Wood Chips

C. Magnusson (1), B. Økland (2) and H. Solheim (2)

(1) The Norwegian Crop Research Institute, Plant Protection Centre, Høgskoleveien 7. N-1432 Aas, Norway; (2) Norwegian Forest Research Institute, Division of Ecology, Høgskoleveien 12, N-1432 Aas, Norway

Abstract
Important pest organisms in wood chips include the fungus Phellinus weirii, the pine wood nematode (Bursaphelenchus xylophilus), the longhorn beetle Monochamus spp; Anoplophora glabripennis, and Tetropium fuscum, as well as Ambrosia beetles in the genera Gnathotrichus and Trypodendron. Insects may spread from wood-chip piles to nearby forests, and some species can transmit nematodes and fungi. Handling wood chips containing pest organisms in forest environments assists in disease spread. The fungus P. weirii is a serious threat to the northern coniferous forest in Europe. Establishment of the pine wood nematode would cause tree mortality in southern areas. Any establishment would result in negative effects on trade, regardless of area. Introduced insect species may attain a different status as a pest in a new environment. Due to the present lack of economically, feasible treatments in wood chips against PWN, its vectors, and other pathogens, regulations of trade seems to be the only safe strategy to avoid phytosanitary hazards.

Introduction
Movement of pathogens, in plants and wood through trade can lead to unforeseen consequences. A drastic example from North America is the outbreak of chestnut blight, caused by the fungus Endothia parasitica. Another example is the pine wood nematode (PWN), Bursaphelenchus xylophilus, the causal agent of pine wilt disease (PWD), which has reached epidemic proportions in Japan. PWN most probably was introduced into Japan in infested wood from North America. Another example is the Asian longhorn beetle (Anoplophora glabripennis), which attacks maple trees in North America and is thought to have been introduced in packaging wood.

Figure 1. Chip Terminal in Savannah, GA, USA. Photo: C. Magnusson.

With regard to PWN, the trade in softwood chips (Fig. 1), from countries where the nematode is known to occur, has been affected by strong phytosanitary regulations. For the United States of America and Canada, millions of dollars have been lost annually due to the embargoes imposed.

The objectives of this paper are to identify: (a) pest organisms potentially occurring in wood chips; (b) modes of spread to forests; (c) phytosanitary risks from such a transfer; and (d) options for risk management.

Pests in wood chips
Some species of fungi classified as quarantine pest organisms could potentially occur in wood chips. Among these is the wood-rotting fungus Phellinus weirii, which causes laminated root rot. Laminated root rot is common in the northwest of North America and is also found in Japan and China.

Figure 2. The pine wood nematode, (PWN) Bursaphelenchus xylophilus. Original C. Magnusson.

The pine wood nematode (PWN) (Fig. 2) is one of the most important pests in the wood trade and is considered native to North America. As mentioned above, the nematode is the causal agent of pine wilt disease (PWD), which causes extensive forest death in Japan, and has also been detected in China, Taiwan, Korea, and Portugal. Wood chips have a high innate risk of PWN infestation, since chips are often made from low-grade wood. PWN has repeatedly been intercepted in North American wood-chip export consignments to Nordic countries.

Several insect species, especially beetles, tunnel into the wood during their larval or pupal stages. Some species spend much of their time as larvae between bark and wood, before they burrow into the wood for pupation. In other cases, the larvae even make a gallery in the wood, e.g., pine sawyers (Monochamus sp.) that are the main vectors for PWN. It is likely that individuals of Monochamus could survive and follow wood to harbors in foreign countries, even when the wood is chopped into pieces.

Other insects, which could be expected to occur in wood chips, include the longhorn beetles A. glabripennis and Tetropium fuscum and Ambrosia beetles in the genera Gnathotrichus and Trypodendron. There is incomplete documentation of which insect species can be brought with wood chips.

Modes of spread
The fungus P.weirii and laminated root rot seem mostly to be spread by root contact, so movement over longer distances is most likely to occur by transport of infested coniferous logs or chips. Handling wood chips infected by this fungus in the forest environment can lead to establishment of this disease.

The pine wood nematode (PWN) is transported from dying or dead pine trees to healthy trees and timber by longhorn beetles in the genus Monochamus. Nematodes are transferred to shoots fed on by adult beetles, and by the oviposition of beetles on timber or dying trees. Wood products made from infested trees are the principal means of spread over longer distances.

