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these two sources runs into many millions of dollars annually. Recent study of losses due to these agents shows conclusively that it is economically possible to avoid about 50 per cent of this annual loss by adopting better sanitary storage methods, by a better knowledge of the environment of the insects, by so designing structures as to avoid conditions favorable to decay, and by properly using wood preservatives and species with natural decay-resisting properties.

While preservative treatment of wood can not be recommended for all purposes, it is most desirable for many purposes and will prevent to a great extent the losses that are due to these two destructive agencies.

Damage to seasoned, finished wood products, of course, causes relatively greater loss than damage to crude forest products. Where the products are damaged after being put in place, the cost of replacement involves additional loss of labor and time, as well as the cost of the original and replaced products, a loss far greater than the value of the raw material. Sometimes such replacement should be charged to both wood-destroying fungi and wood-boring insects, and not to one agency alone, since there is often a close relationship between these forms of life in the destruction of timber. However in many instances either insects or wood-destroying fungi alone are responsible for the destruction.

DECAY

The annual loss of timber in service because of decay alone, amounts to about one-fifth of the annual cut of lumber. Iron and stone disintegrate through inorganic processes, the result of which is seen in the form of rust or crumbling sand and stone litter. In wood the process of disintegration through decay is entirely different, being organic, and the resulting action becomes evident in the form of powdered or soft, crumbling wood mass. (Fig. 1.)

Investigations show that in all instances decay is caused by low forms of plant life called fungi, which consist, for the most part, of very fine threadlike filaments that penetrate the wood in all directions. Certain substances in the wood constitute the food of the fungi. As these substances are dissolved the wood structure is broken down, until it becomes rotten.

In the course of time the fungous threads grow out to the surface to form compact masses called fruiting bodies. Their further growth is described in the Preservative Treatment of Farm Timbers, United States Department of Agriculture Farmers' Bulletin No. 744, as follows:

Since there are many kinds of fungi, there are many kinds of fruiting bodies. The various forms of " toadstools," "punks," "brackets," or "dog ears," which are so frequently found growing on trees and wood, are examples of fruiting bodies. Their presence generally means that decay has made considerable progress in the wood. All fruiting bodies produce spores, which are to the fungus what seeds are to higher plants. Millions of spores may be produced by a single fruiting body, and they are so small that they are able to float long distances in the air. When a spore drops upon a piece of wood and conditions are favorable, it germinates and the fungus begins its destructive action.

1 May be obtained from the Superintendent of Documents, Government Printing Office, Washington, D. C., price, 5 cents.

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Another way in which decay spreads is by the fungus growing from one piece of wood to another. When a piece of decaying wood is in contact with a sound untreated piece, the latter may rapidly become infected and be ruined. The four requirements for the growth of fungi are moisture, air, a favorable temperature, and food.

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FIGURE 1.-Section of a porch which has given way because of decay caused by wood destroying fungi. The woodwork had been thoroughly painted, and decay was not apparent from the outside. This is the danger line in construction of this nature. Material used for such purposes should be treated.-Courtesy Forest Products Laboratory

A damp condition of the wood is most favorable to decay. Wood can be either so wet or so dry that the fungi can not live in it. When submerged in water, wood has been known to last hundreds of years, and in perfectly dry situations it will last indefinitely. Wood in contact with damp ground usually contains the right amount of moisture for the development of decay. Also, where timber is in contact with wood or other material, water frequently

collects in the joints and keeps the wood moist for long periods of time, thus favoring decay at these points. Familiar examples of this are decay in the tops of posts in board fences, in the joints of various kinds of buildings, in porch columns, and in sills resting on wood or masonry piers.

There are very few places where the fungus can not get air enough for its needs. When wood is buried deep in the ground, especially in compact or clayey soil, it tends to become saturated with moisture, so that decay is prevented; but within 2 or 3 feet of the surface there is usually enough air for the growth of the fungus. Conditions are most favorable, of course, just at the surface of the ground, and it will be noted that the point of greatest decay in a fence post is usually near the ground line. Above this line moisture conditions are less favorable, and below it the air supply is less favorable for the rapid development of decay. In loose or sandy soils, however, which under good drainage contain more air than compact soils, decay may extend to the bottom of the post.

Wood-destroying fungi can not grow at very high or very low temperatures; but there are few, if any, climates in which the temperature during at least part of the year is not favorable to their growth.

The wood itself supplies the fourth requirement of the fungi, which is food.

It is interesting to note, therefore, that decay in wood is due to the removal of certain parts of wood to serve as a food supply for these fungi.

In order to prevent decay, it is necessary to deprive the fungus of one or more of its four requirements. In ordinary situations it is out of the question to deprive it of air and warmth, and although moisture can sometimes be eliminated to a certain extent, this can not be done when the wood is exposed to the weather. In general, therefore, the most effective method of preventing decay is to poison the food supply; and upon this principle the successful use of wood preservatives is based.

