PRESERVATION OF TIMBER

Fungi.

Wood degradation is caused by very small organisms called fungi. Deadwood conks and mushrooms are visible examples of the fruiting bodies of fungi from which reproductive spores are produced and disseminated. Some fungi merely discolor wood, but wood-rotting fungi can change the physical and chemical properties of wood, thus reducing its strength. Therefore, the many wood-inhabiting fungi can be divided into two major groups, depending on the damage they cause:

 Decay fungi

 Stain fungi (sap stain fungi and molds)

All fungi produce spores (reproductive cells) that are distributed by wind and water. The spores can infect moist wood during storage, processing and use.

All fungi have certain basic requirements:

 

 Adequate oxygen-fungi cannot live without oxygen. That is why saturated or sunken logs do not decay.

 Food source-wood substance (cellulose, hemicelluloses, lignin).

Decay Fungi.

In most species of wood both the sapwood and heartwood are susceptible to decay. Decay fungi grow in the interior of the wood or appearon wood surfaces as fan-shaped patches of fine, threadlike, cottony growths or as rootlike shapes. The color of these growths may range from white through light brown, bright yellow and dark brown. The spore-producing bodies may be mushrooms, shelf-like brackets or structures with a flattened, crust-like appearance. Fine, threadlike fungal strands (called hyphae) grow throughout the wood and digest parts of the wood as food. In time, the strength and other properties of the wood are destroyed (See figure below)

 

Decay may be thought of as a reversal of the wood-growing process. You may recall that during growth, the action of sunlight on the leaves of a tree, combined with water and carbon dioxide, forms sugars (mainly glucose). This sugary solution is transmitted to all growing parts of the tree where it is converted chiefly into cellulose, which forms the cell walls. Some of the sugars combine to form starch used as a
reserve form of “stabilized glucose” to restart the growth processes when needed, usually in the spring.
During decay, cellulose and starch are broken down by enzymes into sugars and eventually into carbon dioxide and water. These sugars in wood substance are a source of food that sustains the fungi for further growth and other life processes.

Once decay has started in a piece of wood, the rate and extent of deterioration depend on the duration of favorable conditions for fungal growth. Decay will stop when the temperature of the wood is either too low or too high or when the moisture content is drier than the fungi’s requirements. However, decay can resume when
the temperature and moisture content become favorable again. Early decay is more easily noted on freshly exposed surfaces of unseasoned wood than on wood that has been exposed to and discolored by the weather.

The greatest fungal risk to untreated wooden items comes when they are used in or on the ground. Another risk to unprotected wood is its use in fresh or slightly salty “brackish” water (such as exists near the mouth of rivers), inside water cooling towers and in very humid structures like greenhouses. For many construction uses, wood will be too dry for successful fungal attack.

Wood decay fungi can be grouped into three major categories: brown rot, white rot and soft rot.

Wood decay fungi can be grouped into three major categories:
1.brown rot
2. white rot
3.  soft rot.

Figure

Life cycle of a typical wood-inhabiting fungus: Microscopic airborne spores, produced by fruiting bodies on wood undergoing decay, are carried by winds and air currents to potential hosts-logs, lumber, wood products, etc. If conditions are suitable, the spores germinate producing filament-like hyphae, which elongate, branch and multiply, spreading through the wood or forming a cottony surface mat or mycelium. Advanced stages of the fungus produce fruiting bodies, often appearing as shelf-like or bract-like conks on wood surfaces. In tiny crevices on the undersides of the conks, myriads of spores are produced which, when mature, are released into the air and carried to the next potential host.

Brown rot.

Fungi that cause brown rot are primarily able to break down the cellulose component of wood for food, leaving a brown residue of lignin. Wood infested with brown rot can be greatly weakened even before decay is visible. The final stage of wood decay by the brown rots can be identified by:

 The dark brown color of the wood

Excessive shrinkage
 Cross-grain cracking

The ease with which the dry wood substance can be crushed to a brown powder

Brown rot fungi are probably the most prevalent cause of decay of softwoods used in above ground construction. Brown rot-decayed wood, when dry, is some-times called “dry rot.” This is a poor term, because fungi must have moisture and will not cause decay when wood is dry. A few fungi that can decay relatively dry wood have water-conducting strands that are able to carry water from damp soil-to wood in lumber piles or buildings. These fungi can decay wood that otherwise would be too dry for decay to occur. They are sometimes called the “dry rot fungi” or “water-conducting fungi.”

