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Datasheet

Bursaphelenchus xylophilus
(pine wilt nematode)

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Datasheet

Bursaphelenchus xylophilus (pine wilt nematode)

Summary

  • Last modified
  • 19 September 2022
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Natural Enemy
  • Preferred Scientific Name
  • Bursaphelenchus xylophilus
  • Preferred Common Name
  • pine wilt nematode
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Metazoa
  •     Phylum: Nematoda
  •       Family: Parasitaphelenchidae
  •         Genus: Bursaphelenchus
  • Summary of Invasiveness
  • The pine wood nematode, Bursaphelenchus xylophilus, is widespread and abundant on native pine forests of North America, particularly in the eastern USA. It is ubiquitous in a wide range of climates, from subtropical to sub-boreal, but is...

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Pictures

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PictureTitleCaptionCopyright
Bursaphelenchus xylophilus (pine wilt nematode); SEM showing the spicule (centre) which identifies the species, a destructive pinewood nematode. Original x 5000.
TitleSEM
CaptionBursaphelenchus xylophilus (pine wilt nematode); SEM showing the spicule (centre) which identifies the species, a destructive pinewood nematode. Original x 5000.
Copyright©USDA-ARS
Bursaphelenchus xylophilus (pine wilt nematode); SEM showing the spicule (centre) which identifies the species, a destructive pinewood nematode. Original x 5000.
SEMBursaphelenchus xylophilus (pine wilt nematode); SEM showing the spicule (centre) which identifies the species, a destructive pinewood nematode. Original x 5000.©USDA-ARS
Bursaphelenchus xylophilus (pine wilt nematode); distinctive spicule of male nematode. USA.
TitleMale
CaptionBursaphelenchus xylophilus (pine wilt nematode); distinctive spicule of male nematode. USA.
Copyright©A. Steven Munson/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); distinctive spicule of male nematode. USA.
MaleBursaphelenchus xylophilus (pine wilt nematode); distinctive spicule of male nematode. USA.©A. Steven Munson/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); vulva flap of female nematode. USA.
TitleFemale
CaptionBursaphelenchus xylophilus (pine wilt nematode); vulva flap of female nematode. USA.
Copyright©A. Steven Munson/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); vulva flap of female nematode. USA.
FemaleBursaphelenchus xylophilus (pine wilt nematode); vulva flap of female nematode. USA.©A. Steven Munson/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); tail of male, showing characteristic spicule. Japan.
TitleMale
CaptionBursaphelenchus xylophilus (pine wilt nematode); tail of male, showing characteristic spicule. Japan.
Copyright©L.D. Dwinell/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); tail of male, showing characteristic spicule. Japan.
MaleBursaphelenchus xylophilus (pine wilt nematode); tail of male, showing characteristic spicule. Japan.©L.D. Dwinell/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); gravid female. Japan.
TitleFemale
CaptionBursaphelenchus xylophilus (pine wilt nematode); gravid female. Japan.
Copyright©L.D. Dwinell/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); gravid female. Japan.
FemaleBursaphelenchus xylophilus (pine wilt nematode); gravid female. Japan.©L.D. Dwinell/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); general features of head region. Japan.
TitleFeatures
CaptionBursaphelenchus xylophilus (pine wilt nematode); general features of head region. Japan.
Copyright©L.D. Dwinell/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); general features of head region. Japan.
FeaturesBursaphelenchus xylophilus (pine wilt nematode); general features of head region. Japan.©L.D. Dwinell/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); tee mortality on Mason pine (Pinus massoniana), caused by the nematode. Anhui Province, China.
TitleField symptoms
CaptionBursaphelenchus xylophilus (pine wilt nematode); tee mortality on Mason pine (Pinus massoniana), caused by the nematode. Anhui Province, China.
Copyright©William M. Ciesla/Forest Health Management International/Bugwood.org - CC BY-NC 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); tee mortality on Mason pine (Pinus massoniana), caused by the nematode. Anhui Province, China.
Field symptomsBursaphelenchus xylophilus (pine wilt nematode); tee mortality on Mason pine (Pinus massoniana), caused by the nematode. Anhui Province, China.©William M. Ciesla/Forest Health Management International/Bugwood.org - CC BY-NC 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); pine wilt symptoms, one year after death of tree. USA.
TitleField symptoms
CaptionBursaphelenchus xylophilus (pine wilt nematode); pine wilt symptoms, one year after death of tree. USA.
Copyright©USDA Forest Service/Region 2- Rocky Mountain Region/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); pine wilt symptoms, one year after death of tree. USA.
Field symptomsBursaphelenchus xylophilus (pine wilt nematode); pine wilt symptoms, one year after death of tree. USA.©USDA Forest Service/Region 2- Rocky Mountain Region/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); symptoms in a field setting - note dead trees and browning of foliage on others. China
TitleField symptoms
CaptionBursaphelenchus xylophilus (pine wilt nematode); symptoms in a field setting - note dead trees and browning of foliage on others. China
Copyright©Juan Shi/Beijing Forestry University/Bugwood.org - CC BY-NC 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); symptoms in a field setting - note dead trees and browning of foliage on others. China
Field symptomsBursaphelenchus xylophilus (pine wilt nematode); symptoms in a field setting - note dead trees and browning of foliage on others. China©Juan Shi/Beijing Forestry University/Bugwood.org - CC BY-NC 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); the nematode is mycophagous and feeds on fungi in the wood, including bluestain fungi transmitted by engraver and other bark beetles. Japan.
TitleSecondary association
CaptionBursaphelenchus xylophilus (pine wilt nematode); the nematode is mycophagous and feeds on fungi in the wood, including bluestain fungi transmitted by engraver and other bark beetles. Japan.
Copyright©L.D. Dwinell/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Bursaphelenchus xylophilus (pine wilt nematode); the nematode is mycophagous and feeds on fungi in the wood, including bluestain fungi transmitted by engraver and other bark beetles. Japan.
Secondary associationBursaphelenchus xylophilus (pine wilt nematode); the nematode is mycophagous and feeds on fungi in the wood, including bluestain fungi transmitted by engraver and other bark beetles. Japan.©L.D. Dwinell/USDA Forest Service/Bugwood.org - CC BY 3.0 US

Identity

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Preferred Scientific Name

  • Bursaphelenchus xylophilus (Steiner & Buhrer, 1934) Nickle, 1970

Preferred Common Name

  • pine wilt nematode

Other Scientific Names

  • Aphelenchoides xylophilus Steiner & Buhrer, 1934
  • Bursaphelenchus lignicolus Mamiya & Kiyohara, 1972

International Common Names

  • English: pine wood nematode; pinewood nematode
  • French: nématode des pins; nématode du bois de pin

Local Common Names

  • Germany: Kiefernholznematode

EPPO Code

  • BURSXY (Bursaphelenchus xylophilus)

