In the past, different pesticide formulations and active ingredients have been used for control of wood-destroying organisms (WDOs), beetles, decay fungi and termites above ground. These products often presented problems. Some had strong odors, none penetrated wood well and the effective residual life was short. Fumigation, while still required in many cases, is expensive and provides no residual protection against reinfestation. borate Insecticide is a product that can effectively solve many of these problems. It is easy to mix and apply and has no odor. Its residual life is indefinite if the treated wood is protected from running water. borate penetrates far better than do other residual insecticides used to control wood-boring insects. TIM-130R, used alone or in conjunction with other techniques, offers a most cost-effective solution to wood-destroying organisms without creating customer complaints or concerns.

In order to understand the potential for attack on wood by insects and fungi, it is necessary to become familiar with the relative importance of the specific classes and parts of wood, and the moisture in the wood. This understanding will also provide information on how boric acid for pesticide penetrates wood.

Wood classes and structure

As a tree grows in circumference (diameter), it does so in the cambium, a thin cell layer just below the bark (Figure 1-A). In most of the country, there is rapid growth in the spring (earlywood) and a much slower growth in the summer (latewood). This results in pronounced differences in the texture of the wood and annual growth rings consisting of rather wide, light colored earlywood and darker, more dense latewood (Figure 1-H). These growth rings are not obvious in all species of wood, particularly some hardwoods.

That brings us to the designation of classes or types of wood used in structures. These are referred to commercially 115 hardwoods and softwoods. Hard woods come from trees that have leaves: oaks, maples or walnut, etc. and softwoods come from trees having needles: the pines, firs and spruces.

When alive, the wood directly under the bark that is involved in the movement of sap is called sapwood (Figure 1-D). The wood in the center of the tree is often darker in color, and is called heartwood (Figure 1-E). Generally the sapwood and earlywood are the target of insect attack. Stored products in wood cells attract beetles and termites attack earlywood. Heartwood, however, has stopped functioning in the movement of sap, and has had a number of substances deposited in it, which also impart resistance to attack by insects and decay fungi. It is often more resistant to the movement of water and is more difficult to penetrate with insecticides.

Cross Section of Typical Hardwood (Figure1)

A. Cambium - living cells that form new wood
B. Outer bark - dead, protective layer
C. Inner bark - living: transports food from the leaves
D. Sapwood - living and dead cells: transports sap and stores food
E. Heartwood - inactive, dead cells containing extractives
F. Pith - spongy center of original stem
G. Wood ray - cells which move sap across the grain
H. Annual growth ring - early - and latewood produced in one year

Wood moisture levels

When freshly cut logs are processed into usable lengths of wood, the wood then used in construction is kiln dried, having moisture levels of 15% or less. However, while wood is in storage or after it is in use, it slowly reacts to the moisture in the air around it and assumes equilibrium moisture .......... 

There are many factors that influence the final moisture level reached. Also, within structures the levels may vary during the year. Such things as ventilation, drainage, heating and air conditioning, humidity and condensation, etc., influence moisture content of the building and, therefore, of its wood. In addition, wood-boring organisms attacking structural wood are often dependent on relatively high moisture contents to initiate attack and in some cases, to continue their attack on the wood. Termites will attack "dry" wood but bring moisture into the wood. Wood decay fungi need even higher moisture contents to begin their attack (Table I, Page 6). That will continue as the wood becomes wetter until the cell cavities are filled with water.

The moisture needs of these organisms are important to understand, since the wood moisture content will affect the rate and depth to which borate will diffuse into wood. At low moisture contents, some diffusion will occur, but surface amounts of borate are extremely high. However, treating wood with a high moisture content means that boric acid for pesticide will diffuse deeply into the wood to protect against or eliminate WDOs.

