The study of forest insects has not progressed far enough to enable one to make more than a rough approximation of the number of the important species that attack our common trees. However, more than five hundred species are known to afflict the sturdy oak, while four hundred prey upon the bending willow. The birches supply food to about three hundred of these predacious fellows, while poplars feed and shelter almost as many. The pines and spruces are compelled permanently to pension or provide for about three hundred families of sucking, chewing parasites.
The recent ravages of the chestnut-tree blight and the appalling depredations of the gypsy and brown-tailed moths, together with other evils, suggest at once the bigness of these problems and the importance of their study and solution. The insect army is as innumerable as the leaves in the forest. This army occupies points of vantage in every part of the tree zone, has an insatiable appetite, is eternally vigilant for invasion, and is eager to multiply. It maintains incessant warfare against the forest, and every tree that matures must run a gantlet of enemies in series, each species of which is armed with weapons long specialized for the tree's destruction. Some trees escape unscarred, though countless numbers are killed and multitudes maimed, which for a time live almost useless lives, ever ready to spread insects and disease among the healthy trees.
Every part of the tree suffers; even its roots are cut to pieces and consumed. Caterpillars, grubs, and beetles specialize on defoliation and feed upon the leaves, the lungs of the trees. The partial defoliation of the tree is devitalizing, and the loss of all its leaves commonly kills it. Not only is the tree itself attacked but also its efforts toward reproduction. The dainty bloom is food for a number of insect beasts, while the seed is fed upon and made an egg-depository by other enemies. Weevils, blight, gall, ants, aphids, and lice prey upon it. The seed drops upon the earth into another army that is hungry and waiting to devour it. The moment it sprouts it is gnawed, stung, bitten, and bored by ever-active fiends.
Many forest trees are scarred in the base by ground fires. These trees are entered by insects through the scars and become sources of rot and insect infection. Although these trees may for a time live on, it is with a rotten heart or as a mere hollow shell. A forest fire that sweeps raging through the tree-tops has a very different effect: the twigs and bark are burned off and the pitches are boiled through the exterior of the trunk and the wood fortified against all sources of decay. This preservative treatment often gives long endurance to fire-killed timber, especially when the trees killed are yellow pine or Douglas spruce. Many a night in the Rocky Mountains my eager, blazing camp-fire was burning timber that forest fires had killed forty and even sixty years before.
In forest protection and improvement the insect factor is one that will not easily down. Controlling the depredations of beetles, borers, weevils, and fungi calls for work of magnitude, but work that insures success. This work consists of the constant removal of both the infected trees and the dwarfed or injured ones that are susceptible to infection. Most forest insects multiply with amazing rapidity; some mother bark-beetles may have half a million descendants in less than two years. Thus efforts for the control of insect outbreaks should begin at once,—in the early stages of their activity. A single infested tree may in a year or two spread destruction through thousands of acres of forest.
Most insects have enemies to bite them. The ichneumon-fly spreads death among injurious grubs. Efforts to control forest-enemies will embrace the giving of aid and comfort to those insects that prey upon them. Bugs will be hunted with bugs. Already the gypsy moth in the East is being fought in this way. Many species of birds feed freely upon weevils, borers, and beetles. Of these birds, the woodpeckers are the most important. They must be protected and encouraged.
There are other methods of fighting the enemy. A striking and successful device for putting an end to the spruce-destroying beetle is to hack-girdle a spruce here and there in the forest at a season when the physiological make-up of the tree will cause it to change into a condition most favorable for the attraction of beetles. Like carrion, this changed condition appears to be scented from all quarters and afar. Swarms of beetles concentrate their attack upon this tree and bury themselves in it and deposit their eggs. The multiplied army will remain in the tree until late spring. Thus months of time may be had to cut and burn the tree, with its myriads of murderous guests. The freedom of the big trees from insect attacks suggests that man as well as nature may develop or breed species of trees that will better resist or even defy insects.
Insects are now damaging our forests to the extent of not less than one hundred million dollars annually. This we believe to be a conservative estimate. Yet these figures only begin to tell the story of loss. They tell only the commercial value of the timber. The other greater and higher values cannot be resolved into figures. Forest influences and forest scenes add much to existence and bestow blessings upon life that cannot be measured by gold.