Insect and Disease Problems

Insect pests such as the gypsy moth, spruce budworm, southern pine beetle, and Douglas fir tussock moth destroy thousands of trees each year. In the 1930s the American chestnut, once one of the most attractive and commercially important trees in the United States, was virtually eradicated by the chestnut blight. The American elm has suffered much the same fate because of the Dutch elm disease.

Dutch elm disease is caused by a fungus which is spread by the elm bark beetle. It was first identified in the Netherlands in 1921 and, since then, has been found all over the Europe and North America. The first symptoms appear in early June. The shoots wilt, the leaves shrivel and die, and branches die back from the tips. The leaves shrivel and die, and branches twisted. Finally the whole branch of the tree may die.

On cutting through a cross section of dead wood, brown spots can sometimes be found. Vertical black streaks can be seen on the outside of the wood. The disease can be controlled by cutting off the branches several yards away from any signs of the disease. Fungicides can be used to kill the elm bark beetle. The fungus is also spread to healthy elms by underground roots which have grown together. To prevent this, trenches must be dug to stop contact between healthy and unhealthy roots. The ground can be injected with fungicide and even the trees themselves can be injected to protect the healthy elms and to prevent the spread of the fungus before the trees are injected.This photo (figure 2.1) is all too typical of the devastation caused by Dutch elm disease. Once a tree in a row is infected, the disease can move through connected root systems to kill the entire row.

As a rule, older, slower-growing trees that have lost their vigor, and trees that have been damaged by fire or drought, are most vulnerable to attack by insects and disease. Often, however, insect and disease infestations grow to epidemic proportions in weak stands and then the pests attack nearby healthier stands.

Forest scientists continually search for ways to control enemies of the productive forest. They have created many special chemicals to help control forest pests and diseases. Much ecological research is being done to encourage natural enemies of insect pests in order to control destructive populations.

The Control of Fire

For thousands of years forest fires (figure 2.2) have scorched the earth. They both devastated towns and villages, and parched acres of land. Fires, sparked by both natural causes and human accidents, still rage during the hot months in almost every region of the world. However, it was not until the last century that the practice of fire fighting came into widespread use in America. Now, land management organizations employ hundreds of young, strong firefighters to combat the flames. The evolution of fire suppression, however, has also had many negative ecological effects. Ecosystems worldwide suffer from both man-made fires, and man’s effort to suppress forest fires.

Protection against forest fires is often a matter of education. Many fires started by people to burn trash or leaves or dead grass get out of control and burn forests as well. Sometimes campfires or cigarettes cause fires. Arson is another common cause of fires in some areas.

Most forests are watched during the spring and fall fire seasons to detect fires as early as possible. Fire damage can be held to a minimum by quick action. Fire roads are cut through forests to allow firefighters (figure 2.3) to quench blazes before they get hot enough to reach the tops of trees. Ground fires may damage hardwood trees, but they rarely do severe damage to mature pines. Crown fires, on the other hand, kill all trees.

Lightning is still a common cause of fires. If these fires are in remote areas, they can damage large parts of a forest before they are discovered and controlled. Firefighters sometimes have to be flown to the fire in helicopters. Sometimes firefighters called smoke jumpers parachute into battle against forest fires.

Once a stand has been damaged or killed by fire, usable wood is removed quickly to prevent the buildup of destructive insect populations and to prepare for the reforestation of the area. As destructive as fire can be, foresters have learned to set fires that imitate nature to benefit the forest. Fires set by lightning destroy climax forests and allow a new succession of tree species, starting with softwoods, to begin. Natural fires also serve to reduce competition in softwood forests. Forest managers sometimes use controlled fire to control unwanted hardwood growth in pine stands.

This leaves more moisture and nutrients for the pines. It also reduces the ground litter of leaves and branches that might fuel a hotter fire, and it exposes the soil, allowing grasses and brush to grow and feed wildlife.

2.2 Give Russian equivalents: gypsy moth, spruce budworm, southern pine beetle, and Douglas fir tussock moth chestnut blight, Dutch elm disease, elm bark beetle, shrivel, cross section, brown spots, black streaks, fungicide, weak stands, firefighters, to combat, the flames, man-made fires, damage.

2.3 Find the answers to the following questions in the text 1:

1. What tree diseases are mentioned in the text?

2. How can Dutch elm disease be controlled?

3. What do cause forest fires?

4. What is controlled fire in the forest?