unless it occurs in association with other defoliating species. It has combined with 
the forest tent caterpillar in heavily defoliating oaks in south-central Texas. 
The western tent caterpillar, M. californicum (Packard), occurs primarily in 
western America. However, spotted infestations also occur eastward across central 
Canada to Quebec. It has also been collected in New Hampshire, New York, and 
northwestern Minnesota (//49). Alder, serviceberry, willow, common chokecherry, 
birch, apple, plum, cherry, and quaking aspen are known to be attacked. Full-grown 
larvae are predominantly black and yellow or yellow-orange. The head is mottled 
blue-black and sparsely covered with fine yellow-orange setae. The dorsum of the 
abdomen is marked by a stripe formed by a series of elongate bluish-white, 
somewhat pointed dashes, one per segment. 
Eggs are laid in flat, clasping masses covered with brown or dark-brown 
spumaline. The tents are similar to those of the eastern tent caterpillar. The species 
is of little or no economic importance in eastern America. Larvae may be controlled 
with Bacillus thuringiensis (S63). 
The forest tent caterpillar, M. disstria Hubner, occurs throughout most of the 
United States and Canada and feeds on a wide variety of hardwoods (7/). In the 
North and West, quaking aspen is preferred; in the South, water tupelo, blackgum, 
sweetgum, and various species of oak are most heavily defoliated. The adult is 
stout-bodied, light buff-brown, and has a wingspread of about 25 to 37 mm. The 
forewings have two darker oblique bands near the middle (fig. 82A). Full-grown 
larvae have light-blue heads mottled with black and sparsely covered with fine, 
whitish setae. Each abdominal segment is marked dorsally with a yellowish-buff, 
keyhole-shaped spot which may be divided to form an anterior spot and a smaller | 
posterior spot (fig. 82B). The venter is blue-gray to dark gray, usually with a 
median spot on each segment, and often with a dark-gray area running the full 
length of the body between the bases of the legs. 
Winter is spent as a complete embryo within the eggshell and hatching occurs in 
the spring, about the time the buds on the host tree begin to swell. Young larvae 
feed on expanding buds; older ones devour the foliage, often completely defoliating 
the tree. During the first four to five instars, the larvae are gregarious. At first, all of 
those from one egg mass cluster on one leaf or one group of small expanding 
leaves. Later, they become more widely dispersed on surrounding foliage. Al- 
though this species is referred to as a tent caterpillar, it does not construct tents. 
However, larvae do lay down strands of silk along which they travel. They also form 
silken mats on the trunks or branches on which they congregate in masses to rest or 
to molt (fig. 82C). As they approach maturity, they tend to wander individually over 
the trees and other vegetation 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 that encircle twigs up to 37 mm in diameter. The eggs are 
cemented together and are coated with dark-brown spumaline. There is one genera- 
tion 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 1962, several general 
outbreaks, some of which covered thousands of square kilometers and lasted for 2 
to 6 years, occurred in the Northern United States and eastern and western Canada. 
Since the 1930’s several outbreaks have also occurred in Virginia, South Carolina, 
Mississippi, and Louisiana (577). Outbreaks occurred repeatedly in Alabama 
204 
