FOUR BILLION FEET OF BEETLE-KILLED SPRUCE

N. D. WYGANT, ARTHUR L. NELSON.

Ten years or so ago the Engelmann spruce forests in the higher Rocky Mountains of Colorado were a sight to behold. They were a reservoir of unexploited virgin timber, the summer homes and playground of thousands of people who love the mountains. Tall, green, silent, majestic, these forests were a gift of God, an important asset to our natural wealth and welfare.

Today, on much of that ground stand millions of dead trees graceless, lifeless, valueless. They will stand there 20 years more, ghost forests and tragic evidence of how fast and silently a tiny insect can do its damage when once a combination of favorable factors brings about a sudden increase in its numbers.

THE INSECT is the Engelmann spruce beetle, the Dendroctonus engelmanni Hopk. Without the spectacular features of smoke or fire or explosion, but as devastatingly, the beetle built up its population, mostly in the inner bark of living trees, where it fed and bred. Those trees died; then new beetles emerged and attacked other trees. No person even suspected what was happening until the outbreak was well under way and approaching its peak. Then it was too late to do much: Between 1942 and 1948, 4 billion board feet of stumpage had been killed.

Four billion board feet can furnish lumber for 400,000 five-room frame houses. The value in standing trees is estimated at 8 million dollars. It might someday have been made into products valued at 200 million dollars. The insects were more destructive than forest fires in the 6 years, 16 times more timber was destroyed than was killed by fire in the past 30 years in the Rocky Mountain region.

And to those who love the mountains and the trees there was another kind of heartbreaking loss: Damaged for a generation were parts of our most beautiful National Forests, the White River, Grand Mesa, Routt, Arapaho, Uncompahgre, San Juan, and Dixie. On a large part of the White River National Forest, nearly all spruce of saw-timber size-2,900 million board feet of it was killed.

IN ITS ADULT STAGE, the Engelmann spruce beetle is a small, cylindrical, hard-shelled beetle, about one-fourth inch in length, about the size of an ordinary housefly. When the adults leave the dead trees and start to fly in June and July, they are reddish brown to black in color. They soon settle on recently felled or standing green trees and bore through the outer bark into the living inner bark. This attack extends over most of the lower main stem of the tree.

The beetles work in pairs of male and female, each pair raising separate broods. The female makes the entrance, followed by the male, and bores a tunnel between the bark and wood, which usually extends in a vertical direction and parallels the grain of the wood. This tunnel is known as the egg gallery. The eggs are laid in alternate groups along the sides of the gallery, and the galleries are packed with boring dust mixed with pitch. There are usually 3 to 4 groups of eggs and a total of about 125 eggs in each gallery. On the average there are from 6 to 8 such egg galleries for each square foot of bark.

When the eggs hatch in 3 or 4 weeks, the larvae feed on the succulent inner bark and cut mines that run at right angles to the egg gallery. This larval feeding continues through the late summer and fall. When winter arrives they are still in the inner bark, where they become dormant.

The following spring the larvae resume feeding. As summer advances they become mature, transform to pupae, and then into adult beetles. This new adult stage is reached by midsummer, and the beetles first start feeding on the inner bark of the tree in which they were reared. By August or September they appear to be mature. Then some of the beetles emerge and congregate under the bark around the base of the tree. Others remain under the bark where they developed. In both cases they rest quietly during the second winter, and when warm weather returns the following spring they are ready to take flight and attack other living trees.

While the development of the insect is going on, the trees that have been attacked die, usually by the end of the first season of the attack. Death of the trees is caused by the girdling action of the egg galleries and the larval mines and by a blue-staining fungus that permeates the sapwood and stops sap conduction in the tree. This fungus is carried by the beetles and is always found in trees that are successfully attacked by the insects. The foliage of Engelmann spruce does not change color until about a year after the trees are attacked; then it fades to a yellowish green and the needles drop within a short period.

IN NATURE many forces operate to keep beetle populations at a low level over long periods. There is always a high mortality during the brood-development period from eggs to new adults. The number that reach the full-grown larval stage has been found to average from 215 to 360 to the square foot of bark. By the time the new adult stage is reached and the beetles emerge, this average has been reduced to about 150 to the square foot. These averages vary widely. The larger trees tend to produce heavier emergence than smaller trees. Then, when the beetles take flight, they are exposed to all sorts of hazards from wind, weather, and birds.