White rot ,

White rot fungi, which break down both lignin and cellulose, have a bleaching effect that may make the damaged wood appear whiter than normal. Affected wood shows normal shrinkage and usually does not collapse or crack across the grain as with brown rot damage. It loses its strength gradually until it becomes spongy to the touch. White rot fungi usually attack hardwoods, but several species can also cause softwood decay.

Soft rot,

Softrot fungi usually attack green (water-saturated) wood, causing a gradual and shallow softening from the surface inward that resembles brown rot. The affected wood surface darkens, and this superficial layer, up to 3-4 mm deep, becomes very soft, giving the decay its name.

Stain and mold fungi

Sapstain fungi.

Sapstain or stain fungi, which live on the starch in wood cells, may discolor the sapwood entirely or in patches without breaking down the cellular structure of wood. The color of the stain depends on the kind of fungus and the species and moisture content of the wood. Stains may be yellow, orange, purple, red or blue. Most common are the bluestain fungi which turn the wood a bluish or gray-black color. Although blue-stained lumber may experience a reduction in impact strength or shock resistance, other important properties such as compressive and bending strength are not affected. However, it is important to prevent sapstain because it spoils the appearance of lumber, lowers its grade and reduces its commercial value. Sapstain fungi may also provide an environment in the wood that may be conducive to attack by wood-destroying fungi. Sapstain is best prevented by prompt kiln drying or by dipping or spraying with a chemical solution immediately after green wood is sawn. Typical sapstain cannot be removed by planing or brushing.

Mold fungi.
These fungi first become noticeable as green, yellow, brown or black fuzzy or powdery surface growths on the wood. Discoloration of wood surfaces by molds and mildew is superficial, so the discoloration usually can be removed by brushing or planning. On open-pored hardwoods, however, the surface molds may cause stains too deep to be easily removed. Fresh-cut or even seasoned stock piled during warm, humid weather may be noticeably discolored with mold in less than a week. Molds do not reduce wood strength; however, they can increase the capacity of wood to absorb moisture, thus increasing the potential of attack by decay fungi. Fortunately, molds and mildew growth can be prevented by promptly drying green or moist wood and keeping it dry (below 19 percent MC). Pressure-applied preservatives or other chemical treatments also will effectively prevent their growth.

Chemical stains.
Chemical stains may resemble blue or brown stains but are not caused by fungi. These stains result from chemical changes in the wood of both soft woods and hardwoods. Staining usually occurs in logs or in lumber during seasoning, and with a brown sapstain caused by fungi. The most important chemical stains are brown stains, which can downgrade lumber for some uses. They usually can be prevented by rapid air drying or by using relatively low temperatures during kiln drying.

Insects

 Subtemean termites

 Drywood termites

 Wood-boring beetles

 Carpenter ants
 Carpenter bees

Subterranean  termites

Subterranean termites are by far the most destructive insect pests of wood. They are found in all states except Alaska but are most destructive in milder regions of the country. Subterranean termites are social insects that live in colonies located in the ground, with each colony generally consisting of three forms or castes of termites: adults or reproductive, soldiers and workers. The mature workers and soldiers are wingless, greyish white in color, blind, sterile and are similar in appearance. The soldiers, which protect the colony from intrusion, have large heads and heavy jaws that are helpful in their protective duties. Soldiers are approximately 1/4 inch in length. The worker is the colony member that destroys wood by tearing off small wood particles with its heavy jaws. These small particles are ingested and ground into very fine particles in the termite's crop. The ground particles then pass to the rear gut where enzymes secreted by protozoa reduce the wood cellulose to usable food for termites. The worker is the one generally seen in termite-infested Wood.

The sexual adults or reproductive are found in colonies that are two years old or older (see Figure ). They have yellow-brown to black bodies, thick waistlines, two pairs of long whitish translucent wings of equal size, and are approximately 1/2 inch in length. Reproductive will generally swarm after the first few warm days of spring, flying to a new location where they shed their wings, mate and start a new colony Termite damage is often not noticeable on the attacked wood surface. The exterior surface must by stripped away in order to see the extent of damage. Subterranean termites normally first attack the less dense springwood portion of wood and, when this is depleted, they feed on the denser summerwood.

