The hidden threat In firewood: Invasive forest pests

Invasive, non-native, insects and diseases are a direct threat to trees and forests in Oregon. We know that invasive forest pests are being transported throughout the continent in untreated firewood. Examples of these non-native threats include the emerald ash borer, Asian longhorned beetle, Sirex wood wasp, sudden oak death, oak wilt and pitch canker, all of which can survive months in firewood.

In Oregon, native forest pests are generally not a concern unless the firewood is being taken from Oregon to another region. However, there are nonnative pests established in Oregon. For example, emerald ash borer and the Mediterranean oak borer are now established in the Willamette Valley, and moving firewood is a major way to disperse the insects.

In Curry County there is a quarantine zone where the non-native invasive disease sudden oak death, caused by Phytophthora ramorum, is killing trees. It is unlawful to transport tanoak from inside the quarantine to other regions in the state.

Thousand canker disease and Dutch elm disease are also present in Oregon, and moving wood with bark intact is a major threat to the area’s remaining black walnut and American elm trees.

The purpose of this publication is to alert Oregonians to the risk of introducing or dispersing invasive forest pests through firewood. Although Oregon law restricts the import of untreated commercial firewood, there still is a risk that people moving firewood will introduce or spread pests. Interstate transport of firewood by people is one of the most important avenues for dispersal of many invasive forest pests. This has prompted public awareness campaigns, with slogans like “Buy It Where You Burn It!” and “Don’t Move Firewood!” from such groups as the Continental Dialogue on Non-Native Forest Insects and Diseases.

Firewood can contain pests, invaders

In 2012, scientists purchased over 400 bundles of firewood at convenience and grocery stores in four Western states. The bundles were placed in specialized rearing chambers, and checked for live insect emergence for 18 months. Insects emerged from 47% of these bundles, with an average of 11 insects from each bundle. Researchers collected 24 families of beetles, with bark beetles, ambrosia beetles and wood borers being the most common. The study showed that insects can hitch rides on even dried-looking wood.

The primary concern in Oregon and the Pacific Northwest is the movement of firewood into the region. There are many pests that could be very destructive if introduced into Oregon. In addition, we would not want our forest pests transported to other regions. Below are some examples of notorious firewood hitchhikers that could be particularly damaging to our region’s forests as well as to trees in urban settings.

Pests established elsewhere in North America but not yet in Oregon that have the potential to invade the state:

• Asian longhorned beetle: Native to Asia, the Asian longhorned beetle is active in Massachusetts and Ohio, despite ongoing eradication efforts. Eradication efforts rely on destroying the host, which has significantly impacted homeowners and neighborhoods.
• Sirex wasp: The Sirex wood wasp is established in pine forests of New York state but is a significant pest of planted pines throughout the world, including ponderosa pine in Argentina. We don’t know the threat this insect poses to our native ponderosa pine, but it is feared the wood-boring wasp, and its associated fungus, could pose a threat to our native pines.
• Oak wilt: Oak wilt is a fungal disease that kills oak trees. It is found in many Midwestern states.

Non-native pests that have established in Oregon:

• Emerald ash borer: The emerald ash borer has devastated ash (Fraxinus species) trees in the eastern United States, killing almost every tree. In Oregon, we expect the emerald ash borer to kill upwards of 90% of all Oregon ash. This will have a huge impact on Western Oregon riparian forests.
• Mediterranean oak borer: The Mediterranean oak borer (Figure 2) is well-established in Oregon. Currently, it is associated with the mortality of large Oregon white oak trees. We do not know if it has the potential to kill most oaks yet. It is also present in California, especially in Napa and surrounding areas, where it hits valley oak and blue oak.
• Dutch elm disease: Dutch elm disease was introduced into Oregon in about 1990 and has spread throughout most cities and towns that have significant populations of American elm. Although elms are non-native in Oregon, they are historical and beautiful components of urban landscapes.
• Bronze birch borer: The bronze birch borer is native to North America but has emerged as a serious pest of European white birch and other ornamental birches in urban and rural areas of Oregon. The loss of these historic and beautiful trees from landscapes is significant.
• Sudden oak death: Sudden oak death was discovered in Oregon in 2001 in Curry County near Brookings, Oregon, and an active eradication and spread-prevention program has been in place ever since. The disease has intensified and spread in the county but is currently still limited to a region in Curry County.
• Thousand canker disease of walnut: Thousand canker disease of walnut has been in Oregon for several years. It is spread by the walnut twig beetle. In the past, it was limited to Western states, but has now been discovered in the eastern U.S. and threatens black walnut across its native range (Figure 1).

Native insects and pathogens can also be a problem in firewood

While exotic invaders are the primary concern, insect pests that are native to Oregon can persist in firewood and then spread to nearby living trees. This may be a particular problem with bark beetles that inhabit freshly cut or killed pine trees. Examples include the mountain pine beetle, pine engraver, and California five-spined ips. In these cases, there may be concerns if the tree was still alive when cut or recently killed by bark beetles. Ambrosia beetles are also common on recently dead wood (Figure 3), while some wood may have root disease or wood decay fungi already present, such as Armillaria species (Figure 4) or other root, butt, and stem rots (Figure 5).

Some moisture is required for these fungi to survive, but wood stored directly on the ground will likely maintain decay fungi and the community of organisms associated with them.

Insects and fungi are part of the life cycle of a tree

Throughout their long lives and after they die, trees harbor many insects, fungi, and other organisms. This community changes as time elapses, from organisms that utilize leaves, live roots, and cones to organisms that inhabit dead wood after the tree’s death. When a tree is weakened, the first insects on the scene are small bark beetles, which are adapted to feeding on fresh bark. Once the inner bark begins to decompose, the beetles are no longer interested. Bark beetles transport fungi that colonize the sapwood and bark of the freshly dead tree.

