Bark Beetles in Forests

F. P. Keen.

The Engelmann spruce beetle in 1942 began to increase its numbers in wind-thrown spruce in the high mountains of western Colorado. From the windfalls, new beetle progeny emerged to attack living trees. With each generation, increasing hordes of beetles developed and attacked more and more spruce, until more than 4 billion feet has been killed.

During the flight period of 1949, the air was so full of beetles that the ones that happened to fall into a small lake in the infested area and washed ashore (although only a minor fraction of the total flight) were so numerous as to form a drift of dead beetles a foot deep, 6 feet wide, and 2 miles long. Those flying southeast over 18 miles of open country settled on a plateau of previously uninfested forest and killed 400,000 trees in one mass attack.

That is one example of the destructiveness of bark beetles, which, no bigger than small beans, can quickly bring about the death of a majestic pine, fir, spruce, or hemlock in its forest setting and can devastate extensive forests with much the same outcome as a forest fire.

Other species can be destructive, too: The Black Hills beetle often becomes epidemic in the Rocky Mountain region and kills hundreds of thousands of board feet of ponderosa pine. The mountain pine beetle has laid waste hundreds of miles of lodgepole forests in the Rocky Mountain and Pacific Coast regions and also kills valuable sugar pines in California. The western pine beetle killed 25 billion board feet of ponderosa pine saw timber in the Pacific Coast region between 1917 and 1943 and has been a serious competitor of lumbermen for available timber supplies.

Many others, such as the Douglas-fir beetle, the Sitka-spruce beetle, the southern pine beetle, the red turpentine beetle, the pine engraver, and the fir engraver, have added their toll of timber killed, to make this group of bark beetles one of the most destructive of forest enemies.

The association of bark beetles with dying trees has been known for a long time, but not until 1900, when the national forests were being created, was their seriousness as destroyers of forest trees fully realized. A timber-survey party under the direction of Gifford Pinchot then discovered bark beetles running rampant through the Black Hills forest reserve of South Dakota. A. D. Hopkins, then of the West Virginia University, found that this bark beetle was new to science. He named it the Black Hills beetle, Dendroctonus ponderosa. Before the epidemic waned, it had killed more than a billion board feet of ponderosa pine timber in the reserve.

Research on the habits and methods of controlling forest bark beetles has been unflagging since 1902. The results have been invaluable in developing methods of protecting forests from unnecessary damage by these native insects.

The usual picture of a researcher at work is that of a figure in white bending over his flasks and test tubes but not so with the field-going forest entomologist. His garb is that of a woodsman or lumberjack. His laboratory most of the time is the forest and field. There he carries on his studies, using the native materials at hand. Through observation and experiments under woods conditions he tries to determine the role of forest insects. Sometimes, but not often, he brings his materials into the laboratory for special tests.

At first most attention was given to delving into the life histories and habits of the various species of bark beetles, in determining points they had in common and in determining differences. Because the beetles never seem to perform so well in the laboratory as in the woods, most of this biological work was carried out in the forest. Much of the information was obtained through careful observation. Periodically small sections of bark were removed from infested trees and the progress of gallery construction, egg laying, and larval development noted. Later, as the new broods neared maturity, screen cages were attached to the trees and the emerging adults collected.

The observations showed that adult bark beetles bore through the bark and into the cambial region between the bark and the wood. Here they construct egg galleries, tunnels, or cavities in which the eggs are laid. The only weapon of resistance the tree has is its pitch flow. If that is copious enough, the attacking beetles are drowned out. If not, they deposit their eggs.

Most species of bark beetles also introduce the spores of fungi, blue stains, or yeasts, which clog the cells in sapwood and hinder conduction. The beetles, by introducing fungi, sometimes cause such diseases as the Dutch elm disease, which in itself is fatal to the tree. The young, tiny larvae or grubs feed on the inner bark tissue and fan out to destroy more growing tissues. When they are plentiful enough they eventually girdle the tree. It takes several hundreds or thousands of attacking beetles to overpower a tree, but when they do they make short work of it and the tree is dead within 2 weeks or a month.

