PINE BARK BEETLES

F. P. KEEN.

Pine bark beetles are small, dark-colored, hard-shelled insects of the size of a grain of rice or a medium-sized bean. They bore under the bark of various pines and dig egg tunnels, mostly in the inner bark, which cut the cambium layer a tree's most vital tissue. Eggs laid along the sides of these tunnels hatch into small, white, legless grubs. Under the bark also the attacking beetles introduce fungi, blue stains, and yeasts, which penetrate the sapwood and plug the sap stream from roots to foliage. The tree is hurt in the same way that an animal would be injured or killed if worms were to bore into it and stop up all veins and arteries.

When the larvae complete their feeding in the inner bark, they change into pupae, the resting stage, then to new adults. These adults later emerge from the bark and fly off to attack other pines. Thus they perpetuate their species and continue their destructive course. The new adults may attack the green trees nearby, or they may fly several miles to find trees to attack.

A great many different kinds of beetles work into and under the bark of pines. The most destructive bark beetle enemies of American forest trees are the so-called pine beetles (Dendroctonus spp.), which attack primarily the more mature trees, and engraver beetles (Ips spp.) , which prefer young trees or the tops of older ones. Species of Dendroctonus and Ips are found throughout North America.

The more important species of Dendroctonus that attack pine are the western pine beetle (D. brevicomis Lee.), which attacks ponderosa pine and Coulter pine in the Pacific States, Idaho, Montana, and British Columbia; the southern pine beetle (D. frontalis Zimm.) , which attacks all species of pines and spruce from Pennsylvania south to Florida and west to Arkansas and Texas; the mountain pine beetle (D. monticolae Hopk.), which attacks lodgepole pine, western white pine, sugar pine, and other pines in the Pacific States and northern Rocky Mountain regions; the Black Hills beetle (D. ponderosae Hopk.), which attacks ponderosa and lodgepole pines in the southern and central Rocky Mountain regions and in the Black Hills of South Dakota; the Jeffrey pine beetle (D. jeffreyi Hopk.) , which attacks Jeffrey pine in California; and the turpentine beetles (D. valens Lec. and D. terebrans Oliv.), which attack all species of pines but usually can overcome only weakened and injured trees.

The engraver beetles (1ps spp.) attack all species of pines, breed readily in the tops of recently felled trees and in slash, usually develop large populations, and then move into the tops of living pines, frequently killing trees in large groups. Various species are found in different parts of the country, but they all have similar habits.

Forestry was young in America when it was discovered that pine bark beetles were forest destroyers of the first magnitude. In the first official report on forestry, submitted to Congress in 1877, Franklin B. Hough, the first Government forest officer in the United States, directed attention to the considerable injury done to the pines of South Carolina by bark beetles, and referred to an occurrence of their activity as early as 1802. Again, during the first survey and classification of lands of the newly created Forest Reserves by the - United States Geological Survey in 1898, H. B. Ayres reported serious damage caused by pine beetles to the white pine stands in Montana. In 1900 the first field organization of foresters, working under Gifford Pinchot, found bark beetles killing thousands of trees in the Black Hills. As a result of this epidemic, which killed more than a billion board feet of pine timber, Dr. A. D. Hopkins, State entomologist of West Virginia, was called on to investigate and recommend measures of control for this and other forest pests.

So began in the United States a problem in forest protection that ever since has challenged the ingenuity of entomologists and foresters. For it quickly became evident that these were not isolated cases of insect damage but typical examples of what a group of insect enemies could do in many forest stands. Over a long period, the havoc that bark beetles have wrought has resulted in a greater total drain of commercial pine timber than has been sustained from any other destructive agency.

IN PRIMITIVE, UNMANAGED FORESTS, pine bark beetles act as nature's forest managers and loggers. Young stands that have become too crowded and suffer from competition and stagnation are frequently thinned by outbreaks of engraver beetles. In the older stands, the weak, intermediate, and suppressed trees are cut out by pine beetles. And as growing forests reach maturity, the old trees that have escaped fire and storm are harvested by pine bark beetles, and young trees then come up to replace them.

In the development of forest succession, pine beetles often have a prominent part. When fir-hemlock stands of the Cascade Mountain Range are wiped out by fire, for example, lodge-pole or western white pine come in as temporary species to reestablish a forest cover. When these stands get to be about 100 years old, the more tolerant fir and hemlock again become established under them. Then the mountain pine beetle appears to act as nature's forester. An epidemic conveniently eliminates about 95 percent of the pine overstory and thus aids the process of reestablishing the fir-hemlock climax.

On the other hand, the western pine beetle in ponderosa pine makes a selection cutting of certain intermediate, suppressed, and codominant trees that are growing too slowly. In the forest, group killings make holes, which are filled in by young seedlings. This process tends toward the development and maintenance of uneven-aged stands.

