Treatment options for solid wood packing under the International Standards for Phytosanitary Measures (ISPM-15) have not changed since the requirements were instituted a number of years ago. The only approved treatment methods are heat-treating and fumigation with methyl bromide.
Researchers at Virginia Tech are investigating the use of vacuum technology as an alternative treatment method, and their test results look promising. For years vacuum technology has been used in wood processing operations as either a method of drying wood or to pressure treat wood with preservative chemicals. It allows companies to work with a proven technology that may not be hard to adapt to this new application.
Existing methods have some drawbacks. Methyl bromide is regulated internationally; it is prohibited in developed countries for all but quarantine and preshipment use as part of the United Nations Environment Program. Much research has been directed towards identifying alternatives to methyl bromide and reducing its use. The other approved method, heat-treatment, can be expensive due to its high energy consumption and the need to invest in costly equipment.
Virginia Tech researchers believe there is a need for a globally acceptable treatment method that provides the assurance of eliminating the risk of infested wood materials with the benefits of reduced costs and being environmentally benign. They believe vacuum technology may be a viable solution to meet these challenges.
Vacuum technology consumes little energy, does not release ozone-depleting chemicals into the atmosphere, and does not affect the appearance or properties of wood. Large volumes of wood can be treated with low pressure using a sealable container, a vacuum tube, or even shrink wrap.
Vacuum treatment kills insects by removing water from their bodies. Figure 1 is a photograph of an emerald ash borer larvae under the bark of an ash log after vacuum treatment.
The objective of the research conducted at Virginia Tech was to determine the efficacy of vacuum treatment for killing emerald ash borer larvae and pinewood nematodes in naturally-infested wood at room temperature.
A system that can maintain vacuum conditions was used to conduct vacuum treatments. The treatment system consisted of a vacuum pump, vacuum gauge, and vacuum containers.
The vacuum pump can create a continuous vacuum of less than 5 mm of mercury (Hg). The performance of this type of pump is unaffected by moisture loss from the wood and insect larvae during treatment.
Figure 2 is a photograph of the vacuum treatment set-up used in this research. A flexible vacuum container made of high tenacity woven fabrics, coated and impregnated with specialty synthetic rubber compounds, was used as the test chamber.
Experiments testing the efficacy of vacuum treatment to kill EAB in naturally infested stems (small diameter logs) were conducted at the U.S. Forest Service Northeastern Research Station at
Infested stems were inserted into the flexible container before vacuum was applied. During the treatment process, room temperature, pressure and relative humidity within the flexible container were recorded. The pressure was approximately 6-8 mm Hg, and relative humidity was recorded around 6.1% within the flexible container. The room temperature was controlled at 25 degrees C (77 degrees F).
At periodic intervals, the stems were removed from the flexible container. The larvae were removed from the stems and maintained at room temperature. Larvae were considered dead if no movement was observed within three hours of removal from the stem. If any live larvae or nematodes were found in the stems, the stems were put back into the vacuum container, and vacuum treatment was continued.
Results showed that the efficacy killing EAB larvae and PWN in small diameter logs at room temperature using low pressure vacuum treatment was approximately 100 to 200 hours, depending on log size and wood moisture content.
The critical variables for killing pests in wood using vacuum treatment are:
1) The size of the logs or lumber being treated.
2) The moisture content of the wood.
3) The temperature used during treatment.
Vacuum treatment times will substantially decrease using sawn lumber, such as pallet parts. The thinner the lumber, the less vacuum time required to treat.
Where wood moisture content is concerned, the lower the moisture content of the wood being treated, the less time required to treat. At ambient temperatures wood moisture content decreases substantially during vacuum treatment because more time is needed to kill the insects.
Furthermore, the addition of heat into the system significantly reduces treatment time. Shorter treatment times have less of an effect on moisture content.
Subsequent experiments at Virginia Tech used steam to hasten the treatment times of vacuum treated pallet parts. Depending on the moisture content of the wood before treatment, treatment times to kill insects have been reduced to a range of only four hours and, in some cases, less than one hour.
With increasing international trade of commodities, successful vacuum treatment practices could be approved and included in ISPM 15 as a cost effective and efficient technique to minimize the spread of potentially harmful forest pests and improve the use of wood packaging materials in global trade.
The faculty and staff at the
For more information contact Dr. Zhangjing Chen (chengo@vt.edu) or Dr. Marshall S. White (mswhite@vt.edu) at the
(Editor’s Note: Zhangjing Chen is a research scientist at the