Although, wood chips have small dimensions, the possibility remains that some of the larger pieces could contain pupal chambers with adult pine sawyer beetles carrying PWN dispersal stages. Also, the presence of pine sawyers and some other beetles in nematode-infested pine chips may trigger the formation of dispersal stages, and result in infection of pine sawyers. In wood chips, dispersal stages, of PWN may also develop in response to changes in temperatures. This indicates that in-transit hitchhiking pine sawyer beetles may be infected with PWN dispersal stages. Similarly, pine sawyers or other beetles in an importing country that accidentally get trapped in chips during unloading could later function as nematode vectors to trees and timber. In forested countries like Norway, harbors and pulp mills often are located close to forests, where potentially infested pine sawyers can feed and oviposit.

All uncontrolled end-use of nematode-infested wood chips represents a potential danger of transmission of PWN. This nematode has been observed to move from infested wood chips into freshly cut stumps of pine seedling trees. Infested wood chips used as mulch around trees constitute an obvious risk of transmission. A similar example would be the use of wood chips as soil cover on running and skiing tracks.

Damage potential
The root rot fungus P. weirii could be a serious threat to the northern coniferous forest in Europe. This species is similar to Heterobasidion annosum, which causes severe economic losses in northern European forests. Introduction and establishment of this fungus, especially to Nordic countries, could lead to substantial economic losses.

Figure 3. Japanese black pine, Pinus thunbergii, killed by PWN after feeding by nematode-infested pine sawyers, Monochamus alternatus, caged on branches. Kansai Research Station, Kyoto, Japan.  Photo: C. Magnusson.

The damage potential of PWN depends strongly on the water status of the trees attacked, and this implies that high temperatures are conducive to the expression of PWD (Fig. 3). In laboratory studies of PWN and Scots pine (Pinus sylvestris), PWD has been recorded at temperatures higher than +20ºC, which correlates well with temperature in the regions where PWD occurs.

In Europe, the establishment of PWN is expected to cause tree mortality in the more southern regions. Indeed this has recently been confirmed with regard to Portugal. In central and northern Europe the situation is less clear. Growth chamber simulation suggests, however, that PWD expression would require exceptionally high summer temperatures. It is possible that water stress and air pollution could influence, and maybe enhance, the expression of PWD.

The establishment and spread of PWN in the Palearctic forest regions would threaten pine forests in the southern parts of the area. The establishment of PWN would also severely interfere with trade. This would be of special concern to Russia and Nordic countries due to their large export markets.

The damage potential of insects spreading from imported wood chips may be difficult to assess. However, introduced insect species may attain a different status as a pest organism in the new environment compared with its area of origin. For instance, the longhorn beetle T. fuscum, a species introduced into North America, is a primary pest on spruce in Nova Scotia, while it is considered as a secondary pest in Scandinavia. Insects serving as vectors of other pest organisms, like nematodes and fungi, attain an additional dimension of interest in the context of the wood-chip trade.

Risk management
Pressure impregnation operates at temperatures and pressures sufficient to kill fungi, insects and nematodes. In-transit shipboard fumigation with phosphine has been reported to decrease PWN infestation rates of wood chips. Heat treatment has also been successfully applied in eradication of PWN in wood chips but is so far not operative in full scale. Selection of healthy trees for felling and ensuring processing before the flight period of potentially dangerous insect and nematode vectors like Monochamus spp., is probably not an option with regard to wood chips.

Due to the present lack of economically feasible treatments for wood chips to destroy PWN, its vectors, and other pathogens, regulation of trade seems to be the only safe strategy to avoid phytosanitary hazards.

Suggested reading

Evans, H.F., McNamara, D.G., Braasch, H., Chadoeuf, J., and Magnusson, C. 1996. Pest  Risk Analysis (PRA) for the territories of the European Union (as PRA area) on Bursaphelenchus xylophilus and its vectors in the genus Monochamus. EPPO Bull. 26: 199-249.

Liebhold, A.M., MacDonald, W.L., Bergdahl, D., and Mastro, V.C. 1995. Invasion by exotic forest pests: A Threat to Forest Ecosystems. For. Sci. Monogr. 30. Suppl. For. Sci. 41: 49 pp.

Mamiya, Y. 1984. The pine wood nematode. In: Nickle, W.R. (ed). Plant and Insect Nematodes. Marcel Dekker Inc. NewYork: 589-626.