Summary of Invasiveness

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The pine wood nematode, Bursaphelenchus xylophilus, is widespread and abundant on native pine forests of North America, particularly in the eastern USA. It is ubiquitous in a wide range of climates, from subtropical to sub-boreal, but is not considered as a serious pest on this continent, except for on exotic or damaged pines. When introduced into Eurasia, some local Pinus proved to be exceptionally susceptible, and the pine wood nematode became the major invasive pine pathogen in Europe and East Asia. The pine wood nematode uses pine sawyer beetles of the genus Monochamus as vectors to exit depleted hosts and infect new ones, readily adapting to local Monochamus spp., both in North America and in areas where it is invasive.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Metazoa
  •         Phylum: Nematoda
  •             Family: Parasitaphelenchidae
  •                 Genus: Bursaphelenchus
  •                     Species: Bursaphelenchus xylophilus

Notes on Taxonomy and Nomenclature

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The species first came to widespread attention with the paper of Mamiya and Kiyohara (1972) which described Bursaphelenchus lignicolus as a new species from Japan. Subsequent research revealed that the species had been previously described from the USA as Aphelenchoides xylophilus by Steiner and Buhrer (1934), but had been subsequently overlooked. Later it was described again, as Bursaphelenchus lignicolus, when recognized to be the causal agent of pine wilt disease (PWD) in Japan (Mamiya and Kiyohara, 1972). The synonymy was recognized in 1981 (Nickle et al., 1981). A very similar but non-pathogenic species (Bursaphelenchus mucronatus) was described by Mamiya and Enda (1979), differing morphologically only in minor respects from B. xylophilus and most obviously by the presence in the female of a caudal mucron (finger-like projection) in the former species which was absent in the latter. However, populations of B. xylophilus were subsequently discovered in the USA which also carried a mucron on the tail. The earlier described Bursaphelenchus kolymensis Korenchenko (Korenchenko, 1980) was later found to correspond to the European type of B. mucronatus, the nominated subspecies B. mucronatus kolymensis comb. n., widely distributed in Europe and Siberia, while the East Asian type is the nominate subspecies B. mucronatus mucronatus (Braasch et al., 2011). B. xylophilus and B. mucronatus are morphologically very similar to Bursaphelenchus fraudulentus Ruhm, and it has been suggested that they constitute a species complex or supraspecies, separated from one another by reproductive isolation: B. xylophilus is native to North America, whereas the other two are Palaearctic species, all being able to colonize coniferous trees, although B. fraudulentus is predominantly found in dying or dead deciduous trees (Filipiak et al., 2017). However, only B. xylophilus is considered a tree pathogen. Biochemical studies of several populations of these nematodes have clearly confirmed the distinctness of the three species (Webster et al., 1990; Abad et al., 1991; Filipiak et al., 2017).

Description

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Measurements (after Nickle et al., 1981)

Female lectotypes (n=5): L=0.52 (0.45-0.61) mm; a=42.6 (37-48); b=9.6 (8.3-10.5); c=27.2 (23-31); V=74.7 (73-78)%; stylet=12.8 (12.6-13.0) µm.

Male lectotypes (n=5): L=0.56 (0.52-0.60) mm; a=40.8 (35-45); b=9.4 (8.4-10.5); c=24.4 (21-29); stylet=13.3 (12.6-13.8) µm; spicule=21.2 (18.8-23.0) µm.

Measurements (after Mamiya and Kiyohara, 1972)

Female (n=40): L=0.81 (0.71-1.01) mm; a=40.0 (33-46); b=10.3 (9.4-12.8); c=26.0 (23-32); V=72.7 (67-78)%; stylet=15.9 (14-18) µm.

Male (n=30): L=0.73 (0.59-0.82) mm; a=42.3 (36-47); b=9.4 (7.6-11.3); c=26.4 (21-31); stylet=14.9 (14-17) µm; spicules=27.0 (25-30) µm.

Description (after Nickle et al., 1981)

Female: cephalic region high, offset, with six lips. Stylet with small basal swellings. Oesophageal gland lobe slender and about 3-4 body-widths long. Excretory pore located at about the level of the oesophago-intestinal junction, occasionally at the same level as the nerve ring. Hemizonid about 0.67 of a body-width behind the median bulb. Vulva posterior, the anterior lip overhanging to form a flap. Genital tract monoprodelphic, outstretched. Developing oocytes mostly in single file. Post-uterine sac well developed, extending for 0.75 or more of the distance to the anus. Tail subcylindroid with a broadly rounded terminus. Mucron usually absent, but some populations may have a short, 1-2 µm, mucron.

Male: similar to female in general respects. Spicules large, strongly arcuate so that the prominent transverse bar is almost parallel to the body axis when the spicules are retracted. The apex is bluntly rounded and the rostrum prominent and pointed. The distal tip of each spicule is expanded into a disc-like structure named the cucullus. The tail is arcuate with a pointed, talon-like terminus bearing a small bursa. Seven caudal papillae are present; one pair adanal, a single preanal ventromedian papilla, and two postanal pairs near the tail spike and just anterior to the start of the bursa.

A morphological key to the species of the xylophilus group of the genus Bursaphelenchus has recently been published by Braasch and Schönfeld (2015).

Distribution

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The pine wood nematode is considered to be native and widespread throughout North America. However, distribution is highly variable in the native pine forests in the USA. B. xylophilus was found to be reliably abundant in eastern forests (Pimentel et al., 2014), while its occurrence in western USA is rarer and patchier (Blunt et al., 2014; Pimentel et al., 2014; Atkins et al., 2021). Western and eastern native pine forests are geographically isolated by the Great Plains and western forests tend to be more fragmented as a result of the mountainous terrain and associated climatic gradients. They differ in climate and ecology (e.g. different species of pine and Monochamus), but the role of these differences on the abundance and distribution of the pine wood nematode is unknown. Outside the distribution area of native pines, in the Midwest and Great Plains, the pine wood nematode is considered invasive and pathogenic on susceptible exotic pines that have been introduced mainly in urban settings, particularly Pinus sylvestris and Pinus nigra (Kondo et al., 1982; Malek and Appleby, 1984). The pine wood nematode is ubiquitous in the cold adapted pine forests of Canada, and once again its abundance increases from east to west (Bowers et al., 1992). Its distribution extends to Mexico (Dwinell, 1993), although information on the pathogen this far south is scarce. The exact area of origin of B. xylophilus within the vast and diverse continent of North America is unknown.

In Asia, the pine wood nematode is present in Japan, Korea Republic, Taiwan and China. It is considered spread throughout almost entire Japan since 1982, except the two most northern prefectures of Hokkaido and Aomori (Togashi and Jikumaru, 2007). In Korea Republic, B. xylophilus extends from forests in the southern regions of Busan and Mokpo, up to its most northern limit north of Seoul (Kwon et al., 2011), being also widespread throughout Taiwan (Ma et al., 2002; Chen et al., 2006). In China, B. xylophilus was first introduced to the Nanjing area, in the east of the country (Li et al., 1983). In just a few years reached the Shandong province, over 500 km to the north. It also spread swiftly to the west, presently being distributed from the southern coastal province of Guangdong to the more northern provinces of Hubei and Chongqing (Cheng et al., 2008; Wu et al., 2013).

