INSECTS AND DISEASES THAT DESTROY FORESTS
Forest insects and tree diseases occasion heavy losses each year among the standing marketable trees. Insects cause a total loss of more than $100,000,000 annually to the forest products of the United States. A great number of destructive insects are constantly at work in the forests injuring or killing live trees or else attacking dead timber. Forest weevils kill tree seeds and destroy the young shoots on trees. Bark and timber beetles bore into and girdle trees and destroy the wood. Many borers and timber worms infest logs and lumber after they are cut and before they are removed from the forest. This scattered work of the insects here, there, and everywhere throughout the forests causes great damage.
Different kinds of flies and moths deposit their eggs on the leaves of the trees. After the eggs hatch, the baby caterpillars feed on the tender, juicy leaves. Some of the bugs destroy all the leaves and thus remove an important means which the tree has of getting food and drink. Wire worms attack the roots of the tree. Leaf hoppers suck on the sap supply of the leaves. Leaf rollers cause the leaves to curl up and die. Trees injured by fire fall easy prey before the attacks of forest insects. It takes a healthy, sturdy tree to escape injury by these pirates of the forests. There are more than five hundred insects that attack oak trees and at least two hundred and fifty different species that carry on destruction among the pines.
Insect pests have worked so actively that many forests have lost practically all their best trees of certain species. Quantities of the largest spruce trees in the Adirondacks have been killed off by bark beetles. The saw-fly worm has killed off most of the mature larches in these eastern forests. As they travel over the National and State Forests, the rangers are always on the watch for signs of tree infection. Whenever they notice red-brown masses of pitch and sawdust on the bark of the trees, they know that insects are busy there. Where the needles of a pine or spruce turn yellow or red, the presence of bark beetles is shown. Signs of pitch on the bark of coniferous trees are the first symptoms of infection. These beetles bore through the bark and into the wood. There they lay eggs. The parent beetles soon die but their children continue the work of burrowing in the wood. Finally, they kill the tree by making a complete cut around the trunk through the layers of wood that act as waiters to carry the food from the roots to the trunk, branches and leaves. The next spring these young develop into full-grown beetles, and come out from the diseased tree. They then attack new trees.
When the forest rangers find evidences of serious infection, they cut down the diseased trees. They strip the bark from the trunk and branches and burn it in the fall or winter when the beetles are working in the bark and can be destroyed most easily. If the infection of trees extends over a large tract, and there is a nearby market for the lumber the timber is sold as soon as possible. Trap trees are also used in controlling certain species of injurious forest insects. Certain trees are girdled with an ax so that they will become weakened or die, and thus provide easy means of entrance for the insects. The beetles swarm to such trees in great numbers. When the tree is full of insects, it is cut down and burned. In this way, infections which are not too severe can often be remedied.
The bark-boring beetles are the most destructive insects that attack our forests. They have wasted enormous tracts of pine timber throughout the southern states. The eastern spruce beetle has destroyed countless feet of spruce. The Engelmann spruce beetle has devastated many forests of the Rocky Mountains. The Black Hills beetle has killed billions of feet of marketable timber in the Black Hills of South Dakota. The hickory bark beetle, the Douglas fir beetle and the larch worm have been very destructive.
Forest fungi cause most of the forest tree diseases. A tree disease is any condition that prevents the tree from growing and developing in a normal, healthy manner. Acid fumes from smelters, frost, sunscald, dry or extremely wet weather, all limit the growth of trees. Leaf diseases lessen the food supplies of the trees. Bark diseases prevent the movement of the food supplies. Sapwood ailments cut off the water supply that rises from the roots. Seed and flower diseases prevent the trees from producing more of their kind.
Most of the tree parasites can gain entrance to the trees only through knots and wounds. Infection usually occurs through wounds in the tree trunk or branches caused by lightning, fire, or by men or animals. The cone-bearing trees give off pitch to cover such wounds. In this way they protect the injuries against disease infection. The hardwood trees are unable to protect their wounds as effectively as the evergreens. Where the wound is large, the exposed sapwood dies, dries out, and cracks. The fungi enter these cracks and work their way to the heartwood. Many of the fungi cannot live unless they reach the heartwood of the tree. Fires wound the base and trunks of forest trees severely so that they are exposed to serious destruction by heartrot.
Foresters try to locate and dispose of all the diseased trees in the State and Government forests. They strive to remove all the sources of tree disease from the woods. They can grow healthy trees if all disease germs are kept away from the timberlands. Some tree diseases have become established so strongly in forest regions that it is almost impossible to drive them out. For example, chestnut blight is a fungous disease that is killing many of our most valuable chestnut trees. The fungi of this disease worm their way through the holes in the bark of the trees, and spread around the trunk. Diseased patches or cankers form on the limbs or trunk of the tree. After the canker forms on the trunk, the tree soon dies. Chestnut blight has killed most of the chestnut trees in New York and Pennsylvania. It is now active in Virginia and West Virginia and is working its way down into North and South Carolina.
Diseased trees are a menace to the forest. They rob the healthy trees of space, light and food. That is why it is necessary to remove them as soon as they are discovered. In the smaller and older forests of Europe, tree surgery and doctoring are practised widely. Wounds are treated and cured and the trees are pruned and sprayed at regular intervals. In our extensive woods such practices are too expensive. All the foresters can do is to cut down the sick trees in order to save the ones that are sound.