Insects And Diseases That Destroy Forests
( Originally Published 1922 )
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 saw-dust 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. Fin-ally, 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 con-trolling 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 pre-vents 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.
There is a big difference between tree dam-ages caused by forest insects and those caused by forest fungi and mistletoe. The insects are always present in the forest. However, it is only occasionally that they concentrate and work great injury and damage in any one section. At rare intervals, some very destructive insects may centre their work in one district. They will kill a large number of trees in a short time. They continue their destruction until some natural agency puts them to flight. The fungi, on the other hand, develop slowly and work over long periods. Sudden outbreaks of fungous diseases are unusual.
Heavy snows, lightning and wind storms also lay low many of the tree giants of the forest. Heavy falls of snow may weigh down the young, tall trees to such an extent that they break. Lightning it is worst in the hills and mountains of the western states may strike and damage a number of trees in the same vicinity. If these trees are not killed outright, they are usually damaged so badly that forest insects and fungi complete their destruction.
Big trees are sometimes uprooted during forest storms so that they fall on younger trees and cripple and deform them. Winds benefit the forests in that they blow down old trees that are no longer of much use and provide space for younger and healthier trees to grow. Usually the trees that are blown down have shallow roots or else are situated in marshy, wet spots so that their root-hold in the soil is not secure. Trees that have been exposed to fire are often weakened and blown down easily.
Where excessive livestock grazing is permitted in young forests considerable damage may result. Goats, cattle and sheep injure young seedlings by browsing. They eat the tender shoots of the trees. The trampling of sheep, especially on steep hills, damages the very young trees. On mountain sides the trampling of sheep frequently breaks up the forest floor of sponge-like grass and debris and thus aids freshets and floods. In the Alps of France sheep grazing destroyed the mountain forests and, later on, the grass which replaced the woods. Destructive floods resulted. It has cost the French people many millions of dollars to repair the damage done by the sheep.
The Federal Government does its best to keep foreign tree diseases out of the United States. As soon as any serious disease is discovered in foreign countries the Secretary of Agriculture puts in force a quarantine against that country.
No seed or tree stock can be imported. Further-more, all the new species of trees, cuttings or plants introduced to this country are given thorough examination and inspection by government experts at the ports where the products are received from abroad. All diseased trees are fumigated, or if found diseased, destroyed. In this manner the Government protects our country against new diseases which might come to our shores on foreign plants and tree stock.