Rk. Lawrence et al., WHITE SPRUCE AND THE SPRUCE BUDWORM - DEFINING THE PHENOLOGICAL WINDOW OF SUSCEPTIBILITY, Canadian Entomologist, 129(2), 1997, pp. 291-318
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.