Xylosandrus germanus (black timber bark beetle)

X. germanus is a strongly invasive species. It is capable of flight over distances of at least 2 km (Grégoire et al., 2003) and appears to be able to spread over much longer distances. Initial rates of spread in the USA and Europe suggest several tens of kilometres per year (Henin and Versteirt, 2004). It can also be spread long distances as the result of the transport of infested wood by humans (LaBonte et al., 2005). The mating system, involving sib-mating in the parental gallery, followed by dispersal of the mated females means that a single female is theoretically capable of establishing a new infestation. Attacks occasionally occur on apparently healthy trees (Hoffmann, 1941; Weber and McPherson, 1984a; Katovich, 2004), although it is usually a secondary species attacking trees that are stressed, dying or recently killed, and cut logs. The impact of X. germanus can often be severe, and in Europe, it has become one of the dominant species in forests. In Germany, Haase et al. (1998) found that it was the most common species in dead oak (Quercus petraea) wood in a beech (Fagus sylvatica) forest; Büssler and Müller (2004) reported it as one of the dominant species in oak forests; and Zach et al. (2001) the predominant species on spruce (Picea abies) logs. Wulf and Pehl (2005) note the increasing importance of X. germanus in German forests. In Switzerland, Graf and Manser (1996, 2000) considered X. germanus an important pest of stored spruce and pine (Pinus sylvestris) logs. In Belgium, Grégoire et al. (2001) note the large numbers of X. germanus attacking beech trees, and in France, Bouget and Noblecourt (2005) found it to be the predominant species in oak forests. Thus, within about 50 years following its introduction to Europe, X. germanus has become one of the dominant scolytids in many European forests, on both deciduous and coniferous trees. In the USA, X. germanus has become an important pest of black walnut (Juglans nigra) (Weber, 1979; Weber and McPherson, 1983b, 1984a, 1985) and chestnut (Castanea mollissima) (Oliver and Mannion, 2001).

The female of the species was described from Japan by Blandford (1894) in the genus Xyleborus Eichhoff. It was transferred to Xylosandrus by Hoffmann (1941), but has often been referred to in later literature as Xyleborus germanus. The male was described by Eggers (1926). The only synonym, Xyleborus orbatus Blandford (1894), was recognized as the male of the species by Nobuchi (1981). Further references to the species are given by Wood and Bright (1992), and Bright and Skidmore (1997, 2002). It should be noted that the Scolytidae are now usually treated not as a separate family, but as a subfamily (Scolytinae) of Curculionidae (e.g. Kuschel, 1995; Lawrence and Newton, 1995; Marvaldi et al., 2002).

Small, black beetles, antennae geniculate with an obliquely truncate club, pronotum rounded, elytra about a half longer than the pronotum and with a broadly convex declivity. Adult female: 2.0-2.3 mm long, 2.3 times as long as wide. Frons broadly convex, minutely reticulate, sparsely punctured, the punctures with fine, moderately long hairs. Pronotum subcircular, as long as wide, anterior margin with 8-10 low asperities, summit slightly behind middle, anterior slope coarsely asperate, posterior areas smooth, with a few minute punctures, a tuft of fine hairs at the median basal margin, remaining vestiture sparse. Scutellum large, flat, filling sutural notch, flush with surface. Elytra 1.3 times longer than wide, 1.4 times as long as pronotum, shining, sides almost straight and parallel on basal three-fourths, broadly rounded behind; striae not impressed, punctures small, rather shallow, without setae; interstriae smooth, shining, punctures uniseriate, more widely spaced, granulate; declivity commencing slightly behind middle, rather steep, broadly convex, the ventrolateral margin acutely elevated from apex to interstriae 7, interstrial setae longer than on disc.Adult male: Males are rare, and very occasionally found outside the gallery system. Generally resembling female, but much smaller and more weakly sclerotised, 1.3-1.8 mm long, 2.0 times as wide as long. Frons shining, with weak, scattered punctures, median longitudinal line weakly elevated. Pronotum broadly rounded anteriorly, lacking asperities on the margin, anterior slope with numerous small asperities, shining posteriorly and impunctate. Elytra 1.5 times as long as wide, striae and interstriae seriate-punctate, the punctures larger on the disc than at the sides; declivity less convex, impressed along apico-lateral margins. Egg: White, translucent, shiny, ellipsoidal, about 0.67 mm long and 0.38 mm wide (Hoffmann, 1941).Larva: The larva is described by Weber (1982; see Weber and McPherson, 1982). There is a photograph of a larva in Hoffmann (1941). There are three larval instars (Weber and McPherson, 1983c).Pupa: The pupa has not been described. There is a photograph of a pupa in Hoffmann (1941).

