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L.D. Dwinell and M. Mota Over the past 17 years, the pinewood nematode (PWN) (Bursaphelenchus xylophilus) has been intercepted in pine chips, unseasoned lumber, and packing-case wood imported into Europe from North America and other countries in which the nematode is known. Likewise, the PWN’s insect vectors, Monochamus spp., have been found in pallets, crates, and dunnage. As a result of these interceptions and the prevalence of pine wilt disease in Japan and China, the European Union and other countries regulate the import of all coniferous chips, logs, sawn wood, and solid wood packing material to protect their forests from the PWN and other exotic pests.
NEMATODE Taxonomy. The pinewood nematode (as B. lignicolus) was first reported from North America in 1979. It was found in dead exotic pines in Missouri. The initial reaction of the scientific community was, “We’ve been invaded!” In 1982, B. lignicolus was synonymized with B. xylophilus, a species native to North America. Aphelenchoides xylophilus had been transferred to Bursaphelenchus in 1970. A. xylophilus had been found in association with bluestain and other fungi in logs of long-leaf pine (Pinus palustris) and in the bole of a bark-beetle killed short-leaf pine (P. echinata). Distribution. Indigenous to North America, the PWN has been reported from the United States, Canada, and Mexico. B. xylophilus is an introduced pest in Japan, the People’s Republic of China, Taiwan, South Korea, and Portugal. In Europe, it has only been located in a small (30 km radius) area of southern Portugal, as a result of extensive surveys conducted in 1999 and 2000. A closely related species, B. mucronatus, has been reported from Canada and many Asian and European countries. Several minor species of Bursaphelenchus have been recorded on conifers in Europe (i.e., Italy and Poland) in the past few years. B. fraudulentus has been recorded on hardwoods in Europe. Although the PWN is the causal agent of pine wilt disease, it normally exists in nature independent of the disease. Thus, the distribution of the PWN cannot be equated with the distribution of pine wilt disease. Since the PWN is transmitted during oviposition of the adult female pine sawyer (Monochamus spp.), the PWN is found throughout North America wherever recently killed or dying conifers, particularly pines, are colonized by species of Monochamus. Worldwide, species of Bursaphelenchus phoretic on Monochamus are codistributed with their insect hosts. Why is B. xylophilus such an insidious nematode? When introduced into a country with a Monochamus population, the PWN displaces the aboriginal phoretic associate of the insects (i.e., B. mucronatus). Monochamus are opportunistic, saprophytic insects that are an integral part of the forested ecosystem. Thus, a native longhorned beetle vectors an exotic, pathogenic nematode. This combination spells ecological disaster. The end result is a nematode population that cannot be eradicated, and a disease, pine wilt, that can be suppressed but not really controlled. Pathogenicity. Defining the pathogenicity of B. xylophilus and related species has been difficult. Most of the information on pathogenicity of the PWN is based on seedling pathogenicity tests. Pathogenicity of inoculated conifer seedlings does not imply pathogenicity on established trees. B. xylophilus and B. (=Rhadinaphelenchus) cocophilus are the only species of Bursaphelenchus currently known to be pathogenic in nature, although some researchers suspect that other species may be pathogenic if the right combination of weather conditions and tree susceptibility occur. B. cocophilus causes red ring of coconut palm and is vectored by a palm weevil (Rhynchophorus palmarum). Suscepts. In nature, B. xylophilus is a pathogen of pines. In Japan, it causes extensive mortality of Japanese red pine (P. densiflora) and Japanese black pine (P. thunergii). Suscepts of the PWN in China and Korea include P. densiflora, P. thunbergii, and P. massonia. In Taiwan, the suscepts are P. lucheusis and P. thunbergii. In North America, pine wilt disease appears to be confined largely to exotic pines, especially P. sylvestris, in the eastern United States. VECTORS Insects. Most of the described species of Bursaphelenchus have a phoretic relationship with insects, especially bark beetles and woodborers. The PWN and other closely related species (i.e., B. mucronatus) are vectored principally by cerambycid longhorn beetles (sawyers) in the genus Monochamus. In North America, the transmission stage (dauerlarvae) of B. xylophilus has been recovered from adult beetles of most species of Monochamus. Elsewhere in the Northern Hemisphere, B. mucronatus appears to be the most common species of Bursaphelenchus associated with Monochamus. The biologies of the Asian, North American, and Euro-Siberian Monochamus species are similar although they differ in geographical distribution, host plants, oviposition site preferences, number of instars, and length of life cycle. Adult sawyers are attracted to recently dead or dying trees and freshly felled timber (including logs) for breeding. The cause of the conifer’s mortality is not significant. Sawyers oviposit and the larvae develop only in trees or logs with bark. The female gnaws an irregular hole through the bark (oviposition pit) and inserts her eggs. The Monochamus larva feeds for 1 to 2 months on the cambial fiber layer. Later, the larva bores into the sapwood, forming an oval entrance hole. The tunnel is usually u-shaped, and the pupal cell is located in the sapwood just beneath the outer bark.
