Potential Use of Elevated Temperature to Manage Pests in Transported Wood

L. D. Dwinell

If wood fiber is to move globally without causing environmental harm or ecological disaster, procedures are needed to insure that transported wood and wood products are free of viable pests. Since wood is organic and organic, material provides a food source for a large variety of organisms, including insects, nematodes, and fungi, this is not an easy task. Wood is also a highly variable substrate in terms of nutritional content for and susceptibility/resistance to growth of these organisms. With the exception of very fresh logs, most wood transported is not alive, and most of the organisms carried in wood products are saprobes, adapted for life on dead or dying substrate. These saprobes are not normally pathogenic to healthy plants in their native environment; however, there are examples of organisms becoming significant pests on new hosts in exotic environments.

Wood products such as kiln-dried wood, or paper and panel products that have been hot-pressed, are not likely to contain viable pest organisms. Products that are green (still moist) are the most likely to pose a risk of carrying pests, and this paper will concentrate on these. There are several ways to reduce or kill pests on wood, but heat is widely regarded as the most acceptable and environmentally friendly method. Although there is information on the effects of temperature on the growth and reproduction of wood-inhabiting organisms in laboratory studies, there is a paucity of critical information on using heat to decontaminate wood in practical sense.

Thermosensitivity
Temperature plays a decisive role in the survival, growth, distribution, and diversity of organisms. Most, if not all, pests, including insects, nematodes, and fungi, of forest trees are mesophilic organisms that grow under medium thermal conditions (5 to 37ºC). Most members of this ecological group have optima in the region of 20º to 30ºC. The thermal death point for mesophiles, which is the temperature above which they are unable to survive, is generally 46ºC.

Thermophilic fungi “like” warmer temperatures and have a growth temperature maximum at 50ºC or above and a temperature minimum of 20ºC or higher. Thermotolerant species have a growth temperature maximum of about 50ºC and a temperature minimum well above 20ºC. The ability to only develop at high temperatures is seen in only a few fungi. Phanaerochaete chrysosporium is one example of a thermotolerant hymenomycete that can biodegrade lignin in “composting” woodchip piles. Finally, it is important to recognize the difference between thermophily and thermodurism. The latter term refers to organisms that can withstand high temperatures in a dormant or inactive state. Thus, for example, some mesophilic fungi can withstand heat schedules used in pasteurization. Such fungi are thermoduric, not thermophilic.

Heating wood
Elevated temperatures can be used to pasteurize or sterilize wood because the heat denatures proteins and enzyme systems within the organisms. The heat-resistance of organisms is lower in the presence of moist heat because wet heat causes death by coagulation of proteins, while dry heat relies primarily on an oxidation process. Death of organisms occurs more rapidly when free water is present in the wood, which occurs when the wood moisture content is above the fiber saturation point. This is generally 28 to 30% on a dry matter basis. Additionally, because moist air carries a higher amount of heat than dry air and is a better conductor of heat, wood tends to heat faster in a wet atmosphere or in water.

There are a number of heat sources or heating methods that can be used. These include, the application of artificial heat in a kiln chamber, immersion in hot water, and exposure to electromagnetic radiation (gamma to radio waves). The effectiveness of the heating depends on the type (species and physical form) of wood as well as the type and power of the heat source and the efficiency with which heat transfer happens.

Chips
Combinations and successions of fungi commonly occur during decomposition of organic matter. Some 114 species of fungi in 82 genera have been identified from piled hardwood and softwood chips. Ecologically, there is a shift from mesophilic organisms to thermoduric, thermotolerant, and thermophilic organisms in the interior of piled chips because of thermogenesis during the decomposition process. Chips in the interior of piled softwood chips do not harbor the pine wood (Bursaphelenchus xylophilus) nematode when oxidative processes cause spontaneous heating to 60ºC. The thermal death point of the pine wood nematode in wood chips is 46ºC. Like most mesophilic organisms, the rate of mortality of the pine wood nematode in softwood chips is largely a function of temperature, time, wood moisture content, and heat source. It has generally been concluded that wet heat is more effective than dry heat for eradicating the pine wood nematode in chips. The most rapid method for heating wood chips to the thermal death point of the pine wood nematode is a combination of steam and radio waves. Heat treatment methods for wood chips that have already been considered have not been judged to be economically feasible.

