Weathering The Elements

Borate insecticides have been used effectively to protect structural timber in a variety of situations from attack by wood-infesting insects. Boron concentrations above 1,000 ppm are toxic to beetle larvae and termite workers when this quantity is consumed along with wood as food (Taylor 1967). The penetration of a lethal amount of boron to depths below the surface of treated wood depends primarily on application at the recommended rate, and to the moisture content of the wood (Robinson & Barlow 1993).

Two applications of disodium octaborate tetrahydrate (Tim-bor) to southern yellow pine timber with a high moisture content (16% to 18%) provided a residual amount of boron that was considered lethal to powderpost beetles at depths of ¹/₃₂ to ¹/₁₆ inch. The environmental conditions and moisture content of structural timber in typical attics and crawl spaces are usually favorable for the penetration of borate insecticides to depths of ¹/₈ inch (Robinson & Barlow 1995).

The stability of the element boron ensures that borate insecticides will not break down and should remain in the treated wood to provide long-term protection. However, there is limited data on the permanence of boron in structural wood that is exposed to excessive water in the form of rainfall or flooding.

Although these situations may be uncommon, the professional pest control industry has been expanding services to include cleaning wood used in outdoor decking and modern log houses, and protecting wood exposed to the weather with products that combine the features of a paint or stain with a preservative and an insecticide (i.e. X-100 Natural Seal Wood Care). The water solubility of some borate insecticides makes them convenient to use and enhances their penetration into wood, but there is limited data on the permanence of these materials when treated wood is exposed to excess water.

The objective of the research presented here was to investigate aspects of penetration and permanence of two borate insecticides, X-100 Natural Seal Wood Care (9.5% boron) and Tim-bor (10% boron), when applied to southern yellow pine, a common construction timber. Along with information on the penetration of these insecticides into wood with 8% to 10% and 18% to 20% moisture content, data was also collected on the permanence of the borate residues at the surface and below when exposed to rainfall and short-term flooding. This research was conducted in 1994 at the Urban Pest Control Research Center (UPCRC).

METHODS. The wood used for the evaluations was cut into blocks 2 ´ 4 ´ 6 inches long, and the ends (½ inch) were sealed with wax before treatment. The blocks were stored in humid environments to achieve the 8% to 10% and 18% to 20% moisture contents needed for these evaluations.

• Insecticides. Tim-bor (U.S. Borax Inc., Valencia, Calif.), 10% water-based solution of disodium octaborate tetrahydrate; and X-100 Natural Seal Wood Care (American Building Restoration Products Inc., Franklin, Wis.), 9.5% zinc borate in a paraffinic oil carrier.

• Application. One surface of the block was thoroughly wet (to runoff) with the borate solution, to comply with label directions for each product. Two applications were made to each block, and there were four replicates of each treatment.

• Rainfall. Five days after treatment a set of blocks was exposed in the laboratory to approximately ¼ inch of "rain" for 28 consecutive days.

• Flooding. Sixty days after treatment a set of blocks was immersed in water for 72 hours to simulate flooding.

• Analysis. To determine the boron content of the treated wood, a center cube of wood from each block was removed and sliced from the treated surface to depths of approximately 1, 2, 2½, and 3 millimeters. Analysis of the boron content of the wood slices was conducted by the UPCRC.

RESULTS. Penetration into southern yellow pine at 24 hours: The penetration of the two borate insecticides into southern yellow pine is presented in Table 1 below. The variables in this evaluation are (1) application to wood that had a moisture content of approximately 10% or 20%, and (2) the "solution carrier" of the borate. For X-100 Natural Seal Wood Care, the carrier is a paraffinic oil; for Tim-bor, the carrier is water.

At the surface of the wood and at the depth of 2 millimeters there was no significant difference between the amount of boron in the 10% or 20% wood for either insecticide. At these depths the insecticides penetrated wood with low moisture content and high moisture content nearly equally. The difference in the amount of boron in the wood is most apparent at the 1-millimeter depth; there was more boron detected at this depth following Tim-bor treatment than there was from the X-100 Natural Seal Wood Care treatment.

The differences in the amount of boron at the 1-millimeter depth may be linked to the difference in the borate concentration applied Tim-bor was applied as a 10% concentration, X-100 Natural Seal Wood Care was applied as a 9.5% concentration. At the depths of 2.5 millimeters and 3 millimeters, the amount of boron detected in the wood treated with X-100 Natural Seal Wood Care was slightly higher than what was detected in the wood treated with Tim-bor.

