to threatened whitebark pine populations.