In Europe, the distribution of B. xylophilus is still restricted to Portugal, although it is considered spread throughout all its territory (Vicente et al., 2012).

Species of Monochamus, the main vectors, are found throughout all pine forests in the northern hemisphere, and many have been recorded as having species of Bursaphelenchus as associates; it is assumed, therefore, that most, if not all, Monochamus species would be capable of acting as vectors of B. xylophilus to a greater or lesser extent.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Last updated: 21 Jul 2022
Continent/Country/Region Distribution Last Reported Origin First Reported Invasive Reference Notes

Africa

NigeriaAbsent, Unconfirmed presence record(s)

Asia

ChinaPresentIntroduced1982Invasive
-AnhuiPresent1988
-ChongqingPresent
-FujianPresent
-GansuPresent
-GuangdongPresent1988
-GuangxiPresent
-GuizhouPresent
-HenanPresent, Localized
-HubeiPresent
-HunanPresent
-JiangsuPresent1982
-JiangxiPresent
-LiaoningPresent
-ShaanxiPresent
-ShandongPresent1991
-ShanghaiPresent
-SichuanPresent
-YunnanPresent
-ZhejiangPresent1991
Hong KongPresent
JapanPresentIntroducedInvasive
-HonshuPresent
-KyushuPresent
-Ryukyu IslandsPresent
-ShikokuPresent
South KoreaPresentIntroducedInvasive
TaiwanPresent, WidespreadIntroduced
VietnamAbsent, Unconfirmed presence record(s)

Europe

AustriaAbsent, Confirmed absent by survey
BelgiumAbsent, Confirmed absent by survey
DenmarkAbsent, Confirmed absent by survey
EstoniaAbsent, Confirmed absent by survey
FinlandAbsent, Intercepted only
FranceAbsent, Intercepted only
ItalyAbsent, Confirmed absent by survey
LatviaAbsent, Confirmed absent by survey
LithuaniaAbsent, Confirmed absent by survey
MaltaAbsent, Confirmed absent by survey
NetherlandsAbsent, Confirmed absent by survey
NorwayAbsent, Intercepted only
PolandAbsent, Confirmed absent by survey
PortugalPresent, Localized
-MadeiraPresent, Localized
RomaniaAbsent, Confirmed absent by survey
SlovakiaAbsent, Confirmed absent by survey
SloveniaAbsent, Confirmed absent by survey
SpainPresent, Transient under eradication
SwedenAbsent, Intercepted only
SwitzerlandAbsent, Confirmed absent by survey
UkrainePresent
United KingdomAbsent, Confirmed absent by survey

North America

CanadaPresent, WidespreadNative
-AlbertaPresent
-British ColumbiaPresent
-ManitobaPresent
-New BrunswickPresent
-Newfoundland and LabradorPresent
-Northwest TerritoriesPresent
-Nova ScotiaPresent
-NunavutPresent
-OntarioPresent
-QuebecPresent
-SaskatchewanPresent
-YukonPresent
MexicoPresentNative1993
United StatesPresent, WidespreadNative
-AlabamaPresent
-ArizonaPresent
-ArkansasPresent
-CaliforniaPresent
-ColoradoPresent
-ConnecticutPresent
-DelawarePresent
-FloridaPresent
-GeorgiaPresent
-IllinoisPresent
-IndianaPresent
-IowaPresent
-KansasPresent
-KentuckyPresent
-LouisianaPresent
-MarylandPresent
-MassachusettsPresent
-MichiganPresent
-MinnesotaPresent
-MississippiPresent
-MissouriPresent
-NebraskaPresent
-New HampshirePresent
-New JerseyPresent
-New YorkPresent
-North CarolinaPresent
-OhioPresent
-OklahomaPresent
-OregonPresent1996
-PennsylvaniaPresent
-South CarolinaPresent
-TennesseePresent
-TexasPresent
-VermontPresent
-VirginiaPresent
-West VirginiaPresent
-WisconsinPresent

History of Introduction and Spread

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It is presumed that B. xylophilus was introduced from North America to southern Japan, to the island of Kyushu, sometime around the beginning of the 20th century (Nickle et al., 1981; Mamiya, 1983; Malek and Appleby, 1984). From there it spread, during the 1920s, to the Hyogo prefecture, over 730 km east. Pine wilt disease (PWD) subsequently spread steadily to all coastal areas of southwest Japan up until 1947. In the following decade, the progression of the PWD was slowed due to the implementation of exhaustive control measures, but it quickly resumed, invading inland areas with cooler summers in the southwest, as well as northern coastal areas. The number of newly invaded prefectures increased steadily until the 1980s, the affected area remaining stable since then, with only the two northernmost prefectures of Aomori and Hokkaido being free from B. xylophilus (Togashi and Jikimaru, 2007).

From Japan, B. xylophilus was introduced to other Asian countries (Li et al., 1983). The first occurrence of B. xylophilus and the PWD in Korea Republic was reported in 1988, at Mt. Geumjong in Busan, at the south-eastern coast of the country. However, it is suspected that the introduction occurred before 1986. PWD remained restricted to this area until 1997 when was detected in neighbouring districts. In 2001, it was registered over 100 km west, in coastal Mokpo, and 245 km north, in inland Gumi. In the following years it spread steadily to the north, apparently through pines bordering highways, being recorded around Seoul in 2003. In 2004, diseased Japanese black pines (Pinus thunbergii) were recorded on Jeju Island, the biggest island in Korea. On the continent, PWD kept spreading north, and in 2005 was recorded in the northernmost districts of Gangreung and Donghae. During 2006-2007, PWD was recorded in other northern districts such as Seoul, Gwangju, Namyangju, Pocheon, Wonju and Chuncheon (Kwon et al., 2011).

Bursaphelenchus xylophilus was first recorded in continental China in 1982, in the Nanjing area, located in the east of the country (Li et al., 1983). It reached Shandong province, over 500 km to the north in 1991, the same year that it was recorded in Zhoushan, about 400 km to the southeast. It also spread swiftly to the west, reaching the southern coastal province of Guangdong in 1988, and the more northern provinces of Hubei in 1999 and Chongqing in 2003 (Cheng et al., 2008; Wu et al., 2013). It was detected in Taiwan in 1985 (Ma et al., 2002; Chen et al., 2006).

The nematode was detected in 1999 in Portugal in a coastal forest located south of Lisbon (Mota et al., 1999). Despite strict containment measurements, within a time span of 10 years B. xylophilus was considered to have spread throughout the country (Vicente et al., 2012), being detected in a border area in Spain in 2008 (Abelleira et al., 2011). However, it is not yet considered to have spread outside Portugal. In 2009 the pine wood nematode was detected in the Atlantic Island of Madeira (Fonseca et al., 2012).