The factors that tend to limit beetle populations are parasitic and predatory insects that feed on the broods while they are developing in the inner bark; woodpeckers, which locate the infested trees and chip off the outer bark to feed on the broods that are thus exposed; and good growth conditions in the spruce stands where young and vigorous trees predominate. Woodpeckers are especially effective; when they are abundant they destroy 43 to 98 percent of the brood.

Among the conditions that favor multiplication of beetle populations are windfalls and overmaturity of the spruce stands, since the insects prefer to attack the larger mature trees and produce heavier broods in them. Large bodies of windthrown timber provide highly favorable conditions for the build-up of beetle populations, because green trees that have been blown over have been seriously disturbed in their crown and root functions and can offer little resistance to the attacks of the insects. After strong populations have developed, they will attack green stands of spruce regardless of their condition, and heavy losses usually continue until natural control factors again gain the upper hand.

PREVIOUS OUTBREAKS of the Engelmann spruce beetle occurred in the Rocky Mountain region, but most of them were so long ago that their exact extent is not known. In 1907, A. D. Hopkins, of the Department of Agriculture, found evidence on the White River National Forest of an outbreak that occurred 20 to 25 years earlier. He also estimated that severe outbreaks occurred on the Pike National Forest about 1855 and on the Lincoln National Forest in New Mexico about 1890. An outbreak that killed nearly 100 percent of the spruce volume swept over the Aquarius Plateau in Utah between 1918 and 1928. A localized outbreak was reported in the northwestern part of Yellowstone National Park in 1937. Apparently none of these earlier outbreaks even approached in intensity and total volume of destruction the one that started in Colorado in 1942.

Circumstances beyond the control of foresters and entomologists caused this spectacular outbreak. Several factors made conditions favorable. In June 1939 a severe windstorm swept from the southwestern corner of Colorado, in a northeasterly direction, across the mesa-type plateaus in the State. On thousands of acres many of the shallow-rooted Engelmann spruce blew over. Many of the roots on the down side remained in the soil to keep the trees alive for several years or until the beetles made their attack. As a native species, the Engelmann spruce beetle was present in small numbers in decadent trees in the forest. Those down trees proved to be a fertile breeding place for them.

By 1942, this breeding material had been consumed and the beetles had built up great populations. With a previously unknown reproductive force, coupled with an apparent lack of activity of natural control factors, the beetles invaded the standing spruce. By 1943, when the infestation was first discovered, the number of infested trees was so great that control by destroying the insects with fire or insecticides was economically and physically impossible. The problem then became one of determining the extent and severity of the outbreak so as to prevent its spread into other spruce forests, salvaging the dead timber, and studying the life history and habits of the beetle as a basis for development of control measures.

PLANS WERE MADE in 1943 for a survey of the spruce type in Colorado to determine the extent of the outbreaks and their progress, what action should be taken, and whether spread of the insect into nonaffected areas could be prevented.

A person who has not visited the high Rockies in Colorado can hardly realize the difficulties of making a survey and carrying out control measures in the spruce forests. Engelmann spruce grows at elevations of 9,500 to 11,500 feet, generally in rugged terrain except for the forests on plateaus and in places where few roads have been made. Many of the areas can be reached only by a trail, and then often with as much as a full day's travel on pack animals from the end of a road. An attempt was made in 1944 to use an airplane to scout the forests and locate the incipient outbreaks, but the infestations could not be detected from above because of lack of foliage discoloration. Nor could incipient outbreaks be detected from lookouts and vantage points.

The technique finally developed to detect the infested and killed trees required sample lines to be run through the various areas and the trees viewed at close range. Such a survey has been made annually since 1944, with a crew of three to six men. In addition, a close watch for infestation has been kept by the forest rangers during their summer travels. Although coverage has not been so complete and thorough as one would like, a fairly accurate picture has been obtained of the progress of the outbreaks.

The spruce losses have been phenomenal for such a short period. The end of the losses on the White River, Routt, and Arapaho National Forests is not yet in sight. The surveys indicate a serious flight of beetles from the center of the White River National Forest outbreak to the east across the Colorado River into extensive spruce stands. The outbreak on the Gore Range on the Routt and Arapaho National Forests is rapidly moving northward. The outbreak on these two forests has gained much of its momentum from flights of beetles across the Yampa River Valley.

A large beetle population remains on the White River National Forest, however, and the continuation of a mass flight there is still not beyond possibility. In 1946, at the end of the attack period, 77 percent of the spruce was killed on that forest north of the Colorado River. Nearly all the remaining trees were killed in 1947. The beetles have exhausted their food supply on the White River National Forest, and whether they will perish within the infested area or fly to new areas remains to be seen.