L. David Dwinell, Research Plant Pathologist, Southern Research Station, USDA Forest Service, Athens, GA 30602.

Global transport of wood fiber without causing environmental harm or ecological disaster requires that the wood be free of pests. USDA Forest Service research on decontaminating coniferous chips, green lumber, and logs has concentrated on the pinewood nematode (PWN) Bursaphelenchus xylophilus and its pine sawyer (Monochamus) vectors. Methyl bromide has been the treatment for this and other quarantine pests of logs and other wood products. Mitigation procedures that have been investigated over the past decade include fumigation, irradiation, chemical dips, and elevated temperature.

A native of North America, the PWN has become a destructive introduced pest in the forests of Japan and other Asian countries. Bursaphelenchus xylophilus, carried by pine sawyers, are transmitted to recently felled logs or dead or dying conifers, particularly pines, during oviposition. These pests may be found in chips, green lumber, and logs. The PWN has been intercepted in chips, green lumber and packing-case wood exported from North America. As a result of these and other interceptions, the European Union and other countries regulate all coniferous imports to protect their forests from the PWN and other exotic pests.

USDA scientists from the Southern Research Station of the Forest Service and the former Stored Products Insects Research and Development Laboratory of the Agricultural Research Service determined that metam sodium and aluminum phosphide were effective in eradicating the PWN in southern pine chips. To demonstrate the practicality of in-transit fumigation, they applied phosphine to a shipload of woodchips exported from Georgia to Sweden. The experiment was successful and awaits broader application.

The irradiation of pine chips was considered to be an alternative to fumigation. Southern Research Station and Georgia Institute of Technology scientists treated PWN-infested wood samples in a cesium-137 irradiator and found 0.9 MRAD to be the lethal dosage. Subsequently, Canadian scientists reported that 0.7 MRAD dosage (cobalt 60 gammacell 220 irradiator) will eliminate the PWN in aqueous solution. These data support the contention that a higher dosage is necessary to eliminate the PWN in vivo than in vitro. The scientists concluded that 0.9 MRAD was too high to make this an attractive means of disinfestation for commercial wood.

The Southern Research Station studied the efficacy of dip-diffusion sodium borate treatments for eradicating the PWN in chips and for eradicating the PWN and pine sawyers in debarked pine logs. Neither the liquid nor the powder formulations were effective in controlling PWN or pine sawyers. However, in a companion treatment, PWN and sawyers were not recovered from pine logs fumigated with methyl bromide at the rate of 240g/m³.

Using elevated temperatures to eradicate mesophilic organisms in wood shows the most promise. Mortality of the PWN and its vectors in wood is primarily a function of moisture content, heat source, time, and temperature. For example, conventional heat, live steam, and hot water can raise wood temperatures to levels that are lethal to the PWN (greater than 45 ° C). Research by scientists from Georgia Institute of Technology and the Southern Research Station determined that the mortality of the PWN in pine chips exposed to radio frequency waves was a function of temperature. In a subsequent study, they evaluated the effectiveness of radio waves and steam, alone or in combination, on PWN-infested chips. High temperatures are possible in less time by combining steam and radio waves, suggesting that the relationship between these two heat sources may be synergistic. Cooperative research between Southern Research Station and Canadian scientists found that a radio-frequency/vacuum kiln was effective in eliminating the PWN and its vectors in sawn wood.

Elevated temperature can pasteurize or sterilize softwood chips, sawn wood, or logs. Heating southern pine chips, green lumber, and logs to a corewood temperature of 60 ° C for 30 minutes is sufficient to eliminate such mesophilic pests as PWN, pine sawyers, and pathogenic fungi. A team of scientists from the Southern Research Station, European Union, and Canada concluded that heating coniferous wood to a corewood temperature of 56 ° C for 30 minutes will eradicate the PWN and its vectors. Shorter exposure times can be achieved with thermal treatments, such as live steam and radio waves, that rapidly heat the wood to high temperatures.

Drying coniferous wood at conventional kiln schedules is essentially a wood sterilization process. The time-temperature schedules required to dry sawn wood to commercial standards are much higher than those required to eradicate pests in the wood. This has been confirmed by Southern Research Station, Finnish, Canadian, and South Korean scientists.

Europe now accepts coniferous chips, sawn wood, and logs that are certified to have been kiln-dried or treated at 56 ° C for 30 minutes. South Korea now allows the importation of kiln-dried pine sawn wood. The research and development on elevated temperature as a mitigation procedure for decontaminating transported wood has required the cooperative efforts of several countries. Many individuals within the USDA Forest Service, Foreign Agricultural Service, Agricultural Research Service, and Animal and Plant Health Inspection Service worked together to bring science and politics together. The value of these wood exports to the United States has been estimated at $355 million per year.

Similar control principles are appropriate for export or import of chips, green lumber, or logs. Pest control begins in the forest, but forest practices should be followed by heat or other mitigation procedures in the ideal wood management system.