Risk of Introduction

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Bursaphelenchus xylophilus and its vectors, Monochamus species, are regulated by the European Union (EU, 2000; 2017) and by other EPPO countries (OEPP/EPPO, 2012). It is also treated as a quarantine pest in Asia (particularly China), in South America and in Oceania (Ridley et al., 2000; Mireku and Simpson, 2002). Infested wood is the most probable means of international transport of B. xylophilus, and the species has been detected on a number of occasions in sawn wood, round wood and wood chips imported into Europe from the USA and Canada. The introduction of the nematodes within wood imports does not, in itself, represent a threat to the forests of the EPPO region (Magnusson, 1986). It is necessary for a pathway to be found for the nematode to come into contact with a native vector, and this can probably only be achieved if the nematode first invades wood which contains juveniles or pupae of a potential vector. Several species of Monochamus are present in Europe and have been shown to transmit the closely related B. mucronatus (Tomminen, 1990). Nematodes can move very actively from wood chips or sawdust, and the connection with the vector could be made if such material were to come into contact with tree stumps or cut logs (Skwiercz, 1988). Within the wood processing industry, the same means of transport is sometimes used for collecting both imported and local wood material (McNamara and Støen, 1988).

Bursaphelenchus xylophilus is more likely to be introduced by any given pathway if it is imported together with vector insects. Such insects can only survive if the wood has a sufficient moisture content, greater than that needed by the nematode. The larger the pieces of timber, the longer insects are liable to survive, and therefore round wood and sawn wood present a greater risk than wood chips. Wood chips can have a high moisture content, allowing ready nematode survival, but the processing undergone in their preparation reduces the possibility of vectors surviving (Kinn, 1986).

Soliman (2012) predicted significant impacts caused by B. xylophilus in Europe in Portugal, Spain, southern France and northwest Italy.

Habitat

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Evidence points to the pine wood nematode being able to adapt to any forest, independently of climate or geographical location as long as pine hosts and Monochamus vectors are available. In North America, B. xylophilus was found from sub-boreal to sub-tropical pine forest (Bowers et al., 1992; Pimentel et al., 2014) and in China is presently spreading to northern coldest areas of the country, having already occupied southern subtropical areas (Cheng et al., 2008; Wu et al., 2013). However, there seems to be a niche conservatism in relation to temperature independent of the pine wood nematode geographical origin, with temperatures of 28-29°C maximizing the growth rates of the pathogen, and the number of days during the summer with temperatures between 25 and 31°C is determinant for the build-up of populations of B. xylophilus (Pimentel and Ayres, 2018). Furthermore, dispersion of the pathogen is dependent on the flight of its vector, which lasts longer as we move to southern and warmer locations (Pimentel et al., 2014). This is the probable reason why climate has long been considered the determinant for the onset of pine wilt disease (PWD), which was found to be restricted to areas where average summer temperatures surpass 20°C (Rutherford and Webster, 1987; Rutherford et al., 1990). More recently, the role of interactions with temperature and precipitation has been highlighted, with the expression of PWD at a macro- and microgeographical scale being explained by the average monthly mean temperatures of the warmest 3 months and aridity, with high-risk zones located in hot and dry areas (Ikegami and Jenkins, 2018; Calvão et al., 2019). This is likely because the pathogen kills pine trees by restricting water flow through the xylem (leading to cavitation and pine wilt) and hot, dry conditions promote this process (Yazaki et al., 2018).

It remains unknown how B. xylophilus populations manage to subsist and thrive in areas that are too cold or wet for the development of PWD. It is known that populations of B. xylophilus can be sustained for several years in healthy hosts (symptomless carriers) which has been associated with cool temperatures (Bergdahl and Halik, 2004; Hoshizaki et al., 2016). Life-history adaptations of pine wood nematodes to resist periods of low temperatures include prolonged adult longevity and the production of third-stage dispersal juveniles (JIII) that are particularly resistant to freezing conditions (Zhao et al., 2007). It is thought that environmental triggers such as higher temperature, lower rainfall, or mechanical damage to trees can boost the growth of these sub-lethal populations, leading to the development of PWD (Hoshizaki et al., 2016).

Hosts/Species Affected

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Generally, all species of the genus Pinus are considered hosts to the pine wood nematode, but considerable differences in susceptibility to pine wilt disease have been recorded. In Japan Pinus thunbergii, Pinus densiflora, Pinus luchuensis, have been found to be very susceptible in forest settings (Mamiya, 1983). In China the masson pine, Pinus massoniana, has been reported as very affected by pine wilt disease (PWD) (Juan et al., 2008). In the American Continent PWD is considered to be limited primarily to exotic, often stressed, Austrian Pinus nigra and Scots pines Pinus sylvestris, while native pines are considered to be resistant (Kondo et al., 1982; Malek and Appleby, 1984). In Portugal, the maritime pine, Pinus pinaster, proved to be very susceptible, while the sympatric stone pine, Pinus pinea, proved to be resistant (Pimentel et al., 2017).

Several inoculation experiments with B. xylophilus of pine seedlings have further confirmed differences in susceptibility between pine species, although, sometimes, with somewhat ambiguous results (e.g. Woo et al., 2008; Pimentel et al., 2017; Menéndez-Gutiérrez et al., 2018). On these experiments Pinus radiata, the most planted pine species in the world proved to be very susceptible to the pathogen- similar to P. pinaster- in spite of being native to North America and of not having been registered yet as a host in the field. However, the factors that determine resistance to B. xylophilus in the different host species are still not well known, but differences in the production of chemical defences seem to be important (Pimentel et al., 2017; Menéndez-Gutiérrez et al., 2018).

Other conifers can also act as hosts but reports of damage are rare. Isolated cases of death of Picea and Pseudotsuga due to this nematode have been reported in the USA (Malek and Appleby, 1984).

