Insects in the Forest

The big trees of California are never attacked by insects. This immunity is extraordinary and may be the chief characteristic that enables these noble trees to live so long. Unfortunately it is not shared by other species. The American forests are infested with thousands of species of injurious and destructive insects. These insects, like the forest fires, annually kill numerous forest areas, and in addition leave millions of deformed and sickly trees scattered through the living forest to impair and imperil it. After some general tree studies which have occupied odd times for years and extended through the groves and forests of every State and Territory in the Union, the conclusion has been forced upon me that the forests are more widely wasted by insects than by fire.

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.

Returning to the rotten logs for another grub-filled fragment, I paused to watch some wasps that, like the birds, were feasting upon these grubs. A wasp on finding a grub simply thrust his snout into the grub and then braced himself firmly as he bored down and proceeded to suck his victim's fluids. In throwing a log to one side I disturbed a bevy of slender banqueters that I had not seen. Instantly a number of wasps were effervescing round my head. Despite busy arms, they effectively peppered my face, and I fled to a neighboring brook to bathe my wounds.

While I was at a safe distance, cogitating as to the wisdom of returning for further examination of the logs, a black bear appeared down the opening. From his actions I realized that he had scented not myself but the feast in the log-pile. After sniffling, pointing, and tip-toeing, he lumbered toward the logs. Of course I was curious as to the manner of his reception and allowed him to go unwarned to the feast. Two Rocky Mountain jays gave a low, indifferent call on his approach, but the other birds ignored his coming. With his fore paw he tore a log apart and deftly picked up a number of grubs. All went well until he climbed upon the pile of wreckage and rolled a broken log off the top. This disturbed another wasp feast. Suddenly he grabbed his nose with both fore paws and tumbled off the pile. For a few seconds he was slapping and battling at a lively pace; then, with a woof-f-f-f! he fled—straight at me. I made a tangential move.

The hardwoods are also warred upon by bugs, weevils, borers, and fungi. The percentage of swift deaths, however, that the insects cause among the hardwoods is much smaller than that among the pines; but the percentage of diseased and slow-dying hardwoods is much greater. The methods of beetles that attack oaks, hickories, aspens, and birches are similar to the methods of those that attack pines and spruces. They attack in swarms, bore through the bark, and deposit their eggs either in the inner bark or in the cambium,—the vitals of the tree. The grubs, on hatching, begin to feed upon the tree's vitals. In this feeding each grub commonly drives a minute tunnel from one to several inches in length. Where scores of grubs hatch side by side they drive a score of closely parallel tunnels. Commonly these are either horizontal or vertical and generally they are numerous enough to make many complete girdles around the tree. Girdling means cutting off the circulation, and this produces quick death.

While these beetles are busy killing unnumbered millions of trees annually, the various species of another group of beetles known as weevils are active in deforming and injuring even a greater number. They mutilate and deform trees by the millions. The work of the white-pine weevil is particularly devilish. It deposits its eggs in the vigorous shoots of the white-pine sapling. The eggs hatch, and the grubs feed upon and kill the shoot. Another shoot bursts forth to take the place of the one killed; this is attacked and either killed or injured. The result is a stunted, crooked, and much-forked tree.

Borers attack trees both old and young of many species, and a few of these species with wholesale deadly effect. Birches by the million annually fall a prey to these tree-tunnelers, and their deadly work has almost wiped the black locust out of existence. Borers pierce and honeycomb the tree-trunk. If their work is not fatal, it is speedily extended and made so by the fungi and rot that its holes admit into the tree.

Trees, like people, often entertain a number of troubles at once and have misfortunes in series. A seedling injured by one insect is more likely to be attacked again, and by some other insect, than is the sound seedling by its side. Let a seedling be injured, and relays of insects—often several species at a time and each species with a way of its own—will attack it through the seedling, sapling, pole, tree, and veteran stages of its growth until it succumbs. Or let a vigorous tree meet with an accident, and like an injured deer it becomes food for an enemy. If lightning, wind, or sleet split the bark or break a limb, through these wounds some spore or borer will speedily reach the tree's vitals. In many cases the deadly work of parasitic plants and fungi is interrelated with, and almost inseparable from, the destructive operations of predacious insects. Many so-called tree diseases are but the spread of rot and fungi through the wood by means of an entrance bored by a borer, weevil, or beetle.

The bark of a tree, like the skin on one's body, is an impervious, elastic armor that protects blood and tissues from the poisonous or corrupting touch or seizure of thousands of deadly and incessantly clamoring germs. Tear the skin on one's body or the bark upon a tree, and eternally vigilant microbes at once sow the wound with the seeds of destruction or decay. A single thoughtless stroke of an axe in the bark of a tree may admit germs that will produce a kind of blood-poisoning and cause slow death.

The false-tinder fungus apparently can spread and do damage only as it is admitted into the tree through insect-holes or the wounds of accidents. Yet its annual damage is almost beyond computation. This rot is widely distributed and affects a large number of species. As with insects, its outbreaks often occur and extend over wide areas upon which its depredations are almost complete. As almost all trees are susceptible to this punk-producer, it will not be easy to suppress.

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.