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Heat Treatments to Control Pests on Imported Timber

Exotic or introduced timber pests can have damaging effects on forest ecosystems or timber production areas. North American forests are particularly vulnerable to pests such as fungi, nematodes, or insects introduced through importation of logs, lumber, or unmanufactured wood articles (USDA, 1994a). Because trees produced in temperate areas outside North America are affected by and can introduce a wide variety of pests and diseases that are non-indigenous to this continent, special care is required to ensure that imported wood and wood products are pest-free. The introduction of non-indigenous species could be detrimental to U.S. forest production, recreation, and urban forest resources (USDA, 1991a). Pests from tropical hardwoods, however, pose less of a threat, because tropical hardwood pest habitat requirements cannot generally be met within the temperate forests of the United States (Thomas 1996). These pests can bore into the roots, limbs, or trunk can interfere with a tree's reproductive capabilities, and can cause defoliation, wood damage, or a shift in tree species composition over time. Extensive tree death can have serious impacts on the ecosystem and cause changes in habitat and food supply. In addition, establishment of non-indigenous organisms has clearly been shown to reduce biodiversity (USDA 1994a).

There are several historical examples showing that importation of non-indigenous timber pest species has led to wide-spread blights within the United States. Notable cases this century have included: Chestnut blight (Cryphonectria parasitica, 1904-1955), Dutch elm disease (caused by the fungus Ophiostoma ulmi, mid-1920s), White pine blister rust (fungus Cronartium ribicola, early 1900s), Port Oxford cedar root rot (fungus Phytophthora lateralis, 1923), and the recent Gypsy moth (Lymantria dispar, 1870s) outbreaks. Each of these outbreaks has caused ecological damage such as shifts in species composition, changes in habitat, as well as tree defoliation, stress, and death (USDA, 1994a).

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Methyl Bromide May Not Effectively Control Pests on Imported Logs and Lumber

Currently, many U.S. timber importers rely on methyl bromide fumigation to control pests and pathogens such as Lymantria dispar (Asian gypsy moth), Lachnellula willkommii (Larch canker), and Sirococcus strobilinus (Conifer shoot blight) (USDA 1991a p.6). While methyl bromide is used to fumigate timber and wood products, it may not be the most effective treatment for controlling quarantine pests (e.g., bark beetles and borers, termites, and fungus) on imported logs and lumber (USDA 1994a p. 31 and 19, USDA 1991a p.5-6). Further, it is believed that methyl bromide does not penetrate well into logs, particularly logs with a high moisture content. Cross (1992) found that it is difficult to achieve useful insecticidal doses much beyond a depth of 100 millimeters in green materials using conventional tent fumigation techniques. Likewise, according to the USDA, "there is little scientifically derived efficacy data available to determine the most effective ways to employ methyl bromide fumigation to destroy plant pests associated with imported wood products" (USDA 1994a, p. 31). Additionally, recent test shipments of wood products imported into U.S. that were fumigated with methyl bromide have been found to be infested with fungal pests upon arrival (Forest Service, 1992 in USDA 1994a, p. 31 and Appendix B-81). Methyl bromide, therefore, when used to treat logs and lumber, does not completely eradicate the risk of quarantine pests entering new territory.

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USDA Risk Assessments

In 1990, the Department of Agriculture Animal and Plant Health Inspection Service (APHIS) received its first request to import logs from the Soviet Union. At the time, the associated risks of importing timber, and the lack of APHIS regulations led to concern over the long-term impact of importing foreign timber. Three months later, APHIS turned down the request, and implemented a formal ban on all logs from the Soviet Union until further research could be conducted (USDA 1994a, USDA 1990a). As a result, between 1991 and 1993, the USDA Forest Service conducted three risk assessments on imported timber:

APHIS used these assessments to develop its extensive mitigation measures for minimizing pest introductions in the U.S. during the importation of foreign logs and lumber. The proposed APHIS plan, which has been developed as a result of the Forest Service risk assessments, would allow importers to choose between several pest treatment strategies (APHIS would oversee decisions and evaluate all strategies on the basis of effectiveness). If a strategy cannot be shown to produce negligible risk, then APHIS has the power to deny entry.