Host Plants and Other Plants Affected

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Plant nameFamilyContextReferences
Abies alba (silver fir)PinaceaeUnknown
Abies amabilis (Pacific silver fir)PinaceaeOther
Abies balsamea (balsam fir)PinaceaeOther
Abies firma (momi fir)PinaceaeOther
Abies grandis (grand fir)PinaceaeOther
Abies sachalinensis (Sakhalin fir)PinaceaeOther
Cedrus atlantica (Atlas cedar)PinaceaeOther
Cedrus deodara (Himalayan cedar)PinaceaeOther
Larix decidua (common larch)PinaceaeOther
Larix kaempferi (Japanese larch)PinaceaeOther
Larix laricina (American larch)PinaceaeOther
Larix occidentalis (western larch)PinaceaeOther
Picea abies (common spruce)PinaceaeOther
Picea engelmannii (Engelmann spruce)PinaceaeOther
Picea glauca (white spruce)PinaceaeOther
Picea jezoensis (Yeddo spruce)PinaceaeOther
Picea mariana (black spruce)PinaceaeOther
Picea pungens (blue spruce)PinaceaeOther
Picea rubens (red spruce)PinaceaeOther
Picea sitchensis (Sitka spruce)PinaceaeOther
Pinus (pines)PinaceaeMain
Pinus ayacahuite (Mexican white pine)PinaceaeOther
Pinus banksiana (jack pine)PinaceaeMain
Pinus brutia (brutian pine)PinaceaeOther
Pinus bungeana (lace bark pine)PinaceaeOther
Pinus caribaea (Caribbean pine)PinaceaeOther
Pinus contorta (lodgepole pine)PinaceaeOther
Pinus densiflora (Japanese umbrella pine)PinaceaeMain
Pinus echinata (shortleaf pine)PinaceaeMain
Pinus elliottii (slash pine)PinaceaeMain
Pinus halepensis (Aleppo pine)PinaceaeOther
Pinus hartwegii (Hartweg pine)PinaceaeOther
Pinus jeffreyi (Jeffrey pine)PinaceaeOther
Pinus koraiensis (fruit pine)PinaceaeOther
Pinus lambertiana (big pine)PinaceaeMain
Pinus leiophylla (smooth-leaved pine)PinaceaeOther
Pinus luchuensis (luchu pine)PinaceaeMain
Pinus massoniana (masson pine)PinaceaeOther
Pinus monticola (western white pine)PinaceaeOther
Pinus mugo (mountain pine)PinaceaeOther
Pinus nigra (black pine)PinaceaeMain
Pinus oocarpa (ocote pine)PinaceaeOther
Pinus palustris (longleaf pine)PinaceaeOther
Pinus pinaster (maritime pine)PinaceaeOther
Pinus pinea (stone pine)PinaceaeOther
Pinus ponderosa (ponderosa pine)PinaceaeOther
Pinus pungens (tabel Mountain pine)PinaceaeOther
Pinus radiata (radiata pine)PinaceaeMain
Pinus resinosa (red pine)PinaceaeMain
Pinus strobiformis (southwestern white pine)PinaceaeOther
Pinus strobus (eastern white pine)PinaceaeMain
Wingfield and Blanchette (1983), Dwinell (1993)
Pinus sylvestris (Scots pine)PinaceaeMain
Pinus tabuliformis (chinese pine)PinaceaeUnknown
Pinus taeda (loblolly pine)PinaceaeMain
Pinus thunbergii (Japanese black pine)PinaceaeMain
Liou et al. (1999), Zhao et al. (2003)
Pinus wallichiana (blue pine)PinaceaeOther
Pinus yunnanensis (Yunnan pine)PinaceaeOther
Pseudotsuga menziesii (Douglas-fir)PinaceaeOther
Xanthocyparis nootkatensis (Alaska cedar)Other

Growth Stages

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Vegetative growing stage

Symptoms

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The first indication of the presence of nematodes in the tree is a reduction of oleoresin production in reaction to wounding. Transpiration from the leaves decreases and later stops completely. The first obvious external symptom is the yellowing and wilting of the needles, leading to eventual death of the tree (Mamiya, 1983). The wilting may first appear on only one branch ('flag'), although the whole tree may later show symptoms (Malek and Appleby, 1984).

List of Symptoms/Signs

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SignLife StagesType
Leaves / wilting
Leaves / yellowed or dead
Whole plant / plant dead; dieback

Biology and Ecology

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The pine wood nematode has a complex life cycle, which includes a fungal feeding phase and a phytophagous phase. This is unusual within the plant-parasitic nematodes, which are generally obligatory parasites (Espada et al., 2016). In fact, B. xylophilus can be maintained in laboratory cultures either on fungi or pine tissue and evidence indicates that the nematode can follow one or other life style in nature as an adaptation to environmental conditions (Wingfield, 1983; Espada et al., 2016; Pimentel et al., 2017).

The nematodes are always transmitted from one host to the next by flying Monochamus vectors as fourth-stage juveniles. They can enter recently dead or dying trees during oviposition by the female vector, through a hole in the bark cut by the vector to lay its eggs. On this case B. xylophilus will have a strictly mycophagous life cycle feeding on the hyphae of fungi (usually Ceratocystis species) also transmitted to the wood by ovipositing beetles. Immediately on entering the wood the larval nematodes moult to adult and begin laying eggs. The population is composed of males, females and four juvenile stages in this propagative stage of rapid multiplication. At a certain time after the initial invasion by the nematodes, the population ceases to multiply and begins to decline. A different type of third-stage juvenile is produced; this is called the 'dispersal third-stage juvenile' (Mamiya, 1983), although it is more correctly a survival stage, being capable of resisting adverse conditions. It is likely that the population structure of the nematodes changes in reaction to a reduction in the availability of food, when the fungus has fully exploited the wood. The 'dispersal' juveniles gather in the wood surrounding the pupal chamber of the Monochamus vector, possibly under the influence of substances diffusing from the developing pupa. Close to the time of emergence of the beetle, the nematodes moult into the special fourth-stage juveniles, called the 'dauer larvae'. Fungal hyphae also develop around the pupation chambers. The fungus forms long-necked perithecia projecting into the chamber, and the nematodes gather at the tips of the perithecia. When the young adult beetle emerges, it brushes against the perithecial necks, picking up the nematodes which settle below the elytra and, in particular, in the tracheae. The immature adult beetle then flies from the wood carrying nematodes.

A strictly mycophagous life cycle can be considered the primordial life cycle of B. xylophilus just like that of most other species of Bursaphelenchus, that have phoretic relationships with forest beetles. It has been proposed that this is most frequent in areas where healthy pines are resistant to infection by the pathogen, mainly in its native range in North America, which precludes the phytophagous phase (Wingfield, 1983). However, in Eurasia, and also in North America, wherever the nematode comes into contact with non-native or susceptible species of Pinus, the phytophagous stage is swich on. In this case, the nematodes are transmitted from young adult beetles shortly after emergence from their pupal chambers, when they fly to feed on young Pinus shoots. The nematodes enter the shoots through the feeding wounds. This form of transmission only occurs on susceptible species of Pinus presumably because Pinus species native to the area where B. xylophilus occurs develop physical or biochemical barriers to prevent direct invasion to healthy tissues. In susceptible pine trees, B. xylophilus will multiply within the resin canals the young pine shoots, feeding on their epithelial cells. About 3 weeks later, the tree shows first symptoms of 'drying out', in the form of reduced oleoresin exudation. The nematodes can now move freely throughout the dying tree which becomes attractive to adult insects which gather on the trunks to mate. At this stage, wilting and yellowing of the needles becomes evident. The tree dies 30-40 days after infection and may then contain millions of nematodes throughout the trunk, branches and roots. At this stage B. xylophilus switch to the mycophagous phase, feeding on the fungus that invaded the host tissues before boarding a pupa of its insect vector (Mamiya, 1983).

In the laboratory, B. xylophilus can be maintained on fungal cultures. It reproduces in 12 days at 15°C, 6 days at 20°C and 3 days at 30°C. Laying starts on the fourth day after hatching, and the eggs hatch in 26-32 h at 25°C. The temperature threshold for development is 9.5°C.

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Esteya vermicola Parasite

Notes on Natural Enemies

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The endoparasitic fungus Esteya vermicola (Ophiostomataceae) is being increasingly assessed as a potential biocontrol agent of pine wilt disease (Liou et al., 1999; Wang et al., 2017).

