and tree mortality, after about five years. In sustained outbreaks, 
nearly complete mortality of the merchantable volume of balsam 
fir may occur by the 8th year (55). During widespread outbreaks, 
the magnitude of these losses is great: It is estimated that 225 
million cords of pulpwood were destroyed in an outbreak which 
occurred in Quebec, New Brunswick, Maine, and northern Min- 
nesota from 1910 to 1920. 
Much has been learned about the factors or conditions condu- 
cive to the development and decline of spruce budworm outbreaks. 
For example, it was determined many years ago (700) that out- 
breaks 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 
(313). Much has also been learned about factors tending to hold 
populations in check during intervals between outbreaks or which 
assist in bringing them under control once they are underway 
(543). 
The spruce budworm has many natural enemies, including para- 
sites and predators, birds, mites, spiders, and several pathogenic 
organisms. Their effectiveness in control is always important, but 
it is greatest only after another agent, such as weather or starva- 
tion, has reduced the budworm population considerably (521). A 
number of authors have evaluated the control effectiveness of 
natural enemies in specific outbreaks (206, 207, 400, 483, 79, 80). 
Other important natural control factors affecting the spruce 
budworm are: (1) the loss of young larvae through competition 
for new foliage in heavy infestations; (2) loss of young larvae as 
a result of cooler than normal temperatures and late frosts in the 
spring; (38) loss of large larvae through starvation following 
complete defoliation; and (4) loss of adults through dispersal. 
A number of management practices have been suggested for 
reducing the chances of spruce budworm outbreaks. These in- 
clude: the utilization of balsam fir (700); the regulation of age 
classes to prevent the occurrence of large areas of overmature fir 
(310); and the favoring of less susceptible species, such as 
spruce. The removal of overstory mature balsam fir was suggested 
for preventing outbreaks (757, 496) ; management plans and risk- 
rating systems for selective cutting were presented (497, 498, 
758); and risk-rating systems for the Lake States were devised 
(45, 306). The budworm and forest management in the Maritime 
Provinces of Canada was discussed by Balch (23). 
For further information on the spruce budworm, the reader is 
referred to McKnight’s review of the literature (491) which 
covers 370 citations on the species and the two related species, 
Choristoneura occidentalis Freeman and C. biennis Freeman. 
The jack pine budworm, Choristoneura pinus Freeman, is 
known to occur in Nova Scotia, Ontario, Manitoba, and Michigan. 
Its hosts include various species of pine, especially jack and red. 
The adult has a wingspread of 15 to 28 mm. The head, thorax, and 
forewing are ochreous-tawny; the forewings, distinctly mascu- 
late; and the hindwings, entirely smoky with dark basal lines 
through their white fringes. The full-grown larva has a shiny 
light brown to black head, a dark brown “collar” separated from 
381 