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Heat Treatments are Considered a Viable Method to Control Quarantine Pests

Based on USDA risk assessments, heat treatments of logs and lumber are considered to be more effective than methyl bromide for providing quarantine security and are considered to be an effective alternative to methyl bromide for the control of quarantine pests (USDA 1996). As a result, the use of heat-based sterilization to control biological pests offers great potential for the imported timber industry. Both moist heat (steam or hot water) and dry heat have been shown to effectively control fungi, insects, and nematodes associated with logs and lumber products (USDA 1991a) (Task Force on Pasteurization of Softwood Lumber 1991, Jones 1973, Baker 1969, Snyder and St. George 1924) (Dwinell 1990, USDA 1991; Ostaff and Cech 1978, Ostaff and Sheilds 1978, Parkin 1973, Department of Scientific and Industrial Research, Great Britian 1957, Snyder and St. George 1924, Snyder 1923).

To effectively eliminate pests, heat treatment requires that the internal temperature of the logs and lumber be raised to a specified temperature over a given period of time. Regulations require that all heat treatments be performed at a facility authorized by APHIS or by an inspector authorized by the national government of the country in which the facility is located (USDA 1994a). Core temperatures can be monitored by using thermocouples. Heat treatment techniques may include the use of steam, hot water, kilns (lumber only), microwave energy, or any other method that raises the temperature at the center of the log to at a minimum of 71° C (167° F) for at least 60 minutes.

Several treatment specifications are available for using steam or dry heat to treat logs and lumber. Typically, pressurized steam can be introduced to a chamber, and dry heating (with moisture added to minimize warping and spliting) can be accomplished using a commercial kiln. Because the killing efficacy of heat treatments depends on the time, temperature, and humidity (USDA 1991a p. 17), steam heat will kill pests more efficiently and rapidly than dry heat because the organisms under moist conditions are more susceptible to thermal killing due to the denaturation of proteins, particularly enzymes (USDA 1994a p. 3, USDA 1991, USDA 1990a ). However, despite the effectiveness of steam heat treatments, kiln drying is the most commonly used heat treatment for lumber (Mathews 1996, Griffin 1996, Waggener 1996, Briggs 1996, Morrell 1996, Loromer 1996). The specifications for moisture-reduced heat treatment are the same as those for standard heat treatment, with the added component that moisture must be reduced to 20 percent or less. Penetration of dry heat proceeds at a much slower rate than steam heat, therefore kiln drying requires a much longer exposure time (USDA 1994a).

As is the case with methyl bromide, heat treatment has been found to be effective for killing insects and plant pathogens on and within the regulated article only at the time of treatment (i.e., with no residual protective effect), articles must be protected against subsequent reinfestation. A wide variety of methods, separate or in combination, may be used to control reinfestation, including storage in pest-free warehouses, storage in sealed containers, or the use of prophylactic pesticide sprays or dips (USDA 1994a). Kiln drying, stream heating, and hot water immersion can eliminate deep wood pests and can also make the regulated article less vulnerable to reinfestation.

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Heat Treatment: Effects on Wood Quality

Both steam and dry heat can effectively penetrate logs and raise the internal temperatures to levels that effectively control pests without causing wood damage (USDA 1991a). In general, heat treatments do not have any significant deleterious effects on log quality because controlled heat treatments help to reduce wood damage caused by uneven drying (USDA, 1994). Temperatures up to 82.2° C (180° F) for periods up to one hour do not appreciably affect the properties of wood (USDA 1994a). Depending upon the type of wood and size, some surface damage may be noted. Potential damage to lumber caused by poor drying includes the following: surface checks, warping, uneven moisture content, and discoloration (USDA 1991c). Additional research is needed to determine more clearly the potential deleterious effects of heat treatment on logs and other wood articles (USDA 1994a).

Additionally, the value of wood is often increased by proper heat treatment. Whereas green wood products treated with methyl bromide do not incur any additional value, heat treatments cure the wood and impart value compared to unseasoned or untreated wood. For example, wood that has been milled and heat treated (i.e., kiln dried or steam treated) typically has 30-50 percent greater value than untreated wood (Rice 1996, McDonagh 1996). Because wood treated with methyl bromide must be dried and cured via heat treatment anyway and because heat treatments provide similar pest control benefits compared to methyl bromide, methyl bromide treatments may be superfluous. However, some users prefer to purchase "green wood", re-manufacture the material, then dry it.