Means of Movement and Dispersal

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The pine wood nematode is, as far as presently known, exclusively vectored by pine sawyer beetles of the genus Monochamus. In North America, where the pine wood nematode is native, six of eight native Monochamus species have been recorded as vectors: Monochamus scutellatus, Monochamus marmorator, Monochamus mutator, Monochamus notatus, Monochamus carolinensis and Monochamus titillator (Wingfield and Blanchette, 1983; Bergdahl et al., 1991; Pimentel et al., 2014). Invasive populations of B. xylophilus readily adapt to local Monochamus spp. In Asia Monochamus alternatus has been considered as the main vector, but other pine sawyers such as Monochamus nigromaculatus, Monochamus nitens, Monochamus saltuarius and Monochamus sparsutus were also recorded as being able to act as phoretic carriers (Togashi and Jikumaru, 2007; Wang et al., 2021a). In Portugal Monochamus galloprovincialis, the only pine sawyer recorded in the country, became the only known vector for pine wilt disease (PWD) in Europe (Sousa et al., 2001). The current state of knowledge points to any species of Monochamus, both males and females, being able to act as vector of B. xylophilus, although it is not well understood the impact of species-specific differences in biology and ecology on the epidemiology of the PWD.

Monochamus spp. are generally the larger species of the pine phloem feeding guild, which includes several smaller bark beetles species and other larger wood borers. They can be found throughout pine forests of the northern hemisphere, subsisting in a wide range of climates, from boreal to subtropical. They are considered secondary pests, affecting weakened or already dying pine trees, which are selected by females for oviposition. Eggs are deposited underneath the bark, and the larvae feed on fresh phloem, later boring a ‘U’-shaped gallery into the xylem where they pupate, which can decrease the value of timber. Monochamus spp. adults are generally attracted to host volatiles (e.g. (monoterpenes and ethanol) and bark beetles’ pheromones (e.g. ipsenol and ipsdienol), which indicates a suitable host- a damaged or freshly dead pine tree. In fact, blends of these compounds associated with multiple-funnel traps were found to be highly effective for monitoring pine sawyer beetles, with consistency of results across large geographic areas suggesting similar selection pressures on different species of the genus across different climates. Additionally, mature males of species native to Asia, Europe and North America produce an aggregation pheromone, monochamol (2-undecyloxy-ethanol), which is attractive to both sexes and can be used in trapping protocols (Boone et al., 2019).

Adults of these insects are relatively long lived, up to >2 months, and can reach up to 1000 m in one flight, both in the maturation feeding and oviposition stages (Zhang et al., 2007). Thus, pine sawyers are efficient vectors for the spreading of PWD in different forests. However, the abundance of nematodes on its vectors is higher early in the flight season of the different areas, indicating the importance of seasonality in the infection of hosts, and consequently on the setting of the PWD. B. xylophilus dauers present a highly aggregated frequency distribution on Monochamus, with a few individuals carrying hundreds to thousands of nematodes, with most carrying none (Pimentel et al., 2014; Firmino et al., 2017). A tendency for highly aggregated dispersion has been associated with establishment of pine wilt disease because more nematodes per plant in the initial inoculation yields greater probability of disease (Togashi and Shigesada, 2006).

Other genera of the Cerambycidae (for example, Acalolepta, Acanthocinus, Amniscus, Arhopalus, Asemum, Corymbia, Neacanthocinus, Rhagium, Spondylis, Uraecha, Xylotrechus) and other Coleoptera (for example, Chrysobothris, Hylobius, Pissodes) have been found to carry nematodes in or on their bodies, but there is no evidence that they have any role as vectors (e.g. Wang et al., 2021a).

Seedborne Aspects

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The nematode is not seedborne.

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Containers and packaging - woodPinewood Yes
Land vehicles Yes
Soil, sand and gravel Yes

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
Bark adults; eggs; juveniles Yes Yes Pest or symptoms not visible to the naked eye but usually visible under light microscope
Bulbs/Tubers/Corms/Rhizomes adults; eggs; juveniles Yes Yes Pest or symptoms not visible to the naked eye but usually visible under light microscope
Flowers/Inflorescences/Cones/Calyx adults; eggs; juveniles Yes Yes Pest or symptoms not visible to the naked eye but usually visible under light microscope
Growing medium accompanying plants adults; eggs; juveniles Yes Yes Pest or symptoms not visible to the naked eye but usually visible under light microscope
Leaves adults; eggs; juveniles Yes Yes Pest or symptoms not visible to the naked eye but usually visible under light microscope
Seedlings/Micropropagated plants adults; eggs; juveniles Yes Yes Pest or symptoms not visible to the naked eye but usually visible under light microscope
Stems (above ground)/Shoots/Trunks/Branches adults; eggs; juveniles Yes Yes Pest or symptoms not visible to the naked eye but usually visible under light microscope
Wood adults; eggs; juveniles Yes Yes Pest or symptoms not visible to the naked eye but usually visible under light microscope
Plant parts not known to carry the pest in trade/transport
Fruits (inc. pods)
Roots
True seeds (inc. grain)

Vectors and Intermediate Hosts

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Impact Summary

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CategoryImpact
Animal/plant collections None
Animal/plant products None
Biodiversity (generally) Negative
Crop production None
Environment (generally) Negative
Fisheries / aquaculture None
Forestry production Negative
Human health None
Livestock production None
Native fauna None
Native flora Negative
Rare/protected species Negative
Tourism None
Trade/international relations Negative
Transport/travel None

Impact

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Pine wilt disease was first reported in Japan in 1913 in the Nagasaki region, but the causal agent was only identified as B. xylophilus in 1972 (Mamiya and Kiyohara, 1972). The symptoms were first attributed to wood-boring insects, which are found abundantly on infected trees, but it was then found that first symptoms precede attack by the insects. The disease then began to spread northwards causing very severe losses throughout the country. Over 1,000,000 m³ of wood were being lost per year at the end of the 1940s, but a campaign for destroying infected trees then brought this figure below 500,000 m³ per year (still a substantial loss). However, because industrialization has reduced the availability of manpower for the forests, and because wood has been replaced by oil as a fuel, infected trees are again remaining standing as reservoirs of the nematode. In consequence, the loss curve turned sharply upwards from 1970 and now even exceeds 2,000,000 m³ per year. Almost all the Japanese archipelago is affected, from the Ryukyu Islands in the far south, where P. luchuensis is very susceptible, to the northern part of the island of Honshu, with much colder climates where the mean annual temperature is 10-12°C. It seems probable that this northward spread is due to heavy population pressure from the south. Only the most northern island of Hokkaido is still not affected.

In 1979, B. xylophilus was associated with the death of Pinus in Missouri, USA, primarily of P. sylvestris growing in amenity plantings (Malek and Appleby, 1984), and in the USA in general losses arise almost exclusively among exotic species and in artificial forest ecosystems such as ornamental conifer plantings, wind-breaks and Christmas tree plantations. B. xylophilus is widespread in natural coniferous forests, but significant losses have not been recorded.

Economic Impact

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The spread of B. xylophilus has caused huge economic losses in the affected countries, including direct costs due to a decrease in the area of pine forests and wood production, and the indirect costs of managing and mitigating the epidemics, as well as economic losses due to the loss of export markets and import restrictions.

