How Deep Does It Go?

Foam has become an effective way of applying termiticides to voids beneath concrete slabs, and for treating other difficult-to-reach areas around structures (Thomas & Robinson 1991, Barlow & Robinson 1993a). The increased interest in termiticide foam has resulted in the development of specific foaming agents and several portable foam-making machines for the pest control industry (Robinson & Barlow 1993b).

A few of these machines have been designed to deliver borate foam into wall voids for the control of wood-infesting insects. The need for surface loading and the gradual penetration characteristics of some borate formulations may make the use of a water-based foam an effective means of applying these insecticides.

Foam is a compact mass of air bubbles which are separated by a liquid film. It is 85% to 98% air and only about 5% liquid. Foam may contain a lot of water and be considered "wet," or a little water and be considered "dry."

Foam exists for only a limited time; eventually the liquid drains from between the bubbles, which causes them to break and the foam to "collapse." The speed at which foam collapses depends on the amount of water and other chemicals in it.

The slow collapse of a borate foam may provide the opportunity for increased diffusion into wood. However, there is little or no data on the penetration of borate foam into wood, or the advantages of using borate foam over a liquid.

The objective of this research was to investigate the diffusion of a borate foam Tim-bor, disodium octaborate tetrahydrate into southern yellow pine. The variables included one or two applications, a borate concentration of 10% and 20%, and the wood-moisture content. The current label directions for Tim-bor specify two applications at 10% concentration or one application at 15% concentration.

THE STUDY. Six-inch-long two-by-four blocks of southern yellow pine were treated with either a liquid or foam dilution of disodium octaborate tetrahydrate (Tim-bor, U.S. Borax Inc.). Before treatment, the ends of the blocks (1/2 inch) were sealed with wax and stored in humid environments (laboratory conditions) to achieve the wood moisture contents of 8% to 10% and 15% to 20%.

Treatments included one and two applications of 10% liquid, one and two applications of 10% foam, and one application of 20% foam. Foam was produced by a Pestifoamer PF-6 and Pestifoam surfactant. (Both products are manufactured by Richway Industries, Janesville, Iowa.) The expansion ratio was approximately 22:1. The blocks were placed horizontally, and liquid or foam was applied to one two-by-four surface at the rate of 1 gallon per 200 square feet of wood surface.

The blocks remained horizontal until the liquid or foam penetrated the wood. Following treatment the blocks were stored in humid environments to maintain the initial wood moisture content.

At 24 hours the blocks were planed to remove a continuous layer of wood approximately ¹/₃₂ inch thick, beginning from the treated surface down to a depth of 1/2 inch. A total of four layers (each ¹/₃₂ inch thick), representing the treated surface and depths of ¹/₈, 1/4, and 1/2 inch, were removed from each block.

There were three sample replicates for each depth. The Urban Pest Control Research Center analyzed the samples of each layer for the amount of boron present.

The Findings

 • One Application of 10% Borate Foam. When measured at 24 hours, the amount of boron detected at the wood surface (approximately 4,000 to 6,000 ppm) and at ¹/₈, 1/4, and 1/2 inch below the surface was not significantly different between the wood blocks with 20% and 10% moisture contents. Following only one application of borate, the wood moisture content of the blocks did not seem to influence penetration of the borate. Evaluations conducted four weeks after treatment showed little increase in the concentration of boron at the ¹/₈ inch depth.

• Two Applications of 10% Borate Foam. Following two applications of 10% borate foam there was an increase in the amount of boron at the wood surface (approximately 9,000 ppm), and at the ¹/₈, 1/4, and 1/2 inch depths below the surface. The amount was significantly greater than what was detected following one application of 10% foam (see above).

In the evaluations conducted 24 hours after treatment, the blocks with 20% moisture content provided the best penetration of the foam. Penetration of borate into the wood seemed to be linked to two factors the number of applications and the wood moisture content. Evaluations conducted four weeks after treatment did not show an increase in the amounts of boron detected at the three depths below the surface.

• One Application of 20% Borate Foam. There seems to be a long-term advantage to a single application of a high concentration of borate foam (20%) to the surface of structural wood, whether the wood has a moisture content of 10% or 20%.

When measured at 24 hours, the penetration of a 20% borate foam to depths of ¹/₈ and 1/4 inch was not improved over two applications of 10% borate; the majority of the borate was concentrated at the wood surface. However, evaluations conducted four weeks after treatment showed the amount of boron detected at the ¹/₈ inch depth in the 20% moisture-content wood had significantly changed; it increased from 706 ppm to 1,195 ppm during this time. Apparently, the borate at the surface continued to penetrate the wood with 20% moisture during the four weeks following application.

• Foam vs. Liquid Application. The benefits of using foam may be seen when the penetration following two applications of 10% borate foam is compared to two applications of 10% borate liquid to wood with only 10% moisture. The amount of boron that penetrated to the ¹/₈ inch depth was significantly greater after the foam application (1,510 ppm) than after the liquid application (268 ppm).

The primary advantage of using foam to deliver water-soluble borates to the surface of structural wood is increased penetration, even when the wood has a low moisture content. Professional pest control operators may encounter wood with low moisture content (10%) when treating in attics, crawl spaces, or outdoor decks.

TOXICITY TO INSECTS. The objective of applying borates, whether liquid or foam, to structural timber is to produce a concentration of boron below the surface of the wood that is lethal to the target insect pest. Taylor (1967) determined that a concentration of more than 1,000 ppm of boron was necessary to kill the larvae of the common wood-infesting beetles.

Two applications of 10% borate foam achieved the necessary concentration at the ¹/₈ inch depth below the treated surface of wood with 10% moisture content. New label recommendation s permit one application of a 15% borate solution. Considering that data indicate increased penetration of borates in wood with 20% moisture, higher borate concentrations may be expected at this depth.

William Robinson and Robert Barlow are with the Urban Pest Control Research Center, Department of Entomology, Virginia Polytechnic Institute & State University, Blacksburg, Va. George Rambo is president of George Rambo Consulting Services, Herndon, Va.

REFERENCES

Barlow, R. A. and Wm. H. Robinson. 1993a. Using foam effectively. Pest Control Technology 21(11):39-46.

Robinson, Wm. H. and R. A. Barlow. 1993b. Evaluating the foamers. Pest Control Technology 21(4): 46-52.

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

Thomas, C. and Wm. H. Robinson. 1991. Foam formulations for subslab injection of termiticides. Pest Control Technology19(10):64-67.