Unlike many species in the tribe Xyleborini (Xylosandrus and related genera), the distribution of X. germanus lies outside the tropical zone. (There is a single record from Vietnam (Browne, 1968.) Its distribution is confined to more temperate climates, both in its native habitats, and in areas where it has been introduced.

X. germanus is native to East Asia, from the Kuril Islands to Vietnam, but has been accidentally introduced to North America, Europe and the Caucasus region (Mandelshtam, 2001). In the USA, its spread can be followed in the literature. It was first detected in the USA in New York in 1932 (Felt, 1932). Hoffmann (1941) recorded the species from three further states: New Jersey, Ohio and West Virginia. It was present in North Carolina in 1963 (Schneider and Farrier, 1969), in Missouri in 1968 (USDA, 1968) and in Maryland prior to 1971 (Staines, 1984). It was first recorded from Virginia in 1971 (USDA, 1972), Indiana in 1975 (USDA, 1975) and Louisiana in 1978 (USDA, 1978). Weber and McPherson (1982) add seven further states in the eastern half of the USA. In many of these areas, it is regarded as a pest species. More recently, it has been found for the first time (1999) in the western half of the USA in Oregon (LaBonte et al., 2005) and in 2004 in Maine (Crowe, 2005). In southern Canada, it has been recorded in Ontario (Wood and Bright, 1992) and more recently in Quebec (Bright and Skidmore 2002). The first record in Europe is from Germany (Groschke, 1952), but it has also spread to neighbouring countries. It is recorded from France (Wood and Bright, 1992), and for the first time in Switzerland in 1986 (Maksymov, 1987), Austria in 1992 (Holzschuh, 1993), Belgium in 1994 (Bruge, 1995) and Italy in 1998 (Stergulc et al., 1999; Faccoli, 2000). In the Caucasus region of Russia, it is believed to have been introduced from China in 1939-1941 (Mandelshtam, 2001). In almost all these countries, it has attained pest status, at least locally. It can be expected to spread further on both North American and Eurasian continents where conditions are suitable.

X. germanus should be considered a high-risk quarantine pest. In the tribe Xyleborini (Xylosandrus plus related genera), sib-mating occurs in the gallery system before the new generation of mated females emerges. Thus the introduction of only a few individuals (females) may lead to the establishment of an active population if suitable host plants can be found and environmental conditions are satisfactory. Specific host plants may not be a limiting factor because the adult beetle does not actually feed on the plant material, but uses it as a medium for growing the fungus which is the larval food. Any woody material of suitable moisture content and density may be all that is required. A very wide range of host plants have been recorded for many species of Xylosandrus, including X. germanus. The risk of establishment of species of Xylosandrus should be considered very high.

Natural dispersalThe male adults of X. germanus are flightless, but the females can disperse by flight over relatively long distances. Grégoire et al. (2003) suggest that adults can fly at least 2 km. Longer distances may be covered by a few beetles, especially if they are caught by wind currents. In the USA, X. germanus spread at a rate of several tens of kilometres per year, and its initial rate of spread in Europe seems to have been similar (Henin and Versteirt, 2004). Movement in tradeLong distance spread may also be the result of human transport of infested wood. This is the most likely route by which X. germanus has become established in Europe and North America. LaBonte et al. (2005) suggest that the recent spread of X. germanus from the eastern states of the USA, to Oregon in the West, is probably due to the intracontinental movement of untreated domestic solid wood packing material and other raw timber.Vector TransmissionIt was shown many years ago (Buchanan, 1940, 1941) that X. germanus can transmit the Dutch elm disease fungus (Ophiostoma ulmi), but it is not an important vector (Carter, 1973). It is more often associated with Fusarium spp., which cause dieback, sprouting and stem cankers on affected trees. Such associations have been noted in walnut (Juglans nigra, J. regia) (Kessler, 1974; Weber and McPherson, 1984b, 1985; Stergulc et al., 1999; Faccoli, 2000); sycamore (Acer pseudoplatanus) (Gauss, 1960); and tulip poplar (Liriodendron tulipifera) (Anderson and Hoffard, 1978; Weber, 1980). It is likely that in some cases, spores of Fusarium spp. are carried on the cuticle of the dispersing adult beetles. The ambrosia fungus of the beetle, Ambrosiella hartigii (Batra, 1967), is not pathogenic, although it does cause staining of the wood around the galleries.

