Potential Use of Nonpressure Treatment with Preservatives to Manage the Risks of Pests in Transported Wood

Jeffrey J. Morrell

Application of either prophylactic or therapeutic chemicals represents one approach for mitigating the risk of importing pests on wood packing materials. The chemicals used and the methods by which they are applied depend on the pests of interest, the wood species, and the length of time for which the treatment must be effective.

Chemical application can be accomplished using topical surface treatments applied at atmospheric pressure using dipping, spraying, or soaking. These methods are primarily designed to deliver a protective layer to the wood surface and are very much akin to the preventative foliar sprays used to protect agricultural crops. Treatment creates a toxic barrier against fungal and insect invasion, presumably kills near-surface insects and fungi that come in contact with the chemical, and prevents organisms already established in the wood from either sporulating on the surface or, for insects, completing their life cycle by exiting the product.

The quality of the surface treatment and the effective period can vary widely with treatment chemical, wood species, and application method. At its simplest, wood can be viewed as a collection of vertical and horizontal straws. The vertical straws represent the active conducting elements in the living tree. The horizontal straws are much shorter and represent the ray tissues in the living tree. Fluid flow tends to be 2 to 3 orders of magnitude greater in the vertical or longitudinal direction. Since most packing material is produced in boards or timbers that are longitudinally oriented, this means that the treatment will tend to be deepest at the end cuts and shallowest along all of the other lumber faces. Generally, dip treatments depend on the capillary action of wood cells to “draw” chemical inward from the surface. Fluid movement inward is generally limited by the connections between individual cells, termed pits, the amount of liquid already present in the wood, and the treatment chemical.

Some species, such as red oak and many pines, are highly permeable and readily accept treatment, while others, such as larch or white oak, present a formidable challenge to fluid ingress. As a result, treatment zones resulting from nonpressure treatment can vary from less than 1.0 mm to the complete thickness of the wood. Relatively shallow treatment zones can be more easily damaged during handling, compromising protection.

Moisture content can also affect uptake. Very wet wood tends to sorb less treatment solution than dry wood, producing a correspondingly shallower protective zone. Chemical characteristics also affect treatment. Solvent-based treatments tend to have lower viscosities and can, therefore, move more easily through the wood cells. Some waterborne chemicals, however, have the ability to diffuse with wood moisture and, given adequate time and a sufficient initial loading, have the potential to completely penetrate the wood. The best example of this type of chemical is boron. The drawback of this chemical is that its water solubility makes it susceptible to leaching. For example, packing material stored outdoors can be subjected to considerable wetting that can sharply reduce the residual boron levels on the wood surface.

Treatment Chemicals
For many years, dip treatments used broadly toxic, long-lived pesticides, such as pentachlorophenol or chlordane, but increasing environmental concerns have encouraged the development of an array of less broadly toxic substitutes, including copper-8-quinolinolate, 3-iodo-2-propynyl butyl carbamate, didecyldimethyl ammonium chloride, propiconazole, tebuconazole, carbendazim, chlorpyrifos, and boron. In general, surface protection against both fungi and insects requires that both a fungicide and insecticide be applied. There are many effective treatment chemicals available for protecting freshly sawn wood from fungal and insect attack-the limitation is the cost of treatment in comparison with the value of the finished product. The majority of water-based chemicals function by delivering a surface barrier to the material. The depth of the barrier is usually limited and depends on the moisture content at the time of treatment (wetter wood will produce a shallower treatment zone).

There are also a limited number of chemicals available in solvent form. These include copper-8-quinolinolate, copper naphthenate, and pentachlorophenol usually applied in a mineral spirits type of solvent. These solvent borne treatments are used to a limited extent by the military to treat ammunition boxes, pallets, wire spools and other wood-based materials that must retain their integrity during long-term storage under varying environmental conditions. The high cost of the solvent usually makes these materials less attractive for treating low-value packing materials. While these chemicals may also work well for mitigating the risk of pests, there is little data to support their use for this purpose.

Application
As noted, nonpressure application typically involves spraying or dipping for various periods. As expected, spraying requires relatively little time, but produces the shallowest and least uniform chemical loadings.

