Differential Susceptibility of White Fir Provenances to the Fir Engraver and Its Fungal Symbiont in Northern California

The fir engraver, Scolyfus venrralis LeC., attacks white fir, Abies condor (Gord. and Glend.) Lindl., and other true firs, Abies spp., in western North America. The biology, attack behavior, and ecology of this bark beetle were recently summarized by Berryman and Ferrell(1988). During the summer flight season, the attacking beetles bore into the cambial zone of fir boles, introducing a pathogenic brown-staining fungus, Trichosporium symbioticum Wright. Resistant firs react to the invasion by forming a resinous necrotic wound in the phloem and outer sapwood, which contains the spread of the fungus and repels or kills the beetles. In such interactions, the tree usually survives. This reaction is less intense or absent in susceptible firs, resulting in reproductive success of the beetles, and severe damage or even death of the fir. Sporadic outbreaks of the fir engraver, associated primarily with droughts, have caused widespread mortality of true firs in nearly every decade of this century in western North America.