Like many other related ambrosia beetles, X. germanus attacks a very wide range of host plants, including both deciduous and coniferous trees. Weber and McPherson (1983a) list over 200 host species belonging to 51 plant families. X. germanus is not strongly size-selective, and will breed both in small branches and in large logs and stumps, although there may be some preference for stems of less than 10 cm diameter (Henin and Versteirt, 2004). It is clear that it will attack almost any woody plant stem which is in a suitable condition. Given the great range of host trees attacked, and the differences between geographical areas, it is not possible to distinguish 'main host' trees from 'other host' trees. It may be expected that almost any crop, plantation or ornamental tree in a particular area can be attacked. The list of hosts is only a small selection.

In Europe, X. germanus might be confused with Xyleborus (Anisandrus) dispar but is smaller (2.0-2.3 mm relative to 3.2-3.6 mm long), the elytra are not strongly punctured, and the anterior coxae are widely separated not contiguous. In North America (USA), X. germanus might be confused with Xylosandrus crassiusculus or Xylosandrus compactus. X. crassiusculus can be distinguished by the elytral declivity, which is matt, not shiny, lacks striae and is densely covered by minute granules. X. compactus is smaller (1.7 mm or less), the declivital striae are not impressed, and clearly bear setae which are about one-third as long as the interstrial setae.

X. germanus can be found in temperate zone deciduous and coniferous forests, both in natural forests and in plantations (e.g. Nobuchi, 1981; Weber and McPherson, 1984; Grégoire et al., 2001; Iwai et al., 2001; Büssler and Müller, 2004; Bouget and Noblecourt, 2005). It also occurs in orchards, vineyards and tree nurseries (e.g. Yoon et al., 1982; Osumi and Mizuno, 1992; Oliver and Mannion, 2001; Boll et al., 2005).

Life Cycle

The important pest species in the genus Xylosandrus and the related genera Euwallacea, Xyleborinus and Xyleborus are all ambrosia beetles in the Xyleborini, a tribe with a social organization of extreme polygamy. The sexual dimorphism is strongly developed, and the ratio of females to males is high. All are closely associated with symbiotic ambrosia fungi, which are transported by the female, and form the sole food for both adult and larvae.

Studies of the biology and life cycle of X. germanus have been made by Kaneko et al. (1965), Kaneko et al. (1965) and Kaneko and Takagi (1966) in Japan, Hoffmann (1941), Schneider and Farrier (1969), Weber and McPherson (1983c), and Oliver and Mannion (2001) in the USA, and in Europe by Groschke (1953), Gauss (1960), Heidenreich (1960, 1964) and Peer and Taborsky (2004, 2005).

The species is not strongly size-selective, and breeds both in small branches and in large logs and stumps. There may be some preference for stems of less than 10 cm diameter (Henin and Versteirt, 2004). In Japan, X. germanus also attacks the roots of tea (Kaneko et al., 1965). In small stems, an entrance tunnel cut into the pith or wood is extended into a longitudinal tunnel or an irregular chamber. In larger stems, the gallery may branch once or twice in the transverse plane, with a brood chamber in the longitudinal plane, but not penetrating far into the wood. The female feeds on the ambrosia fungus, Ambrosiella hartigii, which she has introduced into the gallery system before oviposition begins. The eggs are laid loosely in the gallery over some days, and the larvae feed on the ambrosia fungus on the walls of the gallery.