Dipping is the more typical method for surface protection of freshly sawn lumber. In most cases, wood (in units of 100 to 200 pieces) is dipped for 30 to 180 seconds in the treatment solution, allowed to drain for several minutes, then transported to a storage area. Most treatment uptake occurs within the first 30 seconds of dipping, although the wood will continue to sorb solution for varying periods of time afterward.

Ideally, treatments are applied to wood that is in its final processed form (i.e. cut to length) to reduce the risk that later cutting or drilling will damage the treatment envelope. This is feasible for standard pallets and boxes, but is unlikely to happen with dunnage, where on-site fabrication is necessary to ensure that the container securely holds the shipped materials. In these instances, a requirement for brush application of chemicals to cut surfaces would be advisable, although it would be difficult to confirm conformance.

Treatment Results
The effectiveness of a surface treatment depends on the amount of chemical delivered and the depth to which it penetrates from the wood surface. For practical purposes, the end grain will be the best treated; it is the radial and tangential surfaces that will be most poorly treated. The amount of chemical, or retention, must be sufficient to prevent colony initiation by fungi and oviposition by insects. It must also be sufficiently thick to limit the potential for exiting adult insects to chew through to the surface and exit the wood. Finally, it should be sufficiently repellent to termites that might try to tunnel to the surface. Of all of these requirements, the levels required for protection against fungal attack are the best understood, since this approach is similar to the protection of freshly sawn lumber from fungal stain. The levels required for inhibiting oviposition, adult tunneling, and termite attack remain the least well understood. It is likely that chemicals may be less effective against adult beetles (that are not feeding on wood). Further work on the ability of various surface treatments to inhibit beetle and termite tunneling will be required to assess the efficacy of this treatment strategy.

Another problem with assessing a treatment is the inability to measure many actives. With the exception of boron, there are no chemical indicators for detecting actives on wood, and few treatment chemicals permanently color the wood. The sophisticated analytical tools required for chemical detection of many actives are far beyond the means of most treating facilities. One solution to this problem is to require the use of an indicator dye in the treatment solution; however, care must be taken to ensure that the treater does not only use the dye.

Protective Period
In most topical or surface treatment applications, the term of protection is relatively limited. For example, freshly sawn lumber destined for export must be protected for 6 months from the time of sawing to allow for shipping and storage at the port of destination before the wood is dried. The protective period for packing materials remains more vague, owing to the potential that packing materials will serve as storage units upon arrival and that the materials may be reused for other shipments. As a result, the protective period may need to be several years from the time of treatment unless arrangements are made for rigorous adherence to single-use packing. Longer periods of packing usage increase the likelihood that the treatment may be inactivated through combinations of wetting, heating, photodegradation, and physical damage to the wood surface. As a result, prolonged use of packing materials treated by nonpressure methods may require supplemental application of protective chemicals to avoid the risk of harboring pests.

Disposal
Although not often considered as a packing material issue, the utility of a given packing system comes to an end, and this material enters the disposal stream. In many countries, this material is used as fuel, while in more-developed countries it enters the municipal solid-waste stream. In most instances, the relatively low levels of chemical present in materials dip-treated with the fungicides mentioned above should pose relatively little risk if the material is burned at high temperatures or disposed of in a lined landfill. However, few cooking fires achieve the elevated temperatures needed to completely combust many pesticides, and it may be necessary to find other disposal options for treated packing materials. This may be extremely difficult to regulate in countries where wood is in short supply.

Research Needs
The risk of pest introduction on packing materials has steadily increased in importance as the times required for international transit have decreased. For hundreds of years, the months required for transit probably resulted in either insects completing their life cycles or the wood drying below the point where microbial growth could occur. Shorter transit times increased the likelihood that pests could survive to invade their new environment. Unfortunately, the research to develop effective mitigation methods for packing materials has failed to keep pace with the rapid growth in international trade. There is a critical need to develop accurate information on the efficacy of various chemical combinations, the schedules required for effective treatment of wood species used for packing, the protective periods provided by the various treatments, and finally, the relative risks of disposing of treated packing materials.

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