DDT IS A NEW WEAPON. Its greatest value is its residual effect when it is applied as a surface deposit to prevent attack on threatened trees or to catch emerging beetles. In the campaign against the Dutch elm disease, a surface spray of 2 percent DDT in a water emulsion is effective in preventing attacks by the beetle that transmits the fungus causing the disease. To prevent piled ponderosa pine slash from becoming attacked by Ills engraver beetles, a 5 percent DDT spray in fuel oil is effective. Some success has been obtained in spraying infested ponderosa pine trees with a 5 percent DDT spray, which catches western pine beetles upon emergence.

Some of these methods of control were developed at first on a small scale or a "pilot plant" basis, but each had to be taken into the woods and applied under practical field conditions before cost and effectiveness could be determined. That was done on going control projects where labor for felling, burning, spraying, or other treatment of infested trees was available. The effectiveness of the method under test in killing broods could usually be determined by sampling the treated trees, but the effectiveness in reducing timber loss in the area treated could only be determined on an area sampling basis sample plots or sample strips had to be established throughout the treated areas and on neighboring untreated ones. The timber mortality on the sample areas was determined by field crews, usually a compass man and two spotters who located the old and new infested trees. A comparison of timber loss before and after treatment and between treated and untreated areas gave the final answer as to the effectiveness of the control work.

Methods of forest sampling to determine amount and character of insect damage have also been developed and improved. The laborious work of field crews struggling up and down hill through dense underbrush to establish and check sample strips or plots on the ground may give way some day to the photographic sampling of plots from the air. Aerial reconnaissance of forest areas to determine the extent, status, and trend of outbreaks already has found an important place in the forest insect-survey program.

Direct methods of destroying beetles are still used to deal with outbreaks that cannot be handled in any other way. Forest entomologists, however, have come to realize that these "strong arm" methods are no permanent solution to the problem and that more attention should be given to preventing outbreaks rather than to the application of expensive control methods to suppress them after they have occurred. Detailed research work on the western pine beetle has pointed the way towards this objective.

Starting in 1921, a series of large projects directed towards controlling the western pine beetles in ponderosa pine forests of California, Oregon, and Washington were undertaken by the Federal Government and private timber owners. The fell-peel-burn method of destroying beetle populations was used. More than 1 million dollars was expended on these projects in the next decade, and yet the results left much to be desired. The control work reduced losses for the following year or two, but if left alone the bark beetles staged a prompt comeback and the effect of the control work was soon nullified.

Then in 1932 a natural phenomenon occurred that pointed up the futility of direct control measures as a permanent solution to the western pine beetle problem. A cold winter, with temperatures in some eastern Oregon forest areas dropping to 54 below zero, destroyed fully 80 percent of the beetles over extensive areas. The destruction of broods was more uniform and complete for large areas than could possibly be accomplished by the usual method of felling and burning infested trees. Yet despite this widespread mortality, it took only 2 years for the beetles to regain their position and go on killing more thousands of feet of ponderosa pine. Obviously direct control or the killing of the beetles themselves was not the answer.

IN THE MEANTIME, scientists discovered another phase of beetle behavior. It was noted that certain types of pines were susceptible to western pine beetle attack and others were decidedly resistant. It was suspected, therefore, that if the susceptible trees could be removed from the stand, the resistant trees could ward off insect attacks. The problem was to be able to recognize these trees so they could be removed. Starting in 1927, tests and inventory studies were made to define as exactly as possible the more susceptible types of trees. It was found that they were slow growing the crowded intermediate or suppressed trees, older and over-mature trees, and those lacking in vigor.

On that basis a system of classification was set up in which ponderosa pine were classified in four relative age groups, from young to overmature; and in four vigor groups, from most vigorous to decadent. A sampling of 51,409 trees killed by bark beetles in eastern Oregon and northern California showed that this classification was sound, for the dead trees fell largely in the poor-vigor and older-age classes.

Further studies showed that it was not merely the slow-growing trees that were most susceptible but those declining in growth rate because of poor health. That being true, trees were now grouped in four degrees of risk, according to their current health, as indicated by short needles, dying twigs, declining vigor of top, poor color, and other characters.