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Costs

A comparison of the costs of sterilizing logs and lumber with methyl bromide fumigation vs. heat treatment (i.e., kiln-drying and steam treatments) is provided in the Table 1. As shown, the net costs for both kiln-drying and steam treatments are negative because of the increased product value resulting from the heat treatment process. However, it must again be noted that some types of timber and wood products may be damaged by heat treatments, and thus would change the cost listed here. The costs for methyl bromide fumigation include labor, tarp, and chemical costs. The costs for heat treating and steam treating lumber include labor, energy, and equipment costs. The labor and energy costs for both kiln-drying and steam treatments, however, vary according to treatment time. Heat treatment can take up to 25 days, whereas steam treatment takes only 1-2 days. Furthermore, steam heating is less labor intensive than kiln drying. Kiln drying requires that "sticks" (or pieces of wood) be inserted between layers of wood to allow better distribution of heat and air flow during the treatment process, however, these sticks must be removed before shipping. However, many mills now have automatic stackers which markedly reduce labor costs. In steam drying, a lath (or a thin piece of residual wood) is used to separate the layers, and does not need to be removed before shipping. Another factor contributing to the cost discrepancy between kiln-dried vs. steam treated wood is Btu utilization. For dry heat sterilization, low heat (low Btu) is used initially, but then gradually increased until the maximum heat (high Btu) is obtained on the 25th to 26th day (for hardwoods -- conifers are normally dried for 3-5 days). For steam sterilization, however, the same amount of heat used in the final stage of kiln drying is used at the beginning of the process and is sustained by small bursts of heat throughout the short 1 or 2 day treatment process. Furthermore, the chamber vents are kept closed to aid in heat conservation. Therefore, steam sterilization takes both less time and utilizes less heat energy compared to that used in dry heat sterilization, ultimately reducing energy costs.

Table 1. Timber Treatment Cost Comparison ($/1000 bd. ft.)
Cost Considerations Methyl Bromide Heat Treatment: Steam Treatment:
Softwood Hardwood Softwood Hardwood
Product Initial Cost/Value (Pre-Treatment) 500-850 500 850 500 850
Treatment Cost: 1-3 85-155a 100-200b 35-60 41-77
Total Cost 501-853 585-655 950-1050 535-560 891-927
Product Value (Post Treatment) 500-853 655 1275 650 1275
Net Cost 1-3 5-(65) (225-325) (90-115) (348-384)
Notes: Sources: Rice 1996, USDA 1996, Mathews 1996, Milota 1996, McDonagh 1996, McGehee 1996, UNEP 1995.

