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.
[Illustration: SECTION OF A VIRGIN FOREST]
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 damages 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. Furthermore, 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.