The trouble is that beetles are crude forest managers. Often they go too far in thinning and eliminating competing trees. They kill and waste much sound lumber. Holes left in the forest stand may take many years to fill. If we are to maintain and utilize our forest resources, we cannot afford to allow these natural processes to run their course, and vet we are often responsible for starting and encouraging them through forest mismanagement.

BECAUSE BARK BEETLES are constantly at work in pine forests thinning, harvesting, and wiping out entire stands of timber to make room for new ones they destroy on the whole a vast amount of commercially valuable timber. In the long run they are no threat to forest perpetuation, but they do take a tremendous toll of wood that we need badly. In some pine areas, this loss occurs as a slow but steady annual drain of merchantable trees spread over a long period of years. In other areas, the losses are more spectacular because they result from epidemic infestations that kill a fairly high percentage of the stand in just a few years. But regardless of the rate at which they occur, these beetle-caused losses affect directly the potential lumber output of pine-producing areas and indirectly the taxable wealth and pay rolls of entire communities. Estimates based on surveys in the major pine regions of the Western States are that during the period from 1926 to 1946 the western pine beetle, the Black Hills beetle, and the mountain pine beetle killed over 50 million board feet of pine.

Just as important is the damage the beetles cause to scenic and property values in our parks and summer-home areas. Mainly for that reason do people lament the extensive outbreaks of mountain pine beetle in lodgepole pine and white pine stands of Yosemite, Crater Lake, Mount Rainier, Glacier, and Yellowstone National Parks.

True, those forests will be replaced in time by other forests of the same or different types, but for many years these ghost forests of white snags are gaunt lessons of forest destruction rather than forest preservation.

Another bad feature is that the snags left by the beetles increase fire hazards. Vast areas of beetle-killed lodgepole pine are a particularly critical fire menace; some of the worst forest fires on record have occurred in these bug-killed localities.

THE CAUSES of epidemics of pine bark beetles we do not fully understand any more than we understand the reasons for grasshopper plagues or influenza epidemics. We do know that bark beetles can increase their populations at a lightning rate--10, 20, even 500 to 1 in a single generation but usually natural factors like parasites, predators, unfavorable weather, or the lack of suitable food keep them from doing so. When susceptible host material is abundant, however, and natural controls are ineffective, then beetles reproduce to capacity, and an outbreak occurs.

Probably the most important factor in building up beetle populations to epidemic numbers is an abundance of suitable breeding material. Just a forest of pine trees is not enough. The beetles prefer certain trees that are in a susceptible condition for attack. Such trees are the ones that are making poor growth or those that are injured and weakened by fire, windstorms, and by other causes. Recently felled trees are especially attractive to the beetles. Weakened trees can offer little resistance by pitch flow when the beetles attack and bore through to the inner bark. When their populations are low, the beetles continually select and thrive in those weak trees ; when the beetles find many such trees, they usually multiply rapidly. Either natural causes or disturbances of forest conditions brought about by man's activities can cause an abundance of this susceptible host material in pine forests.

Fire-weakened trees are often favored as breeding ground by certain bark beetles, which then turn out large populations that threaten surrounding forests. To the extent that man fails to control or is responsible for forest fires, he can be charged also with the pine bark beetle damage.

Drought frequently weakens pine trees and makes them susceptible to attack. Defoliating insects also slow tree growth, reduce vigor, and make trees an easy prey to the beetles. Most pine stands also become more susceptible as they reach maturity.

In any pine forest a rapid increase of bark beetles may develop in any of these various types of favored food material. When the supply of susceptible trees becomes exhausted, the beetles are forced to turn to healthy and vigorous trees, which they overcome by sheer force of numbers. Bark beetle epidemics, once they develop, continue until brought under control by natural forces or by artificial-control measures.

THE NATURAL-CONTROL FACTORS keep some in check. Besides the limitations of food supply, disease, and unfavorable weather that restrict the populations of bark beetles, they have a number of insect enemies. Parasites and predators feed upon and destroy the bark beetles. Also, many species of birds catch beetles when they are in flight. Certain species of woodpeckers go after beetle larvae which are in or under the bark.

Bark beetles can stand heat up to about 120 F. and so they are rarely killed by hot weather, unless on the top side of a log fully exposed to the sun. But they cannot stand subzero temperatures, unless they have had time to acclimatize themselves. Overwintering broods of western pine beetle start to die at about 5 and are hard-hit at 20 .

And so it is that while the vigorous broods, free from too many parasitic insects, predators, and woodpeckers, are necessary for an outbreak, they must also have ample breeding grounds of slash, windfalls, drought-stricken trees, or susceptible stands. If both sets of conditions are favorable, a major epidemic is inevitable, and much timber will be sacrificed to the hungry hordes.