In 1937, bark beetle control based on susceptibility of trees to attack was put to its first test on the Black's Mountain Experimental Forest of the California Forest and Range Experiment Station. From 15 to 20 percent of the stand, consisting of those trees of highest risk, were removed through logging, sent to the mill, and utilized. Subsequent results surpassed expectations. Bark beetle losses were reduced fully go percent on the treated areas during the first year following logging, even though the logged areas were surrounded with infested stands. Subsequent checks have shown over 70 percent less loss on the treated areas than on untreated areas for a period of10 years or more following treatment.

This method of "bugproofing" ponderosa pine stands has been called sanitation-salvage logging. It has met with enthusiastic response on the part of timber owners, who have applied the method to several thousands of acres of high-hazard forest land. The recognition and identification of susceptible trees have also become an established part of the pine-marking rules on timber sales of the Forest Service, on Indian tribal lands, and on the holdings of many progressive timber owners.

The marked success of this method has given encouragement to the thought that similar methods might be applied to the control of other bark beetles. Studies along this line have been started, but it is already obvious that no two species of bark beetles follow the same rules and that what constitutes a susceptible tree for one may be a resistant type to another. The selective tendencies of each species of beetle will have to be determined through careful study.

For many species of bark beetles, the favorite breeding medium is not slow-growing or overmature trees but freshly felled slash. In such material they produce strong and abundant broods which can attack and kill large numbers of healthy trees. The Ips engraver beetles are characteristically of this type, and slash-bred Ips are the source of much damage to young pine stands in the vicinity. Prompt disposal of slash, therefore, is the clue to the prevention of this type of damage.

Windfalls are still another favorite breeding medium for many species of bark beetles. Many outbreaks, such as the Engelmann spruce beetle outbreak mentioned earlier, can be traced to this cause.

Forest fires, in killing and weakening large numbers of trees, have frequently resulted in subsequent bark beetle outbreaks. The Douglas-fir beetle, for example, bred in the fire-killed trees of the great Tillamook Burn of 1933 and developed such hordes of beetles that in 1935 some 300 million board feet of green timber surrounding the burn was attacked and killed.

Preventing outbreaks and damage from bark beetles that breed in slash, windfalls or fire-killed trees, as I mentioned, is not a matter of destroying beetle populations after they have developed in this material, but of avoiding development of beetles through prompt utilization or clean-up of their potential breeding places.

For the future, more and more effort will be given to finding ways through forest management of preventing bark beetle damage, rather than new ways of suppressing beetle populations after they have become epidemic. For example, control of the western pine beetle through sanitation-salvage logging is giving greater and more lasting reductions in bark beetle loss than suppressive measures ever did. Application of this preventive method is reducing the loss from this bark beetle in commercial stands of California, Oregon, Washington, and Idaho. Forest management of stands of lodgepole, western white, and sugar pine gives promise of solving the mountain pine beetle problem. More intensive forest practices in disposing of slash, salvaging fire-killed trees, and cleaning up windfalls will reduce the threat of many other potentially destructive bark beetles that find such material a. favorite breeding medium.

Research in the future will study more intensively the relation of bark beetles to their forest environment, what causes them to become epidemic, why innocuous bark beetles sometimes suddenly become aggressive, and how beetles can be kept in their proper place through regulation of their host trees and environment, for we are dealing with native forest inhabitants that are just as much at home in the forest as the trees themselves. We must assume that Nature had a purpose in putting them there. Rather than upset the balance of Nature by attempting to eliminate them, we should try to find out what purpose they serve and work with Nature to keep them in their proper role. Foresters and timber owners will not object to bark beetles acting as Nature's caretaker in thinning the forest of a few old decadent trees to make room for new ones. All they ask is that the beetles not be allowed to go on a rampage and kill vast quantities of timber before it can be harvested and utilized. The job ahead challenges the insight and resourcefulness of research foresters and entomologists, who will need to work together to solve this forestry problem.

F. P. KEEN, a graduate of the University of California, is principal entomologist of the Bureau of Entomology and Plant Quarantine in charge of the Forest Insect Investigations Laboratory at Berkeley, Calif. His experience with bark beetle problems in the western pine region covers 37 years. He is author of a number of technical papers and bulletins on forest insects. In 1947 he received the Department's Superior Service Award for his development of a ponderosa pine tree classification, which is used as an index to the susceptibility of pine stands to bark beetle damage and as a guide to tree selection on timber sales.

An illustration of Ips engraver beetles appears in the section of color drawings.