Queen

male

Soldier

Workers

 

Although termites require a constant source of moisture to survive, they are able to live in wood containing less than 20 percent moisture by obtaining their moisture from the ground. This moisture is transported by the termites through flattened, earthen shelter tubes that serve as passageways from the soil to the infested wood. These 1/4- to 1/2-inch-wide mud tubes indicate the presence of termites. Subterranean termites are most numerous in warm, moist soil that contains an abundant supply of food in the form of wood or other cellulose material such as paper, cardboard or cotton. Termites often find these conditions around untreated posts or poles and under buildings where ventilation is poor and where form boards, scraps of lumber, grade stakes, stumps or roots are left in the soil. Most termite infestations in buildings occur because untreated wood touches or is close to the ground, particularly at porches, steps and terraces. Cracks or voids in foundations or concrete floors make it easy for termites to reach wood that is not in close proximity to soil. Protective measures against subterranean termites include poisoning of the soil around buildings, use of physical barriers on foundation and, primarily, use of properly preserved wood, which makes it undesirable as a food source. However, termites can build their mud shelter tubes across treated wood to reach untreated wood.

Drywood termites

Drywood termites behave differently from subterranean termites. Drywood termites do not multiply as rapidly as subterranean termites, and have a somewhat different colony life and habits. The total amount of destruction they cause in the United States is much less than that caused by the subterranean termites. However, once a drywood termite enters wood material in a structure or building, it can live its whole life inside the wood, even at moisture contents as low as 5 percent. They can completely hollow out structural or decorative woodwork, which they use both for a food source and a nesting site. Drywood termites are particularly troublesome in southem Florida and southern California, and also occur along the southern gulf coast of the U.S.

Wood-boring beetles

Beetles that bore into wood cause several types of damage with varying degrees of severity. Sometimes the wood is riddled by holes, sometimes it is so completely pulverized as to be unusable and sometimes the holes or galleries and associated stains are the only cause of reduction in a wood's quality or lumber grade. Holes made by wood-boring beetles vary in size, ranging from minute pinholes less than 1/16 inch diameter to holes greater than 1/4 inch diameter. Sometimes the damage is caused by the larvae, which hatch from eggs laid under the bark or in the wood. Two types of wood-boring beetles, ambrosia and powderpost beetles, are primarilyresponsible for the majority of boring damage. One type requires green wood, the other dry, seasoned wood. Ambrosia beetles. Ambrosia beetles primarily attack green logs. These insects are economically important because they degrade wood, principally by staining.

Ambrosia beetles

Ambrosia beetles are unique because they cultivate and feed upon fungi that they introduce into their excavated galleries. They use the wood principally for shelter, deriving no nourishment from the wood itself.

The boring damage to green wood is done by the adult beetles, which bore across the grain of the wood, forming galleries that are kept clear of any boring dust. Accompanying the galleries is extensive staining caused by the implanted fungi. Ambrosia beetle damage generally does not seriously weaken wood structurally, but the associated staining lowers the value of products made from the wood. Ambrosia beetles will attack both hardwoods and softwoods.

Ambrosia beetle Ambrosia beetle damages

 

Powderpost beetles

Second in terms of economic importance among wood-destroying insects are powderpost beetles, which produce the so-called powderpost damage in wood. Powderpost beetles attack both hardwoods and softwoods, both freshly cut and seasoned lumber and timbers, and use the wood for both food and shelter. Within the irregularly-shaped burrows made by the beetles undigested particles of wood in the form of a very fine powder, or frass, are left. Hence the name powderpost beetle.

The most important and destructive species of powderpost beetles are Lyctus beetles (see Figure. Lyctidrs are small winged insects that lay their elongated eggs in the sapwood pores of certain large-pored hardwoods such as, ash, oak and walnut. The beetle larvae, which develop from the eggs laid by the adult female, tunnel through the sapwood in an irregular pattern, leaving their burrows packed with fine, powdery undigested wood particles. These larvae obtain their food from the starch and reserve food materials stored in the sapwood cells. In early spring, winged adults emerge to mate through small holes 1/32 to 1/16 inch in diameter in the wood surface. The females either fly or crawl to other wood where they deposit their eggs in the large sapwood pores. Following emergence of the winged adults, the fine residue in the burrows falls out of the holes, leaving evidence of the presence of powderpost beetles. Perhaps the most serious aspect of powderpost beetle infestation, and certainly the cause of most concern to homeowners, is the appearance of emergence holes or “shot holes” in valuable furniture, flooring or paneling. Powderpost beetles will also attack manufactured products such as tool handles, gunstocks, barrels and other products made from the sapwood of susceptible species. Applying a protective coating of paste wax, lacquer or varnish to the wood can discourage the female beetles from depositing eggs in the open-pored sapwood.
Powderpost beetles in the family Anobiidae, depending on the species, infest hardwoods and softwoods. Anobiids' life cycle takes two to ten years or more and they require a wood moisture content near or above 15 percent for viable infestation. Therefore, in most modem buildings the wood moisture content is generally too low for anobiids. Exit holes for Anobiid powderpost beetles are 1/16 to 1/8 inch in diameter.

