needles on webbed branch tips turn reddish brown by midsummer. In heavy, 
persistent infestations, all of the new foliage may be consumed (fig. 59) for several 
successive years, and opening vegetative buds and developing shoots may be killed 
in their formative stages. Top-kill usually occurs after about 3 years of severe 
infestation, and tree mortality, after about 5 years. In sustained outbreaks, nearly 
complete mortality of the merchantable volume of balsam fir may occur by the 8th 
year (89, 799). During widespread outbreaks, the magnitude of these losses is 
great. 
F-492901 
Figure 59.—Balsam fir defoliated by the spruce 
budworm, Choristoneura fumiferana. 
Much has been learned about the factors or conditions conducive to the develop- 
ment and decline of spruce budworm outbreaks. For example, it was determined 
many years ago (//82) that outbreaks generally begin in extensive and continuous 
areas of mature and overmature balsam fir. Later, it was learned that even when 
these conditions prevail, at least three or four summers of clear, dry weather are 
necessary for populations to explode (502, 985, 1271). Much has also been learned 
166 
