LESSON 7: ENEMIES OF THE FOREST

South Carolina's forests have many enemies. This discussion will explore some of the most significant: fire, insects, and disease. Some information in this lesson is specific to South Carolina and may not apply in other areas.

Ask the children what they think is the most significant enemy of the forest. Most of them will probably say "fire".

Contrary to what most people think, the most dangerous enemy of the forest is not fire but insects and diseases. Insects and disease may be generally grouped together because they are natural biological enemies of trees, but they should be examined separately.

I. Insects

No one knows how many different species of insects there are, but estimates run from 600,000 up to several millions. This is more than all plant and other animal species combined. About 80,000 species have been identified in North America.

Most people refer to any creepy-crawly animal as an insect or just a "bug". Scientific classification, however, is based on the configuration of the adult body form. Most adult insects have three pairs of legs, three body parts (head, thorax, and abdomen), and an exoskeleton. Most insects have wings: some, like butterflies and roaches, have soft wings; others, especially beetles, have hard wings.

GrasshopperThere are a few species of insects (some mites, for example) which do not have 3 pairs of legs.

All insects originate from eggs and develop by identifiable stages into the eventual adult form. This system of change is called metamorphosis.

When this system of change requires four steps, it is called complete metamorphosis. The stages are egg, larva (caterpillars), pupa (cocoon), and adult. Butterflies and moths develop through complete metamorphosis.

Metamorphosis

A three-step maturation process is typical of grasshoppers and roaches. The insect egg hatches into a nymph which usually resembles a mature adult. The nymph undergoes a number of subtle changes until it posesses all its adult characteristics. There is no pupal stage in incomplete metamorphosis.

Many insects damage trees by feeding on various parts of the tree. Others cause tree problems by excavating nesting cavities to raise their young. Some also carry diseases that are transmitted to the tree as the insects feed or burrow nesting cavities.

In South Carolina, most insect damage is caused by feeding larvae. Different species feed on different parts of the tree; some eat leaves, others eat buds, fruit, or seeds, and some eat the soft cambium tissue just inside the bark of the tree. The most damaging insect is the Southern Pine Beetle whose larvae feed voraciously on the cambium of pines and destroy 2.6 million dollars worth of timber each year.

Control of forest insect pests is frequently very difficult, especially when populations of a certain insect increase to epidemic proportions. Chemical controls may be used in some situations, but such measures are expensive and must be applied carefully to avoid environmental problems. Biological controls are sometimes used to disrupt the target insect's life cycle. This usually involves releasing sterile male insects to breed with females in the wild, resulting in a lower reproductive rate.

In South Carolina, epidemics of Southern Pine Beetles crop up frequently. Control of this insect is best accomplished by conducting a concerted effort to remove the infested trees from the forest. If infested trees are cut before they have a chance to dry out and begin to decay, they can still be used for lumber or papermaking. In remote areas where logging is not practical, fairly good control can be achieved by simply cutting the infested trees and letting them dry out on the ground.

Since healthy, vigorous forests are less susceptible to insect attack, good management can reduce the chances of severe insect damage. In pine forests, this may include periodic thinning to insure that all trees have good access to the sunlight and nutrients needed for rapid growth. Old trees are less vigorous, so they are likely targets for insect attack. Sometimes this is an important consideration in determining when a forest should be completely harvested and regenerated.

II. Diseases

The most important enemy of South Carolina forests is tree disease. Diseases are caused by two general groups of agents: biotic (living) agents called pathogens, and abiotic (non-living) agents. The biotic agents may be fungi, bacteria, viruses, etc. Abiotic agents are called non-pathogenic and include climatic, environmental, and pollution-related diseases.

Of all types of tree diseases, fungi are the most damaging to our forests. Fungi are simple plants which are incapable of making their own food and must take it from a host organism. There are fungi which attack almost every part of a tree, including leaves, buds, stems, roots, and the inner circulatory system.

The single most destructive disease of pines in South Carolina is fusiform rust. This fungus attacks most of the southern pines and loblolly pine is extremely susceptible. (This is especially significant since loblolly is the most important forest tree in South Carolina.)

Fusiform rust creates swellings or galls on pine stems and branches. These galls grow as the infection spreads, killing branches and weakening stems. In the early spring, yellowish blisters appear on the galls; the blisters break open to release masses of bright orange dust-like spores. As these spores are blown by the wind, they land on and infect young, succulent oak leaves. Certain changes occur on the oak leaves and new spores are produced. These new spores are carried by the wind to re-infect pines, starting the cycle again.

Other fungi cause other diseases. Some produce long strands of vegetative material which clog the water-conducting tissue within trees, some cause rot in the root system, and others enter trees through wounds and create rot within the stems and branches.

Treating forest trees for disease is usually neither practical nor successful. Since many disease organisms can enter the tree through breaks in the bark, protecting the trees from damage can provide a degree of prevention . For diseases that are present in the soil, a measure of prevention can be achieved by planting species that are not susceptible to that particular disease. One of the most promising disease prevention methods is producing disease-resistant tree varieties through selective breeding.

III. Fire

Wildfire is universally recognized as an enemy of the forest. While not as economically important as either diseases or insects, wildfire constitutes a double threat. More and more frequently, as populations increase and people settle in rural areas, wildfire constitutes a threat to both the forest resource and the lives and property of the citizens.

Each year Forestry Commission firefighters respond to an average of 6,000 wildfires which burn a total of about 30,000 acres. Of these, 98% are caused either directly or indirectly by the activities of people. Lightning, an important fire cause in the western United States, accounts for only 2% of South Carolina's wildfires. (Since South Carolina does not have a uniform fire reporting system, these figures do not include the wildfires controlled by rural fire departments. The actual number of wildfires may be twice the figure reported here.)

Wildfire damages the forest environment in several ways: it can kill trees outright, especially young trees; it may damage portions of the bark and kill the underlying cambium tissue, making the tree susceptible to insect or disease attack; by destroying needles and leaves, wildfire retards tree growth and vigor; and fire can damage the soil directly or make it more susceptible to erosion. Very little wildlife is actually killed by fire, but some habitat may be temporarily destroyed. The actual impact on wildlife depends on the availability of acceptable habitat nearby, and whether the fire interrupts the animals' reproductive cycle.

Fire Triangle

To understand how firefighters combat forest fire, it is important to understand the elements of fire itself. Fire is actually rapid oxydation of fuel, stimulated by heat. Three components are required for this process to occur: fuel, oxygen (air), and a heat source.

Fuel is anything that will burn in the forest, including dead grass, leaves, pine straw, brush and even the trees themselves.

There is always plenty of oxygen from the air, so all that is needed to create a fire is a heat source . . . a match, a cigarette, a smoldering campfire. The three components of fire are frequently diagrammed as the fire triangle:

Forest firefighters usually attack the fuel side of the fire triangle, using tractors with plows and blades to remove fuel from the path of the fire. Firefighters with rural fire departments may take another approach by spraying water or foam on the fire to remove the heat. Sometimes firefighters take away a fire's oxygen by throwing dirt on it or smothering it with long-handled fire swatters.

Invite a forest firefighter to demonstrate his equipment, discuss the procedures for reporting wildfires, and explain how fire protection works in your county.


Reference Resources / Education

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