Logs
There is little information on using heat to decontaminate freshly cut logs. Dwinell reported that heat-treating naturally infested Virginia pine (Pinus virginiana) logs to a core temperature of 53oC killed Monochamus spp., and 60oC was sufficient to eliminate the pine wood nematode and wood-inhabiting fungi. Laboratory studies during the 1930s on sapwood sticks of loblolly pine (Pinus taeda) artificially infested with three decay fungi found that the mortality of the test fungi was a function of temperature and time. The three decay fungi tested are killed if the temperature in the wood is maintained for 60 min at 65ºC, 30 min at 77ºC, 20 min at 82ºC, 10 min at 93ºC, or 5 min at 100ºC, providing the moisture content remains above the fiber saturation point. Furthermore, there is some indication that sapwood fungi colonizing Douglas fir (Pseudotsuga menziesii) logs are more sensitive to elevated temperature than heartwood-colonizing fungi. Heating air-dried Douglas fir logs to 65.6ºC for at least 75 min during treatment eliminates any decay fungi in the wood. Heat treatment during or prior to pressure treatment is commonly used on utility poles to eliminate wood-decay fungi that could otherwise rot the center of the poles.

It has long been known that many of the sapstaining ophiostomatoid fungi (Ophiostoma and Ceratocystis spp.) are vectored by bark beetles. These fungi invade living host cells in the sapwood and cause the blue-black discoloration of sapwood commonly referred to as bluestain or sapstain. Although most sapstain fungi are saprophytic, some are pathogens, especially those in the genus Ceratocystis. For example, Ceratocystis virescens causes sapstain of freshly cut logs of certain hardwoods, but on sugar maple (Acer saccharum) causes the disease sapstreak. Sphaeropsis sapinea, the causal agent of Sphaeropsis blight (=Diplodia blight), is a common fungus in northern and southern hemisphere temperate regions and also causes sapstain in trees and freshly cut logs. Sapstain in logs is controlled largely by appropriate wood management techniques and sometimes by chemical treatment of logs with fungicides.

The oak wilt fungus, Ceratocystis fagacearum, can be killed in oak (Quercus spp.) logs by heating the logs in hot air at 43ºC for 48 h or 54ºC for 54 h or by immersing them in hot water at 43ºC for 48 h or 49ºC for 12 h.

Lumber
Several studies in the early 1990s in the United States and Canada showed that the pine wood nematode could be eradicated in unseasoned sawn pine lumber by heating to a core temperature of 59 to 60ºC. In a trilateral study involving Canada, the United States, and the European Union, it was concluded that heat-treating lumber to a core temperature of 56ºC for 30 min eradicates the pine wood nematode and its pine sawyer vectors. This heat pasteurization treatment is the regulatory standard for importation of coniferous wood into Europe from countries in which the pine wood nematode is known. This heat treatment standard was adopted by the Peoples Republic of China for treatment of solid wood packing material in 2000.

Sapstain and mold fungi can be a problem on unseasoned lumber. The principal sapstain fungi are Ascomycetes, especially the ophiostomatoid fungi. Common molds include Zygomycetes as well as species of Penicillium, Trichoderma, Gliocladium, and Aspergillus. Hardwoods are more frequently host to different genera and species of sapstain fungi than softwoods. Furthermore, wood species may differ considerably in their susceptibility to sapstain. Trichoderma viride (= T. lignorum) is the most common mold growing on the unseasoned sapwood of pine. Thermoduric species of Trichoderma and Rhizopus can survive 61ºC for 3 h in short leaf pine (Pinus echinata) blocks with a wood moisture content of about 20%, but not in blocks that have been heated at 56ºC for 7 h (Dwinell, unpublished data). Temperatures above 65ºC are considered to be lethal to most sapstain fungi.

There is little information on using elevated temperatures to control insects in unseasoned sawn wood. Pine sawyers (Monochamus spp.), vectors of the pine wood nematode, have relatively low lethal temperature requirements. The eggs and larvae of Monochamus have a high mortality rate at 38ºC, and pupae are slightly more tolerant, with complete mortality at 40ºC. Monochamus larvae in spruce, pine, and fir lumber are killed once the core wood temperature reaches 50ºC. It has been generally concluded that the temperature schedules that eliminate the pine wood nematode from wood would be effective in eliminating the pine sawyers. Ambrosia beetles in unseasoned western hemlock can be eliminated by a kiln run of 65ºC for 90 min. There has been extensive research on using heat to control insects in stored-products such as grain. The lethal temperature for stored product insects is generally 46ºC.

