Penetration Of Boron Into Structural Wood

Old house borer and powderpost beetle larvae infesting structural wood may feed close to the surface as early stage larvae and deep in the wood as late stage larvae. These insects are important pests because continued reinfestation may lead to severe structural damage. Strategies for their prevention or control include applying insecticides as a gas to the entire structure (fumigation), or as a liquid spray to exposed wood. The depth of penetration and duration of liquid insecticides used for wood protection depends on several factors, including:

1. the wood species treated; for example, pine accepts liquid insecticides better than spruce;
2. the moisture content of the wood; for example, timber in a crawlspace, attic or exposed to the weather may have different moisture conditions;
3. the insecticide formulation, such as water- or solvent-based material.

In recent years there has been increased interest in the use of borate compounds to protect structural timber from wood-infesting beetles. Borates can be applied to wood in sufficient quantities for a toxic amount to diffuse below the surface and provide adequate protection from infestation (Robinson & Barlow 1993). The penetration of these materials depends primarily on the moisture content of the treated wood, and a high moisture content (more than 30%) is most favorable. This requirement has limited the use of borates to the treatment of unseasoned wood, which has a moisture content of about 30%.

Recently, borate formulations such as Tim-Bor (U.S. Borax) and Bora-Care (Nisus Corp.) have been developed for treating seasoned lumber (2 x 4, 2 x 6, etc.) by spraying. To enhance penetration, product label directions usually require a thorough wetting (spray to runoff) of the exposed wood surfaces with two applications within 24 hours.

Structural wood can have a moisture content that ranges from as low as 8% during winter to as high as 18% during late summer. The moisture level depends on the location within the structure and seasonal changes in relative humidity (RH). Although there is data to show that borates can diffuse easily into unseasoned wood with a high moisture content, there is limited information on the diffusion of borates into structural timbers which have a low (less than 30%) moisture content. The objective of the research presented here was to evaluate the penetration of disodium octaborate tetrahydrate (Tim-Bor) into southern yellow pine (a common construction timber) with low moisture content (8-10%), and high moisture content (16-18%).

METHODS. The wood used in this evaluation was cut into small (.5 x 2 x 0.5 inches thick) blocks to make treatment easier. Blocks were stored for several weeks in humid environments to achieve the desired moisture contents. For treatment, one surface of the block was thoroughly wet with the borate dilution to comply with product label directions. Two applications, 24 hours apart, were made to each block, and there were four replicates of each treatment.

Following the borate application, the blocks were maintained in environments with high (85%) and low (50%) relative humidity for 24 hours or six weeks. Then a center cube of wood from each block was removed and sliced from the treated surface to depths of approximately 1/32-, 1/16 d at the same depths. Analysis of the boron content of the wood slices was conducted by the Urban Pest Control Research Center at VPI&SU.

RESULTS. The results of the studies were as follows:

Wood with high moisture content. In general, applying the label recommended rate of a borate material to wood with a high moisture content results in the greatest penetration into wood. Two applications followed by six weeks in an 85% RH environment resulted in the greatest amount of boron (795 ppm) detected at 1/8-inch below the surface. However, Table 1 shows that at the 1/32-inch depth there was more boron detected in wood stored six weeks in the 50% RH environment (3,052 ppm) than in wood stored in the 85% RH environment (1,275 ppm). At 1/16-inch depths there was no difference in the amount of boron detected in blocksanalyzed after six weeks in the 50% RH or the 85% RH environment.

There appears to be continued diffusion of borate insecticide into the wood after application and exposure to environments with high relative humidity. Since the element of boron is stable and there is no degradation, the decrease in boron on the wood surface following application (between 24 hours and six weeks) can be considered the result of diffusion into the wood. This is confirmed by an increase in the amount of boron detected at the 1/8-inch depth after six weeks in 85% RH environment, compared to amounts present at this depth just 24 hours after application (see Table 1).

The applications to wood with high moisture content followed by six weeks in a 50% RH environment is typical of household conditions that may be encountered by pest control operators during summer and fall (Peck 1932, Bois 1951). During the fall, structural wood in house attics, basements and substructure crawlspaces has high (12-18%) moisture content, and during the winter months the RH of the air and moisture content of structural wood decrease. During late summer and fall, old house borer larvae and other wood-infesting beetle larvae are actively feeding. Control strategies, such as spraying exposed surfaces of infested wood with liquid insecticides, are often initiated at this time.

Wood with low moisture content. The amounts of boron detected in the wood with 8-10% moisture content show the influence of wood moisture at time of application and the post-treatment environment (high or low humidity) on borate penetration. There was limited penetration of boron from the surface of the low moisture content wood (see Table 2).

Two applications to wood with low moisture content followed by six weeks in a low RH environment is representative of household conditions during winter and early spring. During this time, structural wood in attics and basements has low moisture content because the surrounding air is also dry. In early spring, powderpost beetles and old house borers emerge from infested wood. Boron residue on the surface and in the upper layers of wood would provide control of these pests at this time.

TOXICITY OF WOOD-INFESTING BEETLES. The effectiveness of surface spraying of borate to protect seasoned wood from insects is linked to the penetration of a lethal amount of boron into lower layers. In the treated wood blocks there was an average of 56 ppm boron detected at all depths. This amount represents the boron that normally occurs in southern yellow pine. Significantly higher concentrations of boron would be necessary to kill beetle larvae normally feed on this wood.

Taylor (1967) reported that in pine sapwood, approximately 1,000 to 1,900 ppm of boron is necessary to prevent attack by first-stage old house borer and powderpost beetle larvae. In the investigations reported here. Two applications of borate to the southern yellow pine blocks with low or high moisture content resulted in concentrations of greater than 1,000 ppm in the top 1/16 inch of wood. After six weeks in 50% or 85% RH environments an amount of born reported lethal to first-stage beetle larvae remained on the surface of the blocks. On the surface or below there is adequate boron concentrations to provide long-term protection of the wood

Diffusion of large amounts of boron deep into the structural wood is probably not necessary for protecting it from an initial insect attack or reinfestation. Lethal quantities of boron are necessary only in the outer layer of the wood to provide control of wood-infesting beetles because it is in this zone that the beetle larvae regularly feed. In this region they are likely to contact the toxic residues of surface-applied borate insecticides.