The bark beetles (Figure 6) typically inhabit the freshly killed tree no more than one year after death, and are quickly followed by flat-headed and round-headed wood borers. Wood borers initially mine the inner bark, then the sapwood, and, depending on species, the heartwood. These insects tend to be around longer than bark beetles, from a year or two up to a decade or more. Wood borers are major helpers in decomposition of wood. They introduce wood decay fungi into the tree directly, and through their boring activity, allow decay fungi and other micro-organisms, including small insects, to invade the dead tree. Some wood borers, such as the emerald ash borer and flat-headed fir borer, can kill trees outright. Lastly, termites (Figure 7) and carpenter ants take harbor in dead trees and can persist for decades, although they may have been present in the living tree. Termites eat wood, while carpenter ants excavate wood for nest construction, although it is thought carpenter ants require wood decay fungi to soften the wood.

Sapwood in a living tree is more resistant to decay than the heartwood because the living tree can respond to invasion in sapwood, but not heartwood. In the dead tree, this is reversed—sapwood decays first, and the heartwood is most resistant to decay due to tree extracts. The fungi that colonize live trees may continue to rot the tree, while new dead-wood decomposers eventually take over (Figure 5). Vertebrates arrive on the scene, perhaps using a cavity created by decay and insects. Or perhaps a bear will destroy the tree looking for carpenter ants or yellow jacket grubs. The moisture content and temperature are key factors in the success of these fungi and insects.

So what does this mean to you? Depending on how long the tree had been dead prior to cutting into firewood, how long the wood has been stored, and the conditions in which the wood was stored, you can be packing various biological passengers in your firewood, no matter what the age.

Can firewood of one species affect other tree species?

Many insects and fungi on firewood are generalists, although some are specialists. Mountain pine beetle will not invade Douglas-fir, for example, and the alder bark beetle won’t invade Douglas-fir. But the host list for the causal agent of sudden oak death is over 100 plant species! The biggest concern is non-native invasive pests, such as emerald ash borer, Mediterranean oak borer, Asian longhorned beetle, and Sirex wasps. The emerald ash borer is limited to ash species (all North American species in the genus Fraxinus are susceptible). However, olive is a close relative to ash and is also infested by the insect. The Mediterranean oak borer has only attacked Oregon white oak in Oregon, but is known to attack other species of trees in California and Europe.

Proper firewood storage

Proper firewood processing, curing, and storage can minimize these problems. Cutting live pine trees into firewood during the peak bark beetle flight season is risky, as the beetles are looking for fresh wood to breed in. The “safe” window for cutting live pine to avoid this problem is late October to early April. It is also important to avoid stacking fresh pine wood against live pine trees; bark beetles, attracted to or already present in the firewood, may move to the live trees. Firewood that has been cut, split, stacked, and left on site for at least one year will ensure the wood has dried or “cured” to the point it is no longer suitable habitat for many woodboring pests. Debarked firewood is even better—its protective “coating” has been stripped so that it is no longer a suitable host for pests, and you have a cleaner, more efficient heating source.

How far can I move firewood?

Currently, we do not recommend moving oak or ash any distance. Please keep it local.

Of particular concern in Oregon is Dutch elm disease in American elm and thousand canker disease in walnut. If an American elm or black walnut dies, it is likely it has disease and will be infested by bark beetles. Debark or destroy American elm and black walnut wood immediately after tree death.

It is safe to move most firewood within subregions of the state, especially conifer wood. However, if there is a known quarantine zone, such as in Curry County, or if you have a known agent in a tree such as emerald ash borer or Mediterranean oak borer, don’t move firewood. Long-distance movement of firewood — especially interstate movement — could introduce new invaders to those final destinations.

Regulations for firewood transport

In other parts of the country, the long-distance movement of firewood has been implicated in numerous cases as the pathway for new invasions of these and other pests. As a result, there are various rules and restrictions in place for commercial firewood sellers. As of 2012, any commercially produced firewood sold in Oregon that originates outside Oregon, Washington or Idaho must be heattreated for a minimum of 60 minutes at 60º C (140º F) and certified, to eliminate any unwanted organisms. However, this rule, which is administered by the Oregon Department of Agriculture, does not apply to individuals who might cut and transport their own firewood.

What can you do? “Buy it where you burn it!” and “Don’t move firewood!”

1. Limit the distance you move firewood to what makes sense for your region. Keep your untreated firewood in the general region it comes from.
2. When you purchase firewood, choose locally cut and processed wood. If out-of-state wood is the only option, make sure that it is labeled “Approved Pest Free” (Figure 8).
3. If you are a manager of a recreation area or campground, purchase wood from local companies. Encourage campground managers to buy firewood locally.
4. Check the general appearance of the firewood you produce or buy. Clean, dry firewood does not mean you will eliminate the transportation of ALL potential pests, but chances are diminished that pests may be moved to another location. Look for grayness or bleached-out wood (split a piece and observe the contrast in color). If not already removed, is the bark loose or separating from the wood? If necessary, remove the bark and look for evidence of pests.
5. Don’t take firewood to other states from Oregon.
6. Split, stack, and cover piles of firewood. Tarps or other forms of plastic will generate heat in the pile and make an inhospitable environment for pests.
7. If you live in a humid area, monitor the wood for mold or other fungal growth.
8. Encourage your out-of-town visitors to leave their firewood at home when they plan camping trips to Oregon. Likewise, when you go camping elsewhere in the country, don’t take firewood from Oregon with you or bring firewood back home.