The rising cost of energy for heat has resulted in a dramatic increase in demand for firewood to fuel fireplace inserts, wood-burning stoves, and outdoor boilers, according to the National Firewood Association. The increased demand, however, raises concerns about transporting firewood unknowingly infested with invasive insects. 

Two Virginia Tech researchers have developed a new method for treating wood that addresses these concerns while saving time, energy, and resources.

Invasive insects have had a devastating effect on trees in various parts of the country. Those insects that bore deeply into trees are especially troublesome to detect and are often inadvertently transported to uninfested areas. 

For example, the emerald ash borer, first detected in Detroit in 2002, has destroyed tens of millions of ash trees, prompting regulations on the transport of ash logs and the establishment of quarantine zones.

The currently available methods for treating wood to kill invasive pests use either chemicals or extreme heat. However, Research Scientist Zhangjing Chen and Professor Emeritus Marshall White of Virginia Tech’s College of Natural Resources and Environment have introduced a vacuum-contained steam method as an efficient way to treat firewood and other wood products.

Their method was tested with firewood from ash infested with emerald ash borer in Cacapon Resort State Park in West Virginia and proved successful at killing all of the insect’s life stages in the wood. The pest destroyed all of the ash trees at Cacapon within only two years; the only ash remaining in the park is firewood.

The vacuum and steam method took less than half the time and 25 percent less energy than the 140-degree, 60-minute heat treatment required by the U.S. Department of Agriculture before wood can be moved off site.

“Steam carries a large amount of heat, and condensation releases the energy to heat up logs faster,” said Chen. “In addition to saving time and energy, two major advantages of the steam and vacuum treatment are it doesn’t require chemicals and it can be portable.”

The chemical used to fumigate wood is methyl bromide. Use of this ozone-depleting material is severely restricted but is currently permitted for the quarantine and pretreatment of logs.

However, even methyl bromide may not penetrate to the center of logs, where other wood-boring pests like Asian long-horned beetle larvae and pinewood nematodes live, “and conventional heat treatment takes a long time to heat the core to a killing temperature,” said Chen. “But the vacuum and steam treatment can raise the core temperature of even large-diameter logs in a relatively short time.”

The system Chen and White designed consists of a vacuum pump, control unit, flexible vacuum container, and steam generator. The wood to be treated is encased in a bladder tank, which looks like a large plastic bag with a zipper. This portable technology can be used by small business operators to treat firewood, pallets, and other products.

“Several thousand dollars investment and you are ready to go, whether you are treating firewood or pallets,” Chen said. Mother Earth Farms in Millersburg, Ohio, is working with Chen and White to build several units intended to treat firewood and other forest products.

Large bladder tanks can be carried and used in the back of a pickup truck for small loads of firewood, but large logs must be treated in large, rigid chambers, typically found at a research or production facility. 

However, in addition to funding more research with firewood, Chen and White are using U.S. Department of Agriculture Animal and Plant Health Inspection Service funding to develop a pilot-scale rigid unit that fits in a truck trailer so that treatment of logs and large amounts of firewood can take place on site.

The two researchers first tested a vacuum and steam treatment to control insects, fungi, and mold in wood used for pallets in 2006. Further research has proven successful with large logs, such as veneer logs for export, without loss of quality, and for other wood products and packing materials. 

They are now researching the use of the vacuum and steam method to combat snails in ceramic tiles.

Chen is also working in partnership with Chinese colleagues on treatments to kill Asian long-horned beetles in willow wood. He has been named a professor at Northwest Agriculture and Forestry University in China and is currently hosting a visiting scholar from Inner Mongolian Agriculture University who is collaborating on modeling heat and mass transfer during vacuum and steam treatment.

“The goal is to develop technology to address urgent issues on a global scale, to eliminate pathways for the transport of insect and disease pests,” said Chen.