The Dendroctonus beetles, the most important group of tree killers, are monogamous one male and one female to each gallery. Many others, such as the engraver beetles of the genera I ps, Scolytus, Pityogenes, and Pityophthorus, are polygamous; each female makes a separate egg tunnel from a central chamber, where the male beetle waits to serve her. When the larvae reach full growth, they transform to the pupa, or resting stage, and then to new adults, which emerge to carry on new attacks in other trees.

Beyond the discovery of these simple facts of behavior and life habits, important questions arose: Why do some species confine their attacks to specific host trees while others feed on a large number of forest tree species? What attracts them to the trees they attack? How far do bark beetles fly? How do they manage to kill trees so quickly? How are they controlled in nature? Cannot a natural parasite or predator be found to keep them in check? Those and other questions had to be answered by research as a basis for control strategy. Even today some of the questions are unanswered, although progress has been made in uncovering the essential points of behavior of the bark beetles.

A few examples of the type of studies carried out might be of interest. Bark beetles were brought into the laboratory, and many tests were run to find out what attracted them. An olfactometer, a Y-shaped tube through which air was passed, was used. The substance to be tested was placed at one prong of the Y and the beetles at the base of the Y. They then had their choice of the substance or fresh air, and the selection made by the beetles was determined and recorded. Another method was to place the attractant in one corner of a large cage in which the beetles were liberated. Thus they were given more freedom of movement than in the olfactometer. The studies showed that beetles were repelled by the oleoresins, first line of defense of the trees, and were attracted by the odors given off from fermenting inner bark--the deterioration of the tree in some way attracted the beetles.

The researcher who tried to develop large numbers of predators had quite a problem on his hands, for the predators of bark beetles are also cannibalistic and will quickly eat each other if they are not confined to individual quarters. It was found that this method of control was not practical. Nor was it worth while to import parasites from foreign countries, for practically all American bark beetles are native to this hemisphere and have a full complement of natural parasites and predators here with them.

It was discovered that extremely low winter temperatures kill some bark beetles, so experiments were made with the western pine beetle and mountain pine beetle to determine just how low temperatures had to be to prove fatal. Running tests under laboratory conditions was comparatively comfortable work, but the tests did not fully explain the effect of temperature on broods on forest trees as modified by wind, topography, elevation, bark thickness, and tree temperature. It was necessary therefore to take this study into the woods. In the forest, researchers took hourly bark-temperature readings while temperatures dropped to 26 F. below zero, and also skied over snow fields and climbed trees to read thermometers while temperatures hovered around 15 below.

THE CONTROL STRATEGY that first suggested itself was to destroy the developing broods of potentially destructive beetles before they could emerge and attack other trees. That could be done in a number of ways, such as felling the dead infested trees and peeling or burning the bark, or removing the infested trees to millponds where they could be stored until the wood could be cut into lumber. Such control work could be done in the fall, winter, and spring when low temperatures kept the beetles inactive and confined to the trees they had recently killed.

The first large-scale control project, following this control strategy, was started in northeastern Oregon in 1910 against an outbreak of the mountain pine beetle in lodgepole pine. The method was simple and effective enough in killing beetles, but the task of applying it to 2 million infested trees, scattered over 100,000 acres of rough forest land, presented many difficulties and the cost was high.

Much thought and effort has been devoted since to finding easier and less expensive methods of killing the bark beetles. Some that were tried and found wanting were electrocution, burying, and drowning. It was found that an electric current followed the pitch seams of the bark and left the beetles unharmed. Even though buried under 2 feet of soil, beetles emerged from infested bark and found their way to the surface. Beetles could be immersed in water for 6 weeks and still survive. Where logs were sent to millponds and then cut into lumber and the slabs burned, the beetles were destroyed either by drowning or burning.

Sun curing and tree injection have had limited success. In sun curing, trees are felled and laid where they will receive direct rays of the sun, or the bark is peeled and similarly exposed. The method is effective south of latitude 45 , in open timber on flats or southerly exposures where air temperatures reach 80 F. Trees that are not peeled have to be rolled after a few days of exposure. The method has limitations in that it cannot be used in dense shade, on north slopes, or in the cool northerly latitudes.

A great deal of research has been devoted to exploring the possibilities of injecting insecticides into infested trees so as to kill the broods. It can be done but not without girdling the tree so as to get the chemical uniformly distributed through the sap stream, so there is no hope of saving infested trees by use of this method. The method is tedious to apply and works effectively only when trees are still green and the sap is functioning.