The task of locating and reporting insect infestations falls upon the Forest Ranger and other field men of the Forest Service. Since the Rangers are practically the only class of Forest officers that visit all parts of a National Forest during each field season, the Supervisor relies mostly on them to report upon insect infestations. In riding to and from his work, while on fire patrol, while going for mail and supplies, while attending to the timber, grazing and other business of his district, the Ranger does a good deal of traveling and covers practically every part of his district. These are good opportunities to watch for fresh outbreaks of insects, and the wide-awake, progressive Ranger never misses such chances. If he sees reddish-brown masses of pitch and sawdust on the bark of a tree he immediately recognizes it as the work of insects. Or perhaps he may see a pine or a spruce tree with all its needles turned yellow. He knows then that this tree was girdled by bark beetles very recently, probably during the previous summer. A tree whose needles had turned red would indicate to him that the infestation was more than a year old, since trees attacked in the spring of one year usually do not show the results until the following summer. These two stages are known by the trained entomologist as the "yellow-top" and the "red-top" stages respectively. The latter is followed by the "black-top" stage. In this stage, insect infested trees stand out very conspicuously as leafless, gray or black snags, and they tell the story of the work of bark beetles that happened years ago.
[Illustration: Figure 47. The first evidence of insect attack are the reddish brown pitch tubes on the bark. Lodgepole pine infested by the mountain pine beetle. Lassen National Forest, California. Photo by the author.]
[Illustration: Figure 48. The last stage of an insect-attacked tree. The tree is dead and the dry bark is falling off. Lassen National Forest, California. Photo by the author.]
Probably the first external evidence of the attack of a bark beetle upon living trees with normal green foliage, is the presence of pitch tubes upon the outer bark. These are small, reddish-brown (later becoming grayish-white) masses of pitch and sawdust, which exude from the small cylindrical entrance made by the adult beetle where it bores through the bark to begin its egg tunnel. Each tube represents the entrance of one or more of these beetles. But we must follow these egg tunnels further, to learn how the actual damage is done to the tree. As soon as the bark beetle has made its entrance through the bark, it starts to work up through the live bark and cambium of the tree, forming a tunnel but little larger than the diameter of the beetle, which is known as the egg gallery, These egg galleries vary in shape from straight to winding, and in length from ten to forty inches. As a rule, male and female beetles work together in one gallery, and the eggs are deposited along the sides of the gallery, often in little pockets. When the tunneling and egg-laying process of the adult beetles is completed, their activity ceases, and they are usually found dead at the upper end of their galleries. The larvæ hatch and begin their work by burrowing across the cambium at right angles to the egg galleries. The complete girdling of the cambium layer is not accomplished until the larvæ have completed their work, and the numerous larval galleries, by joining one another, form a complete gallery around the cambium of the tree, thus cutting off the food supply which is made in the leaves of the tree, from the lower portion of the tree, namely the roots. Since the roots cannot live without nourishment, the tree dies. As soon as the larvæ have completed their development they pupate. Later they develop into adult beetles. These adult beetles issue forth in swarms the following spring, to attack new trees.