Subterranean termites

Subterranean termites, the insect that causes the greatest amount of damage to structural wood in the U.S., will damage both hardwoods and softwoods. It is typical of subterranean termites to consume the softer earlywood first, leaving the layered appearance in the damaged wood (Figure 2). The initial damage begins on the wood surface, but termites can tunnel and produce galleries throughout the wood. In larger dimensioned timbers, these tunnels or galleries can penetrate deeply into the wood. While destroying wood, termites also increase the moisture content of wood in which they are feeding. In some instances, subterranean termites may find a moisture source above ground and survive without any contact with the soil. In these cases the usual soil treatment will not solve the problem, or control can be delayed for months. Locating and eliminating the moisture source is critical for control. Even if the termites are controlled by treatment, wood decay can take over and continue to destroy any of the wet wood. Termites enter the wood from the surface, so the first target treatment area for application is the surface. If an infestation is already present in the wood, the higher moisture content should increase the movement of the borate. borate treatment will control both termites and wood decay even when alternative moisture sources are available. However, elimination of the moisture source is recommended in any case.

Drywood termites

Drywood termites feed internally in both hardwoods and softwoods and can survive in wood that is quite dry (10%). Again, these insects enter from the surface so even though the wood they attack is fairly dry, borate on the surface will prevent initial attack. In established infestations, borate can be injected into the termite galleries or the wood itself to obtain control.

Dampwood termites

Dampwood termites require no ground contact, but attack wood with a high moisture content and often coexist with wood decay. These termites can be controlled by spraying or brushing wood with borate because the high moisture level in the wood enhances borate penetration.

Powderpost beetles

There are two types of powderpost beetle that attack wood in structures. The lyctid powderpost beetle, sometimes called true powderpost beetle, will attack the earlywood of certain hardwoods. Their eggs are laid in rather large pores (wood cells) in the earlywood and the larvae tunnel directly into the surrounding wood. This means that their attack will usually be rather scattered and damage is restricted to particular parts of the wood. As the larvae complete development and prepare for emergence as adult beetles to the outside, they tunnel close to the surface of the wood. Lyctids may attack wood that is quite dry (see Table) but a surface borate treatment will deposit high amounts of borate and thereby prevent initial attack and kill any larvae feeding just beneath the surface. 

The other type of powderpost beetle, the anobiid, will attack both hardwoods and softwoods that have higher moisture content than normal (see Table). There is variation among the anobiid species, but they tend to be problems primarily in damp, poorly ventilated crawl spaces. The eggs are laid on the wood surface and the larvae bore directly into the wood. They usually tunnel within the sapwood and only occasionally into heartwood. As the larvae mature, they also tunnel back to the surface in preparation for the emergence as adults (Figure 3). Applications of borate protect the surface of the wood and stop larvae from entering the surface of the wood. The higher moisture level enables the borate to penetrate and kill larvae in the wood.

Old house borers

Old house borers are found in the sapwood of softwoods. The eggs are laid in cracks in the wood surface and the larvae bore into the wood below. The old house borer only requires 10% moisture to infest wood, so they may be found in wood in drier parts of buildings. They may bore deeply within large timbers, but frequently they tunnel quite near the surface (Figure 4). Again, surface treatment with borate will stop initial infestation and kill larvae tunneling near the surface.

Carpenter ants

Carpenter ants construct nests in hardwoods and softwoods, but do not consume the excavated wood. Though they usually start their nests in damp wood, they can extend their galleries into dry areas. In addition, they sometimes form satellite nests in wood that is no wetter than the surrounding structural members. Because of their habit of moving from outdoor infestations to indoor locations, they crawl over wood surfaces that they have not attacked. This makes them susceptible to picking up borate dust that has been applied outside their nest galleries. As they groom themselves, they take insecticide into their stomachs and are poisoned.

In addition, since they often start their attack on the wood surface, borate solution applications to the surface have proven very effective at preventing carpenter ant penetration.

Decay fungi

The common Brown and White rot fungi require wood moisture of 25% or more to maintain an infestation and to destroy the wood. Their destruction is internal and only in later stages of development will you see the fungus growing on the wood surface. borate is a contact poison to these organisms. Surface treatments will protect the wood against attack. If already under attack, the high moisture will allow the borate to penetrate deeply into the wood, to the source of the problem.

Dr. Harry Moore is an entomologist and consultant on the staff of DM Associates, Monmouth Beach, N.J.

Table