The dauerlarvae B. xylophilus, B. mucronatus, and other phoretic Bursaphelenchus species invade the callow Monochamus adult through the thoracic spiracles and are held in a quiescent state only in the tracheae. After pupation, the adult emerges by gnawing through the sapwood and exits the log. The sawyer’s life cycle is normally one year in southern areas and one or two years in northern areas.
TRANSMISSION Primary transmission. Upon emergence, the adult beetles move to a suitable host to feed on the bark of young branches. Female beetles reach sexual maturation about 14 to 20 days after emergence. Nematode dauerlarvae emigrate from the spiracles and enter the tree through wounds caused by the beetles feeding. This mode of transmission is termed “primary” because primary infection of a susceptible host occurs. Once inside a susceptible host, the nematode migrates throughout the tree, feeds on the parenchyma cells of the ray canals, and reproduces. Such PWN-infected trees exhibit wilt symptoms. The resulting disease is referred to as “pine wilt.” Secondary transmission. Secondary transmission occurs when B. xylophilus enters a recently killed or dying tree or freshly felled timber through oviposition caused by Monochamus vectors. This mode of transmission, also confirmed for B. mucronatus and B. cocophilus, is now considered the most common mode of transgenerational transfer of species of Bursaphelenchus. The realization that Bursaphelenchus is transmitted during oviposition essentially redefined the status of the PWN in North America. If oviposition pits are noted on a recently killed or dying pine and the PWN is isolated, one cannot automatically infer that the tree succumbed to pine wilt disease. The nematode is a secondary invader in these cases and does not contribute to tree mortality. The PWN feeds on the fungi in the wood. As a result of secondary transmission, species of Bursaphelenchus may be found in logs, chips, and unseasoned lumber. To ascertain the risk of the PWN in lumber and solid wood packing material, it important to understand the biology of the PWN-vector-host complex and how the nematode can be transmitted (Table 1). If no grub holes are visible, it is concluded that there are no insects in the sawn wood that will emerge and transmit the PWN. Furthermore, the foreign Monochamus, which require bark with phloem for oviposition, will be unable to breed in bark-free wood, eliminating contamination by the PWN. Table 1. Pinewood nematode transmission models for lumber.
a Grub holes produced by larvae of
Monochamus. In model I, the wood is free of Monochamus grub holes and bark with phloem, but the PWN can be extracted from the wood. In this model, transmission would have to occur outside the evolutionary biology of the PWN and its Monochamus vectors-that is, soil transmission. Several PWN experts have concluded that soil transmission is “an unlikely event in nature.” Basically, model I is a dead-end for the PWN. In model II, the PWN inhabits the xylem, and Monochamus grub holes are present. The grub holes suggest that the plank contains the pupal stage of the insect. If so, the adult could emerge from the wood, fly to the nearest host, and transmit the PWN during maturation feeding. In model III, the PWN is present in the wood, and the plank has attached bark with phloem. There has to be sufficient bark with phloem to attract the vector and support its development. This would not be expected to occur with dimension lumber where there are small patches of bark and the bark has dried out. The greatest possibility of model III occurring would be relative large pieces of fresh pine lumber with bark (i.e., cable spools). Model IV is reflected in models II and III. Essentially, Model IV is equivalent to model II. If the lumber had been dried to 19% moisture content in a conventional or high temperature kiln, the presence of grub holes and bark is a moot point. The greatest risk of the introduction of the PWN is the importation of pine logs with bark (model IV) or without bark (model II) and including both PWN and Monochamus spp. (as evidenced by the presence of grub holes). CONTROL Mitigation. Mitigation procedures investigated over the last 15 years include prevention, host selection, and treatment by fumigation, irradiation, chemical dips, and elevated temperature. Scientists in a trilateral study involving Canada, the United States, and the European Union concluded that heat-treating unseasoned lumber to a core temperature of 56o C for 30 minutes eradicates the PWN and its insect vectors. The pasteurization of coniferous wood is now the primary treatment for the importation of lumber and solid wood packing material into the European Union and the People’s Republic of China. References Dwinell, L.D. 1997. The pinewood nematode: Regulation and mitigation. Annu. Rev. Phytopathol. 35:153-166. Dwinell, L.D. and W.R. Nickle. 1989. An overview of the pinewood nematode ban in North America. Gen. Tech. Rep. SE-55, USDA For. Service. Southeastern For. Exp. Stn., Ashville, NC. 13 pp. Mota, M. Braasch, H. Bravo, M.A. Penas, A.C. Burgermeister, W. Metge, K. Sousa, E. 1999. First report of Bursaphelenchus xylophilus in Portugal and in Europe. Nematology 1:727-734. Wingfield, M.J. Blanchette, R.A., and Nicholls, T.H. 1984. Is the pine wood nematode an important pathogen in the United States? J. For. 82:232-235.
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