Shifting Outbreaks

SHIFTS TOWARD HIGHER latitudes and elevations in phenological synchrony between Douglas-fir budbreak and the emergence of western spruce budworm larvae were associated with a northern expansion of budworm outbreaks.  The increased impacts to northern Douglas-fir forests were associated with increased frequency of epidemic Douglas-fir beetle infestations; however, the link between defoliation and bark beetle infestation was complex.  Increased likelihood of Douglas-fir beetle infestation occurred when budworm defoliation was acute, but not chronic, suggesting that short-term impairment of host-tree defenses, before phloem resources are impacted, are necessary for Douglas-fir beetle outbreaks.