Insects, Diseases, Parasites

Above: Among enemies of forests are bark beetles and dwarf mistletoe.

INSECTS IN THE FOREST: A SURVEY

F. C. CRAIGHEAD, JOHN M. MILLER.

NATURE has always used insects for her own purposes in forests. Some insects are housekeepers. Some are only incidental parts of the forest environment. Some merely prune trees. Others kill living trees, but even they do not destroy the capacity of the forest to restock and produce new stands of trees. We have convincing evidence that vast areas of mature timber were demolished in the past by insect hordes, only to regenerate after the epidemics had run their course. This was nature's way before man went into the woods. Even primitive man could not have been greatly worried by the insects that killed the forests where he got shelter and meat: Wood was plenty for all; time was plenty for young trees to grow up.

But in modern civilization those things have changed : Now the activity of destructive insects upon the trees and in the forests does matter; great areas have been cleared of forest growth for agriculture; increasing populations have increased the use of wood. Now in his search for timber stands to meet the need for sawlogs, pulp, and box shooks, the lumberman finds some areas where insects got there first and harvested the pick of the crop. For the forest resources and the commercial and esthetic values involved, we have joined battle, insects versus man, and man, for all his science and machines, is not yet the winner.

A reason why that is so is to be found in the nature of the insect infestations.

Insect populations and the timber losses they create fluctuate from year to year; only sporadically do spectacular outbreaks occur. The insects normally are present in the forests in small numbers and only occasional trees are injured or killed. A sort of natural balance seems to persist under which the processes that permit forests to reach maximum production go on uninterrupted. Then, all of a sudden, something happens to disturb this balance. A destructive insect pest appears in great numbers over wide areas and for several years its ravages may continue until a high percentage of the forest stands has been killed. Then, even more suddenly than it appeared, the epidemic subsides.

This sporadic behavior of forest-insect populations indicates that complex factors govern the abundance of certain species in the forest. Parasites, predators, unfavorable weather, resistance of the trees due to growth vigor, all tend to hold populations in check. On the other hand, conditions that will tend to weaken the trees, such as drought, preponderance of a favored food tree, failure of parasites and predators, overmaturity, and windfalls and slash, all provide favorable conditions for the destructive species to breed up in numbers. Man, himself, has at times aggravated serious insect outbreaks by his method of using the forest.

Although science has not yet been able to uncover and appraise all the factors that influence the abundance of forest-insect populations, it has shown that there are dominant conditions that must be taken into account in maintaining productive forests free from excessive losses due to insect pests. The most successful control methods that have been developed up to the present time ( and no doubt those that will be used in the future) are based upon the strategy of using nature's methods as far as possible in holding down destructive insect populations.

Furthermore, the kinds of insects that attack forest trees include many species that vary widely in their habits and in the character and amount of damage they do. Some insects attack only the flowers; others the cones and seeds. The activity of these insects does not damage the tree itself, but at times so much of the seed crop is destroyed that reproduction of the forest is retarded. Sucking insects, such as scales and aphids, attack foliage and stems; they rarely kill the tree outright but gradually weaken it and slow down the growth rate. The most effective tree killers, however, are the defoliators and bark beetles, whose activities destroy vital plant organs and bring about an immediate and often fatal effect upon the growth functions of the tree. Other insects that cause great damage are termites and some wood borers, which feed only on the wood after the tree is dying or dead and destroy material that otherwise could be put to use.

Trees are defoliated mostly by the larvae of certain moths and sawflies and to a lesser extent by both the adult and larval forms of some beetles. Defoliators can kill trees by depriving them of foliage, thus stopping the manufacture of the plant food so that the trees slowly starve. Some of the historic defoliations of the past have been recorded not only in the chronicles of the time but also in the annual rings of surviving trees. Outbreaks of the spruce budworm in the New England States and of the fir tussock moth in the West are recent examples of widespread defoliating epidemics.

Insects that feed between the bark and wood find their nutrition in the sugars and starches that are in solution in the cells of the inner bark and cambium. To reach these they mine through the corky bark into the inner bark layer, where they introduce fungi that develop in the sapwood and stop the flow of the sap. The leaves, deprived of water, quickly wilt and the tree dies. Bark beetles make up the bulk of the destructive cambium feeders. Certain species of bark beetles are particularly adapted to mature stands of pine and in a number of Western States take a heavy toll from virgin forests that are the main reserve of timber supplying the Nation's need for high-quality soft pine. In some regions during the past two decades these insects have destroyed more merchantable timber than was cut by the sawmills and destroyed by fires, combined. Characteristic of the bark beetle infestations is their capacity to flare up into epidemics of spectacular proportions.

Termites and wood borers do not kill or damage living trees and, in nature's economy in the forest, may be of benefit in that they accelerate the deterioration and decay of dead trees and snags, which are thus returned to the soil. They compete with man, however, when he decides to utilize the tree, and attack the wood both during the process of manufacture and after it is in the finished product.

Termites and wood borers in their concealed ways work along methodically year after year. Their destruction never flares up in spectacular peaks, but the annual attrition is nonetheless disturbing and serious. Pinhole and worm-hole borers attacking green logs lower grades of lumber; powder-post beetles in tool handles, furniture, and flooring render quantities of finished material worthless; the old-house borer in the rafters of barns and houses and termites in telephone poles and foundations of buildings claim an annual depreciation requiring constant vigilance and replacement of the damaged wood products.

ESTIMATES OF THE MONETARY VALUE of wood material and esthetic values that are destroyed annually by forest insects are subject to many reservations. The money value of the forest products varies like that of other commodities, according to demand, availability, and the buying power of the dollar; and the esthetic value of trees that are killed in parks and recreational areas can seldom be expressed in terms of money. Although some estimates have been made which indicate that Nation-wide timber losses run into millions of dollars annually, they are based on too many assumptions to be of value in this discussion. However, if we consider only the actual board feet or cubic volume of timber that is killed by insects, we find that this can be measured with considerable accuracy for specific areas and periods. Forest-insect surveys have been made to compute the volume of timber destroyed in many areas that have suffered from bark beetle and defoliator epidemics. Such surveys have been made in the New England States, Where the spruce budworm destroyed 250 million cords of fir and spruce, and in the Western States, where bark beetles killed 45 billion board feet of pine in recent epidemics.

All in all, these varied insect activities, involving tree seeds, the natural restocking of the forests, the forest plantations, second-growth and mature stands of timber, green logs and lumber, telephone and telegraph poles, cross ties and buildings, create a substantial loss that must more and more be reckoned with and prevented as our timber resources become smaller. This loss is often compared with that from forest fires even though all such comparisons are difficult and incomplete.

PREVENTION is the starting point. If he is sufficiently interested and acts in time, man can save for his own use much of the timber that insects will otherwise destroy. There are two ways of going about it.

One approach is to initiate repressive measures against the insect populations that are causing the losses. In the case of bark beetles, the broods are destroyed by peeling and burning the infested bark or by applying a toxic penetrative spray to the bark surface. In the case of defoliators, that usually involves spraying the trees with chemicals which will either kill the insects on contact or poison them through their food. These repressive measures have been termed direct control.