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INSECTS

Wood-boring insects are said to cause $45,000,000 worth of damage annually to forest products, a large percentage of which is preventable. Damage to standing timber is done by many species and varieties of insects, each with a preference for certain woods. Damage by insects to timber and lumber in service is done chiefly by the termite, or as it is commonly called, the white ant. These insects, of which there are at least 46 species in the United States, are most prevalent in the southeastern portion, the Gulf States, the Southwest, the Central West, and on the Pacific coast. Damage by termites in the Corn Belt of the Central Western States is almost as serious as in some of the Southern States. Such damage occurs in rural regions as well as in cities. Termites are very prevalent in all tropical countries. They are not true ants, but resemble them somewhat and live in colonies as ants do, many thousands living in a single colony.

Two types of termites are found in continental United States. (Fig. 2.) One is the subterranean or soil-nesting termite which attacks the wood of buildings, trees, or living crops indirectly from burrows in the earth; and the other is the nonsubterranean or dry

For detailed information covering wood-destroying insects, termites, attention is called to the following reports: U. S. Dept. of Agriculture Farmers' Bull. No. 1472, Preventing Damage by Termites or White Ants, by T. E. Snyder, entomologist, Bureau of Entomology, Superintendent of Documents, Government Printing Office, Washington, D. C., price, 5 cents; and U. S. Dept. of Agriculture Leaflet No. 31, Termites in Buildings, by T. E. Snyder, senior entomologist, division of forest insects, Bureau of Entomology, Dept. of Agriculture, free.

wood termite which does not burrow in the ground but attacks the wood of buildings or trees directly.

The subterranean termites are blind and shun the light, so they are seldom seen until their attack has become severe. They can burrow their way into the hardest of woods, provided they have access to moisture in the ground. This moisture is necessary to the digestion of the cellulose in the wood, which is their food. Certain low animal forms called protozoa are always present in their digestive tracts and assist in the process of digestion. In burrowing into the wood they generally follow the grain, eating out the softer thinwalled larger-celled springwood.

The nonsubterranean termites differ from the subterranean species in that instead of burrowing in the ground they fly to and attack the wood directly; they do not require as much moisture; and instead of following the grain of the wood continuously they also eat

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FIGURE 2.-Diagrammatic map of the United States showing northern limits of
damage by subterranean termites (A-A) and nonsubterranean or drywood
termites (B-B).-Courtesy Bureau Entomology, U. S. Department of
Agriculture

short longitudinal chambers. These termites are destructive to the woodwork and furniture in buildings as well as to living trees. While the termites do not eat their way directly through brick or concrete walls, they will penetrate masonry walls where improper grades of mortar have been used in foundations, and will then work their way up through the interior of the walls. They also build shelter tubes over such walls or enter through cracks in the concrete, and travel from the earth to the wooden portions of buildings. Their attacks, however, are not confined to buildings, but have been noted in other forms of timber, particularly telephone and power poles.

Since in most cases timbers in service are attacked by termites from the inside, not from the outside, spraying with chemicals will not protect the wood. It has been definitely established, however, that wood treated with preservatives according to accepted standards, both as to process and chemicals, by which the preservative is

forced into the interior of the wood, is effective against termite attack and should be used in any construction where termite damage may be feared.

The following is an extract taken from a recent publication of the United States, Department of Agriculture entitled "Termites in Buildings" Leaflet No. 31.

A few hundred dollars additional (2 per cent of the first cost) spent in proper construction may save you thousands of dollars in repair and replacement later. It is much simpler and cheaper to keep termites out of a building than to get rid of them and repair damage. The necessary repairs may be too costly for the small householder, but certain methods of construction that will prevent injury are entirely practical.

Proper construction when building is essential and the following suggestions will assist in preventing damage and loss from termite activities in frame construction.3

Wood or fiber products, when an approved preservative has not been forced into the product, shall not be placed in the earth or within 18 inches thereof,

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FIGURE 3.-Creosoted sills and foundation timbers were used in the construction of this residence. Courtesy American Wood Preservers' Association

excepting wood columns or posts over a concrete floor, which columns shall be provided with noncorroding metal or concrete base plates or footings 6 inches above the floor. This applies to steps, which shall be laid over a concrete base, projecting at least 6 inches beyond the supports of the steps.

Timber to be used in contact with the earth shall be thoroughly impregnated by a standard pressure process with coal-tar creosote or other equivalent preservative. Timber should be completely cut to proper dimensions before treatment, whenever possible, but when cutting after treatment is unavoidable all cut surfaces shall be thoroughly coated with coal-tar creosote or other equivalent preservative.

Masonry foundations and footings shall be laid in Portland-cement mortar. Foundations built up of masonry units, whether hollow or solid, shall be capped below woodwork with at least 1 inch of Portland-cement mortar, or mortar and slate, or solid or joined noncorroding metal, or other equally artificial seal. In the case of frame buildings a metal termite shield shall be provided, continuing completely around the top of the masonry foundation, including all pillars, supports, and piping, below the woodwork of the building, on both the inside and outside surfaces. Such a shield may be formed of a strip of noncorroding metal (such as copper, or zinc, or an alloy composed of 28 per cent

3 See fig. 3.

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