The Spell of the Rockies

by Enos A. Mills

Some of Nature's strange ways are exhibited in the interrelation of insects and fires in tree-killing. It is common for the attack of one of these tree-enemies to open the way for the depredations of the other. The trees that insects kill quickly become dry and inflammable and ready kindling for the forest fire. On the other hand, the injuries that green trees often receive from forest fires render them most susceptible to the attacks of insects.

This interrelation—almost coöperation—between these arch-enemies of the forest was impressed upon me during my early tree studies. One day I enjoyed a splendid forest sea from the summit of a granite crag that pierced this purple expanse. Near the crag a few clumps of trees stood out conspicuous in robes of sear yellow brown. Unable to account for this coloring of their needles, I went down and looked them over. The trees had recently been killed by insects. They were Western yellow pine, and their needles, changed to greenish yellow, still clung to them. In each clump of these pines there were several stunted or deformed trees, or trees that showed a recent injury. The stunted and injured trees in these clumps were attacked and killed by beetles the summer before my visit. In these injured trees the beetles had multiplied, and they emerged the following summer and made a deadly attack upon the surrounding vigorous trees. Although this latter attack was made only a month or two before my arrival, the trees were already dead and their needles had changed to a sickly greenish yellow. Amid one of these clumps was a veteran yellow pine that lightning had injured a few years before. Beetles attacked and killed this old pine about a year before I appeared upon the scene. It was the only tree in this now dead clump that was attacked on that first occasion; but some weeks before my visit the beetles in multiplied numbers swarmed forth from it and speedily killed the sound neighboring trees.

These conclusions were gathered from the condition of the trees themselves together with a knowledge of beetle habits. Not a beetle could be found in the lightning-injured pine, and its needles were dry and yellow. The near-by dead pines were full of beetles and their eggs; the needles, of a greenish yellow, were slightly tough and still contained a little sap.

While I was in camp one evening, in the midst of these tree studies, the veteran pine, now dead, was again struck by lightning. As everything was drenched with rain, there appeared to be no likelihood of fire. However, the following morning the old pine was ablaze. In extinguishing the fire I found that it had started at the base of the tree at a point where the bolt had descended and entered the earth. At this place there was an accumulation of bark-bits from the trunk, together with fallen twigs and needles from the dead tree-top. Thus a dead, inflammable tree in the woods is kindling which at any moment may become a torch and set fire to the surrounding green forest. Although fires frequently sweep through and destroy a green forest, they commonly have their start among dead trees or trash.

The pine beetle just mentioned attacks and burrows into trees for the purpose of laying its eggs therein. When few in number they confine their attacks to trees of low vitality,—those that will easily succumb to their attack. The speedy death of the tree and the resultant chemical change in its sap appear to be necessary for the well-being of the deposited eggs or the youngsters that emerge from them. When these beetles are numerous they freely attack and easily kill the most vigorous of trees.

The pine beetle is one of a dozen species of bark beetles that are grouped under a name that means "killer of trees." Each year they kill many acres of forest, and almost every year some one depredation extends over several thousand acres. The way of each species is similar to that of the others. The beetles of each species vary in length from a tenth to a fifth of an inch. They migrate in midsummer, at the time of the principal attack. Swarming over the tree, they at once bore into and through the bark. Here short transverse or vertical galleries are run, and in these the eggs are laid.

In a short time the eggs hatch into grubs, and these at once start to feed upon the inner bark at right angles to the galleries, extending to right and left around the tree. It does not require many of them to girdle the tree. Commonly the tree is dead in two months or less. All these little animals remain in the tree until late spring or early summer, when they emerge in multiplied swarms and repeat the deadly work in other trees.

The depredations of these insects are enormous. During the early eighties the Southern pine beetle ruined several thousand acres of pines in Texas. Ten years later, 1890-92, it swarmed through western North Carolina, Virginia, and West Virginia to southern Pennsylvania and over an area aggregating seventy-five thousand square miles, and killed pines of all species and ages, leaving but few alive. Within the past few years the mountain and Western pine beetles have ruined a one-hundred-thousand-acre lodge-pole pine tract in northeastern Oregon, destroying not less than ninety per cent of the stand. During the past decade the Black Hills beetle has been active over the Rocky Mountains, where in some districts it has destroyed from ten to eighty per cent of the Western yellow pines. In the Black Hills the forests over several thousand square miles are ruined.

These bug-killed trees deteriorate rapidly. In most cases a beetle-killed pine is pretty well rotted in five years and usually falls to pieces in less than a decade. Borers attack upon the heels of the beetles, and the holes made by the beetles admit water and fungi into the wood. This rapidly reduces the wood to a punky, rotten mass.

One day in Colorado I tore a number of wind-wrecked, bug-killed trees to pieces and was busily engaged examining the numerous population of grubs and borers, when some robins and other birds discovered the feast, collected, and impatiently awaited their turn. Perceiving the situation, I dragged a fragment of a log to one side for examination while the birds assembled to banquet and dispute.