SIMULATED RAINFALL. Aspects of the permanence of the two borate insecticides in southern yellow pine is presented in Tables 2 (above) and 3 (on page 50). The variables are wood moisture content (10% or 20%) and the exposure of treated wood to ¼ inch of rainfall per day for 28 consecutive days.

The overall effect of the simulated rainfall on the treated wood was:

(1) The exposure to water (rainfall) removed some of the borate residue that remained on the wood surface following application.

(2) The rainfall induced some further penetration of borates into the wood.

The significant decrease in the amount of boron detected at the wood surface following exposure to the simulated rainfall (a decrease from between 7,142 and 14,201 ppm in Table 1 to between 1,724 and 2,956 ppm in Table 2) did not occur from degradation, since boron is a very stable element. Most of it was probably washed off with the water.

There was an increase in the amount of boron detected at the 1 millimeter to 3 millimeter depths for the wood with 20% moisture content. For example, for wood treated with X-100 Natural Seal Wood Care, at the 1 millimeter depth the change was from 716 ppm (Table 1) to 1,110 ppm (Table 2). The paraffinic oil base apparently was effective in preventing the loss of boron from the wood. The insolubility (in water) of zinc borate may have prevented the loss of boron in this treatment.

The result of the simulated rainfall on wood treated with Tim-bor was to remove some of the residue from the surface and to redistribute boron from close to the wood surface to depths below the surface.

SIMULATED FLOODING. This evaluation was perhaps the greatest challenge for a water-soluble material like boron. The im-mersion of borate-treated wood in water for 72 hours (60 days after treatment) resulted in a significant decrease in the amount of boron detected at the surface and at depths below the surface (Table 3). Slightly more X-100 Natural Seal Wood Care was lost when compared to Tim-bor, but neither insecticide sustained high concentrations of boron.

CONCLUSIONS. The results indicate that the X-100 Natural Seal Wood Care formulation (9.5% zinc borate and paraffinic oils), and the Tim-bor solution (10% borate and water) maintained concentrations of boron lethal to wood-infesting beetles at the surface and immediately below the surface, even following exposure to ¼ inch of rain for nearly a month.

These data show that wood with high moisture content, at least above 10%, will accept water or paraffinic oil formulations of borate insecticides better than wood with low (10%) moisture contents.

Long-term exposure to water such as flooding may remove a considerable amount of borate from treated wood. In these situations reapplication may be necessary to structural timber exposed to such conditions if the effective level of boron that provides protection from wood-infesting insects is to be maintained.

Robert A. Barlow and William H. Robinson are with the Urban Pest Control Research Center, Department of Entomology, Virginia Polytechnic Institute and State University, Blacksburg, Va.

REFERENCES

Robinson, W. H & R. A. Barlow. 1993. Dif-fusion of disodium octaborate tet-rahydrate into southern yellow pine to control wood-infesting beetles. Proc. Inter. Conf. on Insect Pests in the Urban Environ.: 187-191.

Robinson, W. H. & R. A. Barlow. 1995. Penetration of boron into structural wood. Pest Control Technology 24:1.

Taylor, J. M. 1967. Toxicity of boron compounds to the common furniture and house longhorn beetle. Inter. Pest Contr. 9 (1): 14-17.

SIMULATED RAINFALL
INSECTICIDE/WOOD MOISTURE CONTENT		AVERAGE PPM OF BORON PER WOOD SLICE DEPTH BELOW WOOD SURFACE (INCHES) 0 1/32 1/16 1/8
X-100 Natural Seal Wood Care
10%	2,112		760	353	153
20%	2,956		1,110	381	205
Tim-bor
10%	2,348		2,417	2,038	886
20%	1,724		1,526	2,540	2,663

 

PENETRATION AFTER 24 HOURS
INSECTICIDE/WOOD MOISTURE CONTENT		AVERAGE PPM OF BORON PER WOOD SLICE DEPTH BELOW WOOD SURFACE (INCHES) 0 1/32 1/16 1/8
X-100 Natural Seal Wood Care
10%	10,586		603	168	180
20%	7,142		716	257	157
Tim-bor
10%	13,918		2,326	456	81
20%	14,201		1,068	219	78

PCT Magazine, December 1995