6 BULLETIN 255; U. S. DEPARTMENT OF AGEICULTUEE. 



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, molts 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 

 reddish 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 is 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 larva? 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 Sesia 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 yellow pine for the larva? of Vespamima sequoia, which survive 

 under any exposure, presumably on account of this protection. 



