Potential Use of Fumigation to Manage the Risks of Pests in Transported Wood

L. D. Dwinell

Fumigation of raw wood products and solid wood packing material to eliminate pests shows considerable promise. Methyl bromide has been the most widely used fumigant for the treatment of logs and solid wood packing material for phytosanitation purposes. In the past few years, considerable research has been conducted worldwide on alternatives to methyl bromide for pest management.

Methyl bromide
Methyl bromide has been identified as an ozone-depleting substance, and the developed countries signatory to the Montreal Protocol must phase-out its use by 2005. The Montreal Protocol presently exempts the use of methyl bromide for quarantine and preshipment purposes. Methyl bromide is very effective against plant pests, including all stages of insects and nematodes, as well as most fungi.

Methyl bromide is considered to be the most penetrative of the available fumigants. It penetrates oak, Douglas fir, and pine timber by several centimeters. Methyl bromide has been found to eradicate the pine wood nematode (Bursaphelenchus xylophilus) and its Monochamus vectors in southern pine logs and sawn wood. Researchers in the People’s Republic of China have also demonstrated the efficacy of methyl bromide against the pine wood nematode in pine logs.

The efficacy of methyl bromide against insects when properly used has been well established. USDA APHIS Schedule T-404 is a generic treatment for insect control (APHIS Plant Protection and Quarantine Manual, 1985).

Oak wilt, caused by Ceratocystis fagacearum, is a disease of oaks (Quercus spp.) in the United States. The European Union has long been intent on preventing the introduction of oak wilt into Europe. Methyl bromide has been found to kill the oak wilt fungus in oak logs. The exposure periods are 72 and 48 h, respectively, for logs and lumber. The USDA Animal and Plant Inspection Agency has developed Schedule T312 for fumigating oak logs and lumber with methyl bromide (APHIS Plant Protection and Quarantine Manual, 1985). The fumigant has also been shown to eliminate Ophiostoma ulmi, the causal agent of Dutch elm disease, from diseased elm logs intended for use as firewood.

During the past few years there has been significant progress in methyl bromide recapture processes. These systems are designed to reduce the amount of methyl bromide discharged to the atmosphere during ventilation by 90 to 95% without interfering with the normal time cycle for the fumigation. Systems, for example, use activated carbon or a proprietary zeolite to absorb methyl bromide from the ventilation stream. When the carbon is spent, it is sent to a process facility for reactivation and thermal destruction of the methyl bromide. The activated carbon can be reused. When the zeolite is spent, the methyl bromide is desorbed by heat and then condensed using refrigeration and liquid nitrogen. The liquid methyl bromide is then reprocessed. A commercial methyl bromide recapture system using the activated carbon procedure was installed in 1999 at the Dallas/Ft. Worth International Airport.

Phosphine
Phosphine, also known as hydrogen phosphide, is highly toxic to insects, burrowing pests, humans, and other forms of animal life. The gas corrodes certain metals and may ignite in air at concentrations above its lower flammable limit of 1.8% (v/v). Phosphine is routinely used to kill insects found in stored products, but it has no fungicidal properties.

Although phosphine is effective against all insect stages, there is a paucity of information on its efficacy against wood-inhabiting insects. Exposure time differs with the species of insect and temperature and is a key factor for the success of phosphine. It has been reported that fumigation with phosphine was effective against Monochamus sutor in fire-damaged logs in China. Almost total mortality has been noted in all stages of Cryphalus fulvus and Xyleborus pfeili fumigated with phosphine at 2.0 g/m³ for 48 h at 25ºC. The efficacy of phosphine against forest insect pests is much less at 15ºC.

Phosphine has been used extensively for shipboard fumigation to control insects in U.S. export grains. In 1987, this technology was transferred to the in-transit fumigation of pine chips to eradicate the pine wood nematode. The “Florani” experiment showed that phosphine could be successfully used as an in-transit fumigant for eliminating the pine wood nematode from pine chips. This successful in-transit fumigation method awaits broader application.

The “Florani” unloading pine chips in Sweden that had been fumigated in-transit with phosphine in Sweden.

There have been a number of advances in phosphine fumigation delivery systems and formulations over the past few years. Eco2Fume is 2% phosphine with 98% carbon dioxide in pressurized cylinders and is produced by Cytec Canada Inc. The Horn generator, which was developed by Degesch America Inc., produces phosphine by mixing magnesium phosphide powder with water. The efficacy of Eco2Fume and the Horn generator fumigant methodology has been demonstrated for shipboard fumigation of stored-product insect adults and eggs. Eco2Fume received full food registration from the U.S. Environmental Protection Agency in August 2000. New formulations, such as aluminum phosphide tablets based on paraffin wax for use in generators, are being developed.

