one egg mass often cluster on one leaf or one group of small ex- 
panding leaves. Later, they become more widely dispersed on sur- 
rounding foliage. Although this species is referred to as a tent 
caterpillar, it does not construct tents. The larvae do lay down 
strands of silk along which they travel, however. They also form 
silken mats on the trunks or branches on which they congregate 
in masses to rest or to molt (fig. 127C). As they approach maturity, 
they tend to wander individually over the trees and other vege- 
tation in search of food or places to pupate. Pupation occurs in 
pale yellow cocoons spun in folded leaves, in bark crevices, on 
shrubs or other vegetation, and occasionally on buildings. Adults 
appear from late May in the South to late June and July in the 
North. Eggs are laid in masses of 100 to 350 in bands 25 to 37 mm. 
wide which encircle twigs up to 37 mm. in diameter. The eggs are 
cemented together and are coated with dark brown spumaline. 
There is one generation per year. 
The forest tent caterpillar has been an important enemy of 
forest, orchard, and shade trees for many years. During the 
period from 1886 to 1940, five general outbreaks, some of which 
covered thousands of square miles and lasted for several years, 
occurred in northwestern United States and eastern Canada. 
Since the 1930’s several outbreaks have also occurred in Virginia, 
South Carolina, Mississippi and Louisiana (356). In general, 
tree mortality has not been severe, but losses in reduced incre- 
ment following defoliation has been great. Studies in Minnesota 
indicate that about 70 percent reduction in basal area growth 
of aspen occurs during the first year of heavy defoliation. About 
90 percent reduction also occurs during the second year of heavy 
defoliation, plus about 15 percent reduction during the year of 
recovery. Total reduction for the 3-year period averages about 
58 percent (211). Heavy defoliation of bottomland gums in the 
South results in substantial mortality and dieback, as well as 
severe reduction in annual increment. In the Northeast, heavy 
defoliation in sugar maple orchards not only causes serious injury 
to the trees, but also a reduction in the quantity and quality of 
the sap. Outbreaks in recreational areas adversely affect busi- 
ness because of the nuisance created by migrating caterpillars 
and the midwinter appearance of defoliated trees during the 
tourist season. Rose (619) and Hildahl and Reeks (353) discuss 
the effects of defoliation on trembling aspen in Canada. 
Outbreaks usually subside after 3 or 4 consecutive years of 
defoliation. Several adverse environmental factors are responsi- 
ble for population declines. Freezing weather shortly after the 
eggs hatch may kill large numbers of larvae. Excessively high 
temperatures later in the spring may kill large numbers of adults 
and seriously reduce the viability of newly-laid eggs. Mortality 
in the late larval instars may be severe or complete as a result 
of starvation in heavily or completely defoliated stands. A poly- 
hedrosis virus disease sometimes kills enormous numbers of lar- 
vae in the late stages of outbreaks. The sarcophagid parasite, 
Sarcophaga aldrichi Park., often becomes extremely abundant 
during the late stages of outbreaks in the Lake States and greatly 
aids in their termination (355). The tachinid parasites, Zenilla 
332 