The size of the brood varies considerably. Broods from 1 to 54 individuals have been found, with an average of about 16 (Kaneko and Takagi, 1966; Weber and McPherson, 1983c). Pupation and mating of brood adults occurs within the gallery system, the (usually) single male in each gallery mating with his sisters. The new generation of females emerges through the entrance hole made by the parent. It is usually considered that the males of xyleborine ambrosia beetles do not emerge from the gallery system (e.g. Kirkendall, 1993), but Peer and Taborsky (2004) have shown that some males of X. germanus do disperse locally (by walking because they are flightless) to seek additional matings. Total development time from egg to adult is about 25 days at 24°C in the laboratory (Weber and McPherson, 1983c), but in the field in the temperate zone summer, about 55-60 days is required from gallery initiation to emergence of a new generation (Oliver and Mannion, 2001).

Genetics

X. germanus, like other xyleborines (Jordal et al., 2000), has diploid females and haploid males (Takagi and Kaneko, 1966). The diploid number of chromsomes is 16, the haploid number 8 (Takagi and Kaneko, 1966). Unmated females produce only haploid male offspring; mated females lay eggs with a sex ratio of about 9 females: 1 male, and produce adults with the same sex ratio (Takagi and Kaneko, 1966).

Phenology

The number of generations per year depends primarily on environmental temperatures. In its native range in Japan, there are one or two generations per year (Kaneko et al., 1965), in central Europe one generation (Bruge, 1995; Henin and Versteirt, 2004), but in Italy usually two (Faccoli, 2000), and in the USA (North Carolina to Illinois) two generations per year (Weber and McPherson, 1983). The optimum range of temperature for development is 21-23°C (Kaneko et al., 1965). The flight period of the adults is usually between April and August, but may extend into March and September (Kaneko and Takagi, 1966; Weber and McPherson, 1991). Adults overwinter in the host plants, often clustering in galleries (Hoffmann, 1941; Kaneko and Takagi, 1966; Weber and McPherson, 1983).

Environmental Requirements

The biotic and abiotic factors that could affect the distribution of the species are discussed by Henin and Versteirt (2004), and it is concluded that climatic conditions, particularly winter temperatures, play a crucial role - at least in limiting its northward spread. The reasons for the absence of the species from more tropical regions are unknown.

Associations

X. germanus is often found together with other species of scolytid ambrosia beetles in the same trees, e.g. together with Xylosandrus compactus on tea (Kaneko and Takagi, 1966); Xylosandrus crassiusculus on black walnut (Oliver and Mannion, 2001); Ambrosiodmus apicalis on apple (National Horticultural Research Institute, 2002); Xyleborus (Anisandrus) dispar on grapevine (Boll et al., 2005). The distributions of the species on the host tree may differ, for example, in Japan, X. germanus attacks mainly the roots of tea, X. compactus the stems (Kaneko and Takagi, 1966). These additional attacks add to the detrimental effect on the host plants.

No natural enemies appear to have been specifically recorded from X. germanus. Gauss (1960) recorded the parasitoid Tetrastichus sp. from galleries of X. germanus in Germany, but provided no evidence of its relationship to the species. Weber and McPherson (1983c) record one instance of predation by a hemipteran, possibly a reduviid. Fungivorous mites have also been recorded from the galleries (Gauss, 1960; Weber and McPherson, 1983c). A recent, thorough survey of the parasitoids and predators of European scolytids (Kenis et al., 2004) found no records of parasites or predators on X. germanus. In general, the immature stages of ambrosia beetles have few natural enemies. The female parent normally remains in the gallery entrance while the immature stages are developing, preventing the entry of potential predators and parasitoids. Adults will fail to oviposit if the ambrosia fungus fails to establish in the gallery. Provided that the female remains alive and the growth of the ambrosia fungus on which the larvae feed is satisfactory, mortality of the immature stages is likely to be very low. Adults of ambrosia beetles are likely to be predated by lizards, clerid beetles and ants as they attempt to bore into the host tree. Most mortality is probably during the dispersal of the adults and during gallery establishment.