Sulfuryl fluoride
Sulfuryl fluoride (SO₂ F₂) has been used as a structural fumigant since the 1960s to control drywood termites and wood-boring insects. This fumigant has a low boiling point, excellent penetration qualities, low reactivity potential, and rapid aeration.

There has been considerable interest in Japan in sulfuryl fluoride as an alternative for methyl bromide for eradicating pests in imported timber. It is toxic to all stages of insects. The egg stages, however, are more resistant to sulfuryl fluoride than other postembryonic stages. For example, researchers have shown that the egg stages of wood borers and bark beetles are more resistant to sulfuryl fluoride than larval, pupal, and adult stages. The dose level required to attain 100% mortality of the small bark beetle (C. fulvus) eggs, for example, is considered to be too high to be practical (130 g/m³ for 24 h at 15ºC). The eggs of the ambrosia beetle X. pfeili are more resistant than C. fulvus eggs to sulfuryl fluoride. The USDA APHIS has developed a surfuryl fluoride fumigation schedule for use against bark beetles and wood borers (APHIS Plant Protection and Quarantine Manual Treatment Manual (1985)).

Sulfuryl fluoride has also been investigated as an alternative for methyl bromide for eliminating Ceratocystis fagacearum from oak logs and lumber. The oak wilt fungus can be killed throughout the sapwood in large red oak logs from naturally infested trees after fumigation with sulfuryl fluoride at treatment levels equal to or greater than 27,4000 g h/m³. Sulfuryl fluoride fumigation of red oak veneer logs (with bark) to prevent export of the oak wilt fungus is supported by these data. The fumigation of oak logs at the aforementioned level greatly reduces, but does not eradicate, other microorganisms (i.e., fungi and bacteria) from the sapwood region. The use of sulfuryl fluoride as a sterilant for red oak sapwood requires further research.

Gas mixtures
In the past few years, there has been interest in the use of gas mixtures for a quarantine treatment of timber. For example, fumigation with a low dose of methyl bromide targeting the egg stages and sulfuryl fluoride providing high efficacy of larval and pupal stages shows promise. Another gas mixture being investigated is sulfuryl fluoride and phosphine for treatment against wood borers, bark beetles, and ambrosia beetles.

Other fumigants
There are a number of fumigates that are being investigated as alternatives to methyl bromide. However, most of the research has been on agricultural and horticultural crops. Fumigates being studied include chloropicrin, methyl iodide, metam sodium, propargyl bromide, iodinate hydrocarbons, and propylene oxide. Metam sodium can be used as a fumigant to eradicate the pine wood nematode in pine chips (Dwinell, unpublished data). If registered, methyl iodide has the potential of being a drop-in replacement for methyl bromide.

Postfumigation infestations
Fumigation will not prevent future infestation of sapwood by insects (i.e., powder-post beetles) and saprophytic fungi. If the moisture content of the fumigated wood is above the fiber saturation point, the sapwood is susceptible to colonization by sapstain fungi and molds (i.e., Trichoderma spp.).

Conclusions
Although nonchemical methods for managing pests are desirable, they may not always be a practical, efficient substitute for the simple technology of tarp fumigation. Methyl bromide recovery units offer the opportunity to use methyl bromide for preshipment and quarantine without impacting the ozone layer. The potential of fumigants other than methyl bromide for managing pests in transported wood needs to be fully explored.

In October 1998, the U.S. Congress made specific changes to the Clean Air Act that harmonized the United States phase-out of methyl bromide with the Montreal Protocol. Preshipment and quarantine uses are exempt from the phase-out under the Montreal Protocol. Since this action was taken, there has been a significant reduction in research on alternatives to methyl bromide for preshipment and quarantine uses. This trend needs to be reversed.

Acknowledgement. The author thanks Elmer Schmidt, Department of Wood and Paper Science, University of Minnesota, for technical assistance.

Leesch, J.G., Davis, R., Simonaitis, R.A., and Dwinell, L.D. 1989. In-transit shipboard fumigation of pine woodchips to control Bursaphelenchus xylophilus. EPPO Bull. 19:173-181.

Schmidt, E.L. 1996. An overview and update on fumigation of logs and lumber for pest eradication. Pages 109-112 in: Importing Wood Products: Pest Risks to Domestic Industries, Portland, OR, March 4-6, 1996.