WHITE SPRUCE AND THE SPRUCE BUDWORM - DEFINING THE PHENOLOGICAL WINDOW OF SUSCEPTIBILITY

Synchrony of insect and host tree phenologies has often been suggested as an important factor influencing the susceptibility of white spruce , Picea glauca (Moench) Voss, and other hosts to the spruce budworm, C horistoneura fumiferana (Clemens) (Lepidoptera: Tortricidae). We evalu ated this hypothesis by caging several cohorts of spruce budworm larva e on three white spruce populations at different phenological stages o f the host trees, and then comparing budworm performance with host phe nology and variation of 13 foliar traits. The beginning of the phenolo gical window of susceptibility in white spruce occurs several weeks pr ior to budbreak, and the end of the window is sharply defined by the e nd of shoot growth. Performance was high for the earliest budworm coho rts that we tested. These larvae began feeding 3-4 weeks prior to budb reak and completed their larval development prior to the end of shoot elongation. Optimal synchrony occurred when emergence preceded budbrea k by about 2 weeks. Larval survival was greater than 60% for individua ls starting development 1-3 weeks prior to budbreak, but decreased to less than 10% for those starting development 2 or more weeks after bud break and thus completing development after shoot elongation ceased. H igh performance by the budworm was most strongly correlated with high levels of foliar nitrogen, phosphorous, potassium, copper, sugars, and water and low levels of foliar calcium, phenolics, and toughness. The se results suggest that advancing the usual phenological window of whi te spruce (i.e. advancing budbreak prior to larval emergence) or retar ding budworm phenology can have a large negative effect on the spruce budworm's population dynamics.