Effect of Swiss needle cast on Douglas-fir stem ethanol and monoterpene concentrations, oleoresin flow, and host selection by the Douglas-fir beetle

Douglas-fir growing on the western slopes of the Oregon Coast Range are experiencing an unprecedented outbreak of Swiss needle cast (SNC) caused by the fungus Phaeocryptopus gaeumannii. SNC can produce substantial physiological stress in host trees by reducing needle gas exchange and enhancing premature needle abscission, resulting in slower growth. Based on the frequent link between stressed trees and insect activity, we explored the potential influence of SNC on Douglas-fir beetle, Dendroctonus pseudotsugae, activity and some tree physiological parameters that may influence beetle attraction (i.e., constitutive ethanol and monoterpene contents of woody tissues) and host susceptibility (i.e., wound-induced resin flow). Woody tissue ethanol concentrations, wound-induced resin flow, and beetle attraction were all reduced as SNC severity increased. Although trees affected by SNC attracted fewer beetles, the number of attacks did not decline, the attacks were more likely to penetrate to sapwood depth, and the galleries were longer than in healthier trees, most likely due to a weakened oleoresin defense. However, there have been no current reports of increased Douglas-fir beetle activity on SNC stressed trees, and no rapid increases in beetle population numbers, or outbreaks associated with these diseased forests. SNC stressed trees may remain free from attacks because pioneering beetles have difficulty recognizing them as being stressed with low ethanol concentrations. Furthermore, beetle populations may not be increasing since stressed trees appear unsuitable for reproduction, as no eggs, larvae, or adult beetles were observed in excavated galleries on any attacked trees. However, if large volumes of host materials became available as a result of some catastrophic event (e.g. wildfire or wind-throw), and the beetles can reproduce successfully enough to increase population densities then the potential for a devastating outbreak of Douglas-fir beetle in SNC stressed trees might be exacerbated because they have compromised oleoresin defense systems, and may be killed with fewer beetle attacks.