References

Baker, K.F. 1969. Aerated-steam Treatment of Seed for Disease Control. Horticultural Research. Volume 9. Pages 59-73.
Briggs. 1996 (July 16). Personal communication. David Briggs, Professor, University of Washington. Seattle, Washington.
Chidester, M.S. 1937. Temperatures Necessary to Kill Fungi in Wood. Proceedings from the American Wood Preservers Association. Volume 33. Pages 316-324.
Chidester, M.S. 1939. Further Studies on Temperature Necessary to Kill Fungi in Wood. Proceedings from the American Wood Preservers Association. Volume 35. Pages 319-324.
Cross, D.J. 1992. Penetration of Methyl Bromide in Pinus radiata Wood and Its Significance for Export Quarantine. New Zealand Journal of Forest Science. Volume 21. Number 2 and 3. Pages 235-245.
Department of Scientific and Industrial Research, Great Britian, 1957. The Kiln Sterilization of Lyctus-infested Timber. Leafl.13. Princes Risborough, UK: Forest Products Research Laboratory. Page 4.
Dwinell, L.D. 1990. Heat-treating and Drying Southern Pine Lumber Infested with Pinewood Nematodes. Forest Products Journal. Volume 40. Number 11/12. Pages 53-56.
EPA. 1986. Guidance for Registration of Pesticide Products Containing Methyl Bromide as the Active Ingredient. United States Environmental Protection Agency, Office of Pollution and Toxic Substances. Washington, DC.
Griffin. 1996 (July 17). Personal communication. Robert Griffin, United States Department of Agriculture, Animal and Plant Health Inspection Service. Hyattsville, MD.
Jones, T.W. 1973. Killing the Oak Wilt Fungus in Logs. Forest Products Journal. Volume 23. Pages 52-54.
Loromer. 1996 (July 17). Personal Communication. Jennifer Loromer. United States Department of Agriculture, United States Forest Service. Washington, DC.
Matthews. 1996 (July 17). Personal Communication. Jim Matthews, Western Wood Products Association. Portland, Oregon.
McDonagh. 1996 (September 3). Personal Communication. Tom McDonagh, Rex Lumber Company. Acton, Massachusetts.
McGehee. 1996 (September 9,11,13, 17). Personal Communication. Stan McGehee, Willamette Industries, Inc. Sweethome, Oregon.
McLean, J.D., 1952. Preservative treatment of wood by pressure methods. U.S.D.A. Agricultural Handbook 40. Washington, DC.
Milota. 1996 (July 17). Personal Communication. Mike Milota, Oregon State University. Corvallis, Oregon.
Morrell. 1996 (July 16). Personal communication. Jeff Morrell, Professor, Department of Forest Products, Oregon State University. Portland, Oregon.
Newbill, M.A. and J.J. Morrell. 1991. Effects of Elevated Temperatures on the Survival On Basidiomycetes that Colonize Untreated Douglas-fir Poles. Forest Products Journal. Volume 41. Number 6. Pages 31-33.
Rice. 1996 (July 18). Personal Communication. Bob Rice, University of Maine at Orno. Orno, Maine.
Sahle-Demessie, E., K.L. Levien, J.J. Morrell, and M.A. Newbill. 1992. Modeling internal temperature changes of timber poles during ACA treatment. Wood Science and Technology 26:227-240.
Snyder, T.E. 1923. High Temperatures as a Remedy for Lyctus Power-post Beetles. Journal of Forestry. Volume 21. Pages 810-814.
Snyder, T.E. and R.A. St. George. 1924. Determination of Temperature Fatal to the Powderpost Beetle, Lyctus planicollis LeConte, Steaming Infested Ash and Oak Lumber in a Kiln. Journal of Agricultural Research. Volume 28. Number 10. Pages 1033-1038.
Task Force on pasteurization in Softwood Lumber. 1991. The Use of Heat Treatment in the Eradication of the Pinewood Nematode and Its Vectors in Softwood Lumber.
Thomas. 1996 (July 17). Personal communication. Don Thomas, United States Department of Agriculture, Animal and Plant Health Inspection Service. Washington, DC.
Ostaff, D.P. and M.Y. Cech. 1978. Heat-sterilizaion of Spruce-pine-fir Lumber Containing Sawyer Beetle Larve (Coleoptera: Cerambycidae), Monochamus sp. Rep. OPX200E. Ottowa, ON: Canada Forestry Service. Page 9.
Ostaff, D.P. and J.K. Sheilds. 1978. Reduction of Loses to Logs and Lumber Caused by Wood-boring insects. Rep. OPX218E. Ottowa, ON: Canada Forestry Service. Page 15.
Parkin, E.A. 1973. The Kiln-sterilization of timber infested by Lyctus powder-post battles. Journal of Forestry. Volume 11. Pages 32-39.
USDA. 1991a. An Efficacy Review of Control Measures for Potential Pests of Imported Soviet Timber. USDA, Animal and Plant Health Inspection Service. Miscellaneous Publication No. 1496. September 1991.
USDA. 1991b. Pest Risk Assessment of the Importation of Larch from Siberia and the Soviet Far East. USDA, Forest Service. Miscellaneous Publication No. 1495. September 1991.
USDA. 1991c. Dry Kiln Operator's Manual. USDA, Forest Service, Forest Products Laboratory. Madison, Wisconsin. Agricultural Handbook No. 188. August 1991.
USDA. 1992. Pest Risk Assessment of the Importation of Pinus radiata and Douglas-fir Logs from New Zealand. USDA, Forest Service. Miscellaneous Publication No. 1508. October 1992.
USDA. 1993. Pest Risk Assessment of the Importation of Pinus radiata, Nothofagus dombeyi, and Laurelia philippiana Logs from Chile. USDA, Forest Service. Miscellaneous Publication No. 1517.
USDA. 1994a. Importation of Logs, Lumber, and Other Unmanufactured Wood Articles: Environmental Impact Statement.
USDA. 1994b. Proposed Rules: Importation of Logs, Lumber, and Other Unmanufactured Wood Articles. USDA, Animal and Plant Health Inspection Service. 7 CFR Part 319 [Docket No. 91-07403]. RIN 0579-AA47.
USDA. 1996. Importation of Logs, Lumber, and Other Unmanufactured Wood Articles. 7CFR319.40. United States Department of Agriculture, Animal and Plant Health Inspection Service (APHIS). Washington, DC.
UNEP. 1995. Montreal Protocol on Substances that Deplete the Ozone Layer, 1994 Report of the Methyl Bromide Technical Options Committee: 1995 Assessment. United Nations Environment Programme, Ozone Secretariat. Nairobi, Kenya.
Waggener. 1996 (July 16). Personal communication. Thomas Waggener, Professor, University of Washington. Seattle, Washington.

Please note that this publication discusses specific proprietary products and pest control methods. Some of these alternatives are now commercially available, while others are in an advanced stage of development. In all cases, the information presented does not constitute a recommendation or an endorsement of these products or methods by the Environmental Protection Agency (EPA) or other involved parties. Neither should the absence of an item or pest control method necessarily be interpreted as EPA disapproval.

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