Powderpost beetles Powder post beetles damages

Roundheaded borer

A longhorn beetle, commonly known as the old house borer, damages seasoned pine timbers (see Figure 3.4). The larvae bore through the wood. Over many years their tunneling can weaken structural timbers, framing members and other wooden parts of buildings. Contrary to its name, the old house borer most often infests new buildings. It is found in the eastern and gulf coast states.

Larvae reduce sapwood to a powdery or sawdust-like consistency. It may take several years to complete their development. While working in the wood, they make a ticking or gnawing sound. When mature, the adult beetle makes an oval emergence hole about 1/4 inch in diameter in the surface of wood.

 

Roundheaded borer

 

Flatheaded borers.

Flatheaded borers are beetles that infest trees as well as recently felled and dead standing softwood trees. They can cause considerable damage in rustic structures and some manufactured products by mining into sapwood and heartwood.

Typical damage consists of rather shallow, long, winding galleries that are packed with fine powder. Exit holes are 1/8 to 1/2 inch in diam-eter. Adults are often called metallic wood-boring beetles because of their color. They are about 3/4 inch long, with wing covers usually rough, like bark.

Flatheaded borers

Carpenter  ants

Carpenter ants are the only true ants that are of importance in wood degradation (see  Figure  3.5, page 26). Carpenter ants are troublesome particularly in the northeastem and northwestem U.S. They use wood for shelter rather than food, generally attacking the relatively soft spring- wood first. They can usually be found in old stumps and other wood that has been softened by decay. Carpenter ants are commonly found attacking wood in service such as utility poles and structural members of housing such as porch columns, window sills, sill plates and porch roofs. Although carpenter ants can extend their tunnels into dry wood, they must have high humidity in their nesting area.

Like termites, carpenter ants live in colonies, each colony containing three castes: queens, males and workers. They may gain access to buildings directly from the soil by crawling to wooden members set in or on the ground or they may be carried in on firewood. Damage by carpenter ants can be recognized by the presence of large, hollow, smooth-sided tunnels that cut across the grain of the wood. Generally, the tunnels bored by termites contain frass, whereas the tunnels produced by carpenter ants are clean.

Carpenter ants

 

Characteristic	Termite	Ant
Wing size	both pairs of wings of identical size	hindwings much smaller than forewings
Wing veins	very many fine conspicuous veins	few, dark, conspicu- ous veins
Wing stubs	stubs left when wing detaches	no wing stubs remain when wings break off
Body  shape*	no abdominal constriction present	first 1 or 2 abdominal segments constricted to form 'wasp waist"
Antennae*	antennae resemble a string of beads; never  "elbowed"	antennae usually "elbowed";  seg- ments not similar in shape

*These characteristics are also used to separate non-winged forms.

If left undisturbed for a few years, the black or brown carpenter ants will enlarge their tunnels to the point where wood strength is impaired and replacement or extensive repairs will be re-quired.

Carpenter bees

Carpenter bees resemble large bumblebees, but the top of their abdomen is bare of hairs. Carpenter bees are a problem to unpainted and untreated wood in the eastem and southeastem U.S. These insects cannot digest wood, but they use their jaws to chew holes in which to lay their eggs, and the small pieces of chewed-out wood are discarded. The females make large (1/2 inch diameter) tunnels into soft wood for nests.

Because they reuse nesting sites for many years, the bees' nesting tunnel into a structural timber may be extended several feet and have multiple branches. They will nest in stained wood and wood with thin paint films or light preservative salt treatments as well as in bare wood. Tunnels may be injected with an insecti-cide labeled for bee control and after several days plugged. A good paint film or pressure preservative treatment usually protects exterior wood surfaces from nesting damage. Because CCA preservatives may not completely protect wood against their damage, suppliers of CCA-treated wood may exclude carpenter bee or carpenter ant damage from warranties they may offer. The best defense against these insects is spraying with a contact insecticide.