Drying lumber
Drying lumber is done for two reasons: first, most customers want a stable dried product; second, drying is the most effective method of preventing sapstain and molds, since the minimum wood moisture content for infection by fungi is around the fiber saturation point. However, kiln-drying of some hardwoods for lower value solid wood packing material may be uneconomical and impractical. It costs about twice as much to kiln-dry hardwoods as softwoods, and nail holes may have to be predrilled (most hardwood lumber is case-hardened during kiln-drying and will split when nailed). Wood that has been seasoned, either in a dry kiln or in the seasoning yard, will upon wetting again be subject to colonization by stain-producing fungi and molds as well as wood-decay fungi. The fungi colonizing rewetted wood may not be the same as those that inhabited the wood prior to drying.

A kiln in the Pacific Northwest used for drying lumber.

Kiln-drying kills insect infestations, and the wood will not normally be susceptible to attack by insects that are attracted to logs (i.e., engraver and longhorned beetles). However, drying will not prevent future infestation by insects that attack wood in service, such as termites, carpenter ants, and powder-post beetles. These insects can cause extensive losses to seasoned sapwood of hardwood and softwood lumber. They can be eliminated by additional use of elevated temperatures. Heating of whole buildings is being considered to control termites and stored-product insects as an alternative to fumigation. For example, Hylotrupes balufus, an important destroyer of wood indoors in Europe, has a lethal threshold temperature of 45ºC. Reportedly, short exposure (5 min) to elevated temperatures (55º to 56ºC) is lethal to eggs of Lyctus powder-post beetles in dry wood.

Kiln schedules
Bulk heating of wood to control pests is most efficiently done in a well- designed and well-maintained kiln. The heating is usually, but not necessarily, done as part of the drying process. Kilns can be conventional, using hot air and/or steam heating or more specialized equipment. The drying process in conventional kilns consists of supplying heat, which vaporizes the water in the wood, and venting the moist air outside the kiln. This is done in a continuous process balancing heat supply and venting, so the wood dries at an optimum rate. To promote even drying, the stacks are made so warm air circulates through, rather than around, the piles of lumber. The airflow is carefully controlled by the speed and direction of fans. Temperature and humidity are also controlled by kiln control systems, usually computerized, so an established schedule can be followed. Schedules are designed to minimize energy consumption and time to obtain the target moisture content while also minimizing development of defects in the lumber.

The temperatures reached in a kiln depend on the schedule being followed and the particular equipment being used. Typically, the ambient chamber (dry-bulb) temperature is in the order of 77ºC or above, although when specialized products are being dried slowly, or when dehumidifier kilns are used, the temperature can be lower. For example, conventional schedules for southern pine start at 77ºC and take about 72 h to dry to 19% and lower moisture contents. The largest volume of southern pine is dried in high-temperature kilns at above 100ºC (19 to 24 h). Depending on board size and heat conductivity, the temperature in the core of a board will in time equilibrate with the ambient temperature. During industrial drying, quality-control checks of the moisture content and temperature are routinely made, and target core temperatures can be met or exceeded.

Conclusions
The use of elevated temperature to manage pests in transported wood is already used for higher value wood products and kiln-dried wood. Heat treatment also has great potential for the production of pest-free lumber used for solid wood packing material. The thermosensitivity of microorganisms is generally understood and can be effectively applied to use elevated temperatures to decontaminate wood. In wood, the lethal temperature for insects is below those found for the pine wood nematode and many wood-inhabiting fungi. The technology, such as kiln chambers for using artificial heat to season lumber, is well established.

Acknowledgement. The author thanks Tony Byrne, Forintek Canada Corp., Vancouver, BC, Canada, for his in-depth review of the original manuscript.

Selected References

Dwinell, L.D. 1996. Using heat to decontaminate softwood chips, lumber and logs. Pages 91-97 in: Importing Wood Products: Pest Risks to Domestic Industries, Portland, OR, March 4-6, 1996.

Morrell, J.J. 1995. Importation of unprocessed logs into North America: A review of pest mitigation procedures and their efficacy. For. Prod. J. 45:41-50.

Smith, R.S., ed. 1991. The use of heat treatment in the eradication of the pine wood nematode and its insect vectors in softwood lumber. Report of the Task Forces on Pasteurization of Softwood lumber. Forintek Canada Corp., Vancouver, BC, Canada. 72 pp.