After pine wilt disease was initially reported in Japan in 1913 in the Nagasaki region, it spread northwards causing severe damage throughout the country, with a loss of over 1,000,000 m³ of wood per year up till the 1940s (Mamiya and Kiyohara, 1972). A campaign to destroy infected trees then brought this figure below 500,000 m³ per year. However, industrialization lead to a decrease in the availability of manpower to work in forests and the replacement of wood by oil as a fuel, resulting in infected trees being left standing as reservoirs for the nematode. In consequence, losses increased sharply after 1970, exceeding 2,000,000 m³ per year. Almost all the Japanese archipelago is affected, from the Ryukyu Islands in the far south, where Pinus luchuensis is very susceptible, to the northern part of the island of Honshu, with much colder climates where the mean annual temperature is 10-12°C. Only the most northern island of Hokkaido is still not affected.

In China, between 1998 and 2017, the average annual economic loss due to the invasion of B. xylophilus was over a billion dollars, of which, the direct economic losses due to destruction of forest resources accounted for about 250 million dollars, the rest being due to ineffective prevention, control and forest management expenses. Most of the losses occurred in East and South China accounting for about 80% of the total national economic losses (Zhao et al., 2020).

In 1979, B. xylophilus was associated with the death of Pinus in Missouri, USA, primarily of Pinus sylvestris growing in amenity plantings (Malek and Appleby, 1984), and in the USA in general losses arise almost exclusively among exotic species and in artificial forest ecosystems such as ornamental conifer plantings, wind-breaks and Christmas tree plantations. B. xylophilus is widespread in natural coniferous forests, but significant losses have not been recorded. Despite low economic impact due to tree mortality, the USA and Canada were badly hit by subsequent embargoes to coniferous chips, sawn wood, and logs export trade by the European Union (EU), to protect their forests from pine wilt nematode. This embargo became fully effective in 1993, and a trade worth of 1 billion dollars came to a halt (Dwinell, 1997).

In Portugal, B. xylophilus has mostly affected maritime pine, Pinus pinaster (Pimentel et al., 2017), the most important pine species in the country, widely used in forestry, with an annual industrial production of more than 100 million euros. Thus, invasion by B. xylophilus and consequent restrictions to the circulation and export of wood imposed by the EU has led to severe economic consequences for Portugal. Soliman (2012), modelling an unregulated infestation of B. xylophilus in the EU, estimated that the loss of forestry stock over 22 years (2008-2030) would cost 22 billion euros.

Environmental Impact

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Impact on Habitats

Bursaphelenchus xylophilus is a highly invasive species, colonizing and damaging natural or managed pine forests in areas outside its natural range. In Japan, forests of Pinus densiflora and Pinus thunbergii severely affected by B. xylophilus tend to be succeeded by deciduous oak forest: Quercus serrata in central Japan, and evergreen oak forest of Quercus glauca further south (Fujihara et al., 1992; Fujihara, 1996). Pine forest only persists on upper slopes, where damage is less severe (Fujihara et al., 1999). In the southern island of Kyushu (Inoue, 1995), pine is replaced by more tropical evergreen trees (Mallotus japonicus and Rhus succedanea [Toxicodendron succedaneum]). Similarly, in Taiwan, Pinus luchuensis is replaced by the evergreen Mallotus paniculatus (Ma et al., 2002).

A similar situation occurred in China, with pine wood nematode infection accelerating the succession from the native pine plantations, mostly Pinus massoniana, with higher economical values to the zonal evergreen broad-leaved forests which provide more ecological services (Yu et al., 2011).

Impact on Biodiversity

Invasive B. xylophilus can present a direct threat to the survival of the local endemic pine species, such as the case of Pinus armandii var. amamiana [Pinus amamiana] in southern Japan (Kanetani et al., 1999). With the decimation of pine forest, which tend to be monocultures, species diversity is initially reduced, but progressively recovers with the establishment of more diverse broad-leaved forest (Katagiri et al., 1995). The changes in the community structure of woody plants after invasion of B. xylophilus, has cascading effects on upper trophic levels, changing the functional relationship between communities (Wang et al., 2021b).

Impact: Biodiversity

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B. xylophilus presents a direct threat to the survival of the very local endemic Pinus armandii var. Amamiana in southern Japan (Kanetani et al., 1999).

Social Impact

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Destruction of pine forests in Japan initially affected local employment, but this was overtaken by post-war economic development involving a much diminished reliance of local populations on forest products. In fact, this decline in forest use has aggravated losses due to B. xylophilus, as damaged forests tended to be neglected.

Risk and Impact Factors

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Invasiveness
  • Proved invasive outside its native range
  • Has a broad native range
  • Abundant in its native range
  • Highly adaptable to different environments
  • Is a habitat generalist
  • Gregarious
Impact outcomes
  • Ecosystem change/ habitat alteration
  • Modification of successional patterns
  • Negatively impacts forestry
Likelihood of entry/control
  • Highly likely to be transported internationally accidentally
  • Difficult to identify/detect as a commodity contaminant
  • Difficult/costly to control

Detection and Inspection

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In the field, B. xylophilus can be detected and identified in infested trees- cut branches, wood, bark and wood shavings- and in adults of Monochamus spp. Nematodes can be extracted from samples of tree tissues by soaking them in water for some time using an adaptation of the Baermann funnel (see OEPP/EPPO, 2013 for details on the procedures). The detection of B. xylophilus is also possible by capturing live flying adults of Monochamus spp. with baited traps and subsequently extracting phoretic dauer larvae (Pimentel et al., 2014; Firmino et al., 2017). Extracted nematodes can be quantified and species identity confirmed based on male spicule, female vulva flap and tail terminus (Nickle et al., 1981). In the case of the dauers extracted from flying pine sawyer beetles, they should be cultured, to obtain adult nematodes, which can then be identified. Molecular methods can also be applied directly to pine wood samples or extracted nematodes, to assist in detection and identification. This includes real-time polymerase chain reaction (PCR) tests and loop-mediated isothermal amplification (LAMP) tests (Cao et al., 2005; François et al., 2007; Kikuchi et al., 2009; Wang et al., 2011). Information on DNA barcoding of B. xylophilus is described in EPPO Standard PM 7/129 (EPPO, 2016).

These methods of assessment can be used by authorities in entry ports receiving material from infested areas, including green conifer wood, wood packaging material and dunnage from conifers, conifer particle wood large enough to host the larvae and coniferous wood products.

Similarities to Other Species/Conditions

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Bursaphelenchus xylophilus is superficially similar to other species of the genus, although these are not regarded as pests. It can be distinguished from most of these by the unusual shape of the male spicules and the presence of a vulval flap in the female. However, there is a group of Bursaphelenchus species sharing these characters and differentiating B. xylophilus from these species requires considerable expertise (see the EPPO diagnostic protocol for B. xylophilus; OEPP/EPPO, 2001). There is also a species of Laimaphelenchus [Aphelenchoides] recorded from North America and Europe that may be confused with B. xylophilus - the spicule form is similar and the female also has a vulval flap. Molecular methods presently available can help differentiate B. xylophilus from these species (Filipiak et al., 2017).

