6 BULLETIN 205, -U. S. DEPARTMENT OF AGRICULTURE. 
During the second season the larva merely maintains and enlarges 
the established chamber cr tunnel, and, growing in size, as far as 
was possible to ascertain, moits the first time when 1 year old. The 
third season is passed like the second, the larva molting once again 
when two years old. By the end of this period the covering pitch 
tube is about the size of a silver dollar, depending somewhat on the 
shape of the wound inside. 
The third spring after deposition of the eggs the larva, now nearly 
three years old, is ready to pupate. During the last two years the 
larva changes but little in size; the younger is somewhat more 
slender if of the same length as the older. The older larva, how- 
ever, has become so thick skinned that it appears almost entirely 
white, while through the thinner skin of the younger generation the 
veddish intestines are still plainly visible. It is not a simple matter 
and requires a great deal of experience to separate these two genera- 
tions. 
With the exceptions noted for high altitudes and late oviposition, 
exactly three years after the egg was laid the adult appears, complet- 
ing the life cycle and making the generation of the species triennial. 
Although there are no seasons in which this insect is very abundant 
there are none in which it is unusually scarce. 
HABITAT. 
Unlike others of this group of insects, the Douglas fir pitch moth 
prefers the shade. It is most numerous in from 10 to 50 year-old 
Douglas-fir stands with a northerly exposure, and is consequently 
most injurious there. So-called “spruce swamps” are as much 
avoided as are the sun-exposed hillsides. While it may also be found 
to some extent on sunny slopes having a stand of trees sufficiently 
dense to provide practically constant shade, it is usually only trees 
which have been injured by some other cause which are here infested 
first. The insect 1s evidently attracted here from the preferred locali- 
ties by the smell of pitch, just as barkbeetles are attracted by the 
smell of felled or fire-scorched timber. ! 
It may be noted here that larve under pitch tubes which are much 
exposed to the sun are almost invariably killed during the winter 
months. Evidently the larva can not survive when kept active by 
the warmth of the sun while its sustenance is cut off by frost. This 
may probably explain why this insect is not numerous under condi- 
tions which expose it much to the influence of the sun. The pitch 
tubes of Ses?a novaroensis and the bark of Douglas fir at the usual 
points of infestation do not provide the protection against the indi- 
cated influence as do, for example, the big pitch tubes and the bark 
of vellow pine for the larvae of Vespamima sequoia, which survive 
under any exposure, presumably on account of this protection. 