 

Marine Borers

Marine borers are a group of wood-boring marine organisms that attack submerged, unprotected wooden members in salt and brackish waters. Some of these borers may occasionally be found in fresh water. Marine borers attack any untreated wood between the waterline and the mudline. Their boring action plus erosion from wave action generally results in rapid deterioration of wooden structures. There are two distinct groups of marine wood-boring organisms, each characteristic in its general structure and method of attacking wood. One group is the molluscan borers, distantly related to oysters and clams, and the other is the crustacean borers, which are kin to lobsters and crabs.

Molluscan  borers

Molluscan borers consist of three principal genera: Teredo spp. and Bankia spp., (known collectively as shipworms) and Martesia spp or pholads. Shipworms are found in nearly coastal waters . Shipworms enter wood in a tiny worm-like form (see  Figure ).  A pair of boring shells on the head grow rapidly in size as they bore into the wood making larger and longer tunnels which are lined with a chalky, shell-like deposit. The siphon or tail part of the worm remains at the original entrance and their bodies grow behind them within the wood, where they stay confined for life. Their shells rasp the wood, oxygen is extracted from the sea water and enzymes digest the wood, creating ever-larger tunnels. Their tunnels may be up to 3/4 inch in diameter and 2 feet long. In a few months these organisms can do considerable damage to wooden structures and are a constant problem to harbor maintenance engineers because their presence is not readily apparent. Only their protruding siphon tubes are visible at the surface. Another group of wood-boring mollusks are pholads, which clearly resemble clams and therefore are not included with the shipworms (see  Figure ). These are entirely encased in their double shells. The martesia are the best-known species. Like the shipworms, martesia enter the wood when they are very small, leaving a small entrance hole, and grow larger as they burrow into the wood. They generally do not exceed 2- 1/2 inches in length and 1 inch in diameter, but are capable of doing considerable damage.
Crustacean borers

Crustacean borers, in contrast to shipworms, erode timbers from the outside (see Figure). They are small shelled animals related to shrimp. Both the larvae and adults are mobile and can move from one source of wood to another although they usually continue to bore in one place. By chewing on the surfaces of timbers, large numbers of these marine organism can wear the wood away. The wood surface becomes riddled with tunnels separated by very thin walls which are then broken away by wave The mollusc sucks water in through siphon A, absorbs oxygen and tiny plants (plankton) and forces the water out through B  

The most common type of crustacean borer is Limnoria,.Limnoria, known sometimes as gribbles, are about 1/8 to 1/6 inch long. Untreated piling can be destroyed by limnoria within a year in heavily infested harbors. Other species such as Chelura and Sphueromu are as widely distributed but not as plentiful as limnoria and do much less damage.

Apart from heartwood timbers sawn from greenheart trees, (now almost unavailable) only heavily-treated wood of pines are suitable for marine use. Tests in marine waters have shown that creosote offers better protection against pholads than CCA preservatives, while treatment with CCA protects wood better against certain crustaceans. For this reason, where coastal structures must have long lives, and where both wood-boring mollusks and crustaceans are present, it is common to specify CCA treatment followed by reseasoning and then retreatment with creosote. This dual treatment is the best form of chemical preservation presently available, especially where pholads are present.

Other methods of preventing attack by marine borers have included wrapping piles with plastic material in an attempt to suffocate the organisms. This can be successful so long as the barrier is not damaged.

Teredo marine borers.

The extremely destructive Teredo navalis or shipworm is bisexual: eggs are fertilized within the adult. After hatching, vast numbers of mature larvae are released into the sea. They settle on wood and, after metamorphosis, bore into it. As the adults grow, they deposit a chalky layer on the inside of the tunnel. Teredo species are one of three groups or genera of molluscan borers.

Adapted from Preservation and Treatment of Lumber and Wood Products. 1987. New York State College of Agricul-ture and Life Sciences. Cornell University, Ithaca, NY
action. Eventually the dimensions of the timbers are reduced so much that they have to be re-placed.

 

 

Martesia are the most common species of pholads, another group (or genera) of wood-boring mollusks that resemble clams.

Adapted from Preservation and Treatment of Lumber and Wood Products. 1987. New York State College of Agriculture and Life Sciences. Comell University, lthaca, NY.

Limnoria, a common type of crustacean borers, are small (1/8 to 1/6 inch long), shelled animals related to shrimp. Crustacean borers, in contrast to shipworms, erode wood from the outside.

 

 

 

 

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