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

EPPO's recommendations to prevent the introduction of B. xylophilus and its vectors cover plants and wood of all conifers, apart from Thuja plicata, from countries where the nematode occurs. Wood, including chips, may be prohibited, with exceptions in the case of previous treatment by heat treated to a core temperature of 56°C for 30 min (Soma et al., 2003). In the case of wood packing, ISPM no. 15 can be applied (IPPC, 2002). Other phytosanitary treatments for wood chips have been proposed, such as steam/heat treatment or fumigation in transit with phosphine (Kinn, 1986). Such treatment can, however, be expensive in relation to the value of the commodity.

Recently cut logs can be protected from pine sawyer beetle oviposition (and thus from introduction of nematodes) by chemical treatment, which can even be more effective in killing insect larvae already present under the bark. Covering logs after felling, placing trap logs nearby and ensuring that felling is conducted outside the flight period of the beetles can also be effective methods of preventing wood from being infected (Raske, 1973; Dominik, 1981).

Eradication

Bursaphelenchus xylophilus has the ability to establish in trees as a saprophyte without symptom expression. The lack of symptoms under cooler conditions or during the latent period can make it difficult to detect B. xylophilus in time to allow for successful eradication (EPPO, 2018). Thus, in areas where the pine wood nematode is well established, eradication is generally not considered possible. However, Muramoto (1999) reports that B. xylophilus has been successfully eradicated from Okinoerabu Island in the Ryukyu Archipelago, Japan.

Containment/Zoning

In Portugal, the present action plan aims to contain B. xylophilus within areas where it is well established, while trying to eradicate it from isolated spots where it was recently detected. Under this programme, trees are assessed for wilting symptoms during the winter all over the Portuguese territory. Whenever wilting host is found, wood samples are collected, and analysed for the presence of B. xylophilus. Infested trees are removed and transported to an authorized destination, where the woody materials are processed to ensure the absence of B. xylophilus or its vector. Monitoring and removal of diseased trees is intensified in areas where B. xylophilus was previously detected. To avoid dispersal of the nematode to the remainder of Europe, a 20 km buffer zone was established along the Portuguese border, totalling 2.5 million ha. Within this zone monitoring occurs during the whole year and all conifers showing any wilting symptoms are immediately removed (Calvão et al., 2019).

Control

So far it has proved impossible to control nematodes once introduced into a tree. Therefore, control of pine wilt disease in affected areas has concentrated on a combination of cultural practices, in removing dead or dying trees from the forest to prevent their use as a source of further infection, and the control of the vector beetles by insecticidal treatment. The Japanese government has spent large amounts of money on extensive control programmes involving aerial spraying and removal of diseased trees (Ikeda, 1984). In the case of individual trees with a particular significance (for example, religious), infection can be prevented by a prophylactic chemical treatment. Research is continuing to try to find alternative means of control, such as biological control agents for both nematodes and vectors, insect attractants, breeding of resistant Pinus clones and inducing resistance by inoculation of non-pathogenic strains of B. xylophilus.

Monitoring and Surveillance (incl. remote sensing)

Remote sensing has been widely applied to assess forest health and decline due to the wide areas generally involved, and the ability of this detection method to quantify changes in tree canopy cover across large regions. As declining/dying trees affected by B. xylophilus are characterized by a browning/reddening of the leaves, which have a spectral behaviour different to that of green healthy leaves, these technics, coupled with field sampling, have great potential for assessing incidence of pine wilt disease. Satellite information from different spectral channels can be complemented with surveys of tree mortality on the field, and drones equipped with a digital camera can also be used to acquire aerial images of declined trees (e.g. Syifa et al., 2020).

References

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Abad, P., Tares, S., Brugier, N., Guiran, G. de, 1991. Characterization of the relationships in the pinewood nematode species complex (PWNSC) (Bursaphelenchus spp.) using a heterologous unc-22 DNA probe from Caenorhabditis elegans. Parasitology, 102(2):303-308. DOI: 10.1017/S0031182000062636

Abelleira, A., Picoaga, A., Mansilla, J.P., Aguin, O., 2011. Detection of Bursaphelenchus xylophilus, causal agent of pine wilt disease on Pinus pinaster in northwestern Spain. Plant Disease, 95(6):776. DOI: 10.1094/PDIS-12-10-0902

Atkins, D.H., Davis, T.S., Stewart, J.E., 2021. Probability of occurrence and phenology of pine wilt disease transmission by insect vectors in the Rocky Mountains. Ecological Solutions and Evidence, 2(e12044). DOI: 10.1002/2688-8319.12044

Bergdahl, D.R., Halik, S., 2004. Persistence of the pine wood nematode in asymptomatic Scots pines. In: The pinewood nematode, Bursaphelenchus xylophilus. Proceedings of an International Workshop, University of Évora, Portugal, 20-22 August 2001 [ed. by Mota, M., Vieira, P.]. Leiden, Netherlands: Brill Academic Publishers. 177-185.

Bergdahl, D.R., Halik, S., Tomminen, J., Akar, H., 1991. Frequency of infestation of Monochamus notatus and M. scutellatus by Bursaphelenchus xylophilus in Vermont. In: Phytopathology, 81. 120.

Blunt, T.D., Jacobi, W.R., Appel, J.A., Tisserat, N., Todd, T.C., 2014. First report of pine wilt in Colorado, USA. Plant Health Progress, (July):PHP-BR-14-0010. http://www.plantmanagementnetwork.org/php/elements/sum2.aspx?id=10774

Boone, C.K., Sweeney, J., Silk, P., Hughes, C., Webster, R.P., Stephen, F., Maclauchlan, L., Bentz, B., Drumont, A., Zhao, B.G., Berkvens, N., Casteels, H., Grégoire, J.C., 2019. Monochamus species from different continents can be effectively detected with the same trapping protocol. Journal of Pest Science, 92(1):3-11. DOI: 10.1007/s10340-018-0954-4

Bowers, W.W., Hudak, J., Raske, A.G., Magasi, L.P., Myren, D.T., Lachance, D., Cerezke, H.F., Sickle, G.A. van, 1992. Host and vector surveys for the pinewood nematode, Bursaphelenchus xylophilus (Steiner and Buhrer) Nickle (Nematoda: Aphelenchoididae) in Canada. Information Report - Newfoundland and Labrador Region, Forestry Canada, (N-X-285):viii + 55 pp.

Braasch, H., 2001. Bursaphelenchus species in conifers in Europe: distribution and morphological relationships. Bulletin OEPP, 31(2):127-142. DOI: 10.1111/j.1365-2338.2001.tb00982.x

Braasch, H., Gu, J., Burgermeister, W., 2011. Bursaphelenchus mucronatus kolymensis comb. n.-new definition of the 'European type' of B. mucronatus. Journal of Nematode Morphology and Systematics, 14(2):77-90.

Braasch, H., Schönfeld, U., 2015. Improved morphological key to the species of the xylophilus group of the genus Bursaphelenchus Fuchs, 1937. Bulletin OEPP/EPPO Bulletin, 45(1):73-80. DOI: 10.1111/epp.12174

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