The economic impact of X. germanus results both from its attacks on trees per se, and through its association with its ambrosia fungus, Ambrosiella hartigii, and with pathogenic fungi, primarily Fusarium spp. The tunnels, although not usually penetrating far into the wood, are accompanied by staining of the wood caused by the ambrosia fungus. This can cause degradation of affected timber. In Switzerland, Graf and Manser (1996, 2000) consider X. germanus an important pest of stored spruce and pine (Pinus sylvestris) logs. The tunnels also provide sites where pathogenic fungi can become established (Kessler, 1974; Weber, 1979, 1980; Katovich, 2004). These fungi may cause dieback, sprouting and stem cankers (e.g. Weber and McPherson, 1985; Stergulc et al., 1999; Faccoli, 2000) and may eventually result in the death of affected trees (Anderson and Hoffard, 1978; Weber, 1980). Attacks sometimes occur on healthy trees, especially walnut trees in plantations (Katovich, 2004). However, Weber (1984) suggests that the long-term effects of attacks by X. germanus on black walnut are minimal both biologically and economically. In China and Japan, X. germanus can be locally important as a pest of tea, attacking especially the roots of living plants (Kaneko and Takagi, 1966; Nobuchi, 1981).

Visual inspection of suspected infested material is required to detect the presence of ambrosia beetles. Infestations are most easily detected by the presence of entry holes made by the attacking beetles, and the presence of frass produced during gallery construction. In X. germanus, the frass is pushed out in the form of a compact cylinder, which may reach a length of 3-4 cm before it breaks off, and forms a useful recognition character for the presence of attacks by this species or the related species, Xylosandrus crassiusculus. Ethanol-baited traps have been used to detect and monitor the presence of X. germanus in Europe, Asia and North America (Klimetzek et al., 1986; Grégoire et al., 2001; Oliver and Mannion, 2001; National Horticultural Research Institute, 2002; Boll et al., 2004, 2005). Both Oliver and Mannion (2001) and Boll et al. (2005) note that the traps are significantly less attractive to X. germanus than to other ambrosia beetles, and are only suited to monitor the presence of the species, not its abundance. Many types of traps are available, both commercial and non-commercial. One simple, cheap design is described by Bambara et al. (2002). Oliver et al. (2004) compare the efficiency of ambrosia beetle collection, and problems associated with the use of a number of different types of trap.

Cultural Control

The removal of infested trees, branches and logs, and their destruction can help to reduce the level of attacks, at least locally. Weber and McPherson (1984a) showed that black walnut (Juglans nigra) trees from different provenances were not equally susceptible to attack. Selection of resistant strains may help to reduce the level of attacks.

Biological Control

Given the apparent absence of pathogens, parasitoids and predators, biological control measures are unlikely to be effective.

Chemical Control

Some studies have been carried out in Japan on the susceptibility of both adults and immature stages of X. germanus to fumigation with various chemicals (methyl iodide, methyl isocyanate, sulfuryl fluoride) (Mizobuti et al., 1996; Soma et al., 1997; Naito et al., 1999). The results suggest that methyl iodide has high potential as a fumigant of imported logs. Attempts to control X. germanus and related species of Xylosandrus using insecticides have had rather limited success (Kaneko, 1967; Hudson and Mizell, 1999). Ambrosia beetles are difficult to control with insecticides because the host tree forms a barrier between the insecticide and the beetle. To be effective, insecticides must either be closely timed with beetle attacks, be applied repeatedly, or have long residual activity (Oliver and Mannion, 2001).

Physical Control

A study in Japan of the effects of irradiation on X. germanus in cut timber (Yoshida et al., 1975) indicated that treatment could prevent progeny surviving to the adult stage, and also further boring damage.

Monitoring

Oliver and Mannion (2001) have pointed out that the catch of X. germanus in ethanol-baited traps may not reflect the actual abundance and frequency of attacks on trees in the neighbourhood. Nevertheless, such traps can be used as an indicator of attacks, as with the related species, Xylosandrus crassiusculus (Mizzell et al., 1998; Oliver et al., 2005). It has been shown that the response of the beetles increases with the concentration of ethanol (Klimetzek et al., 1986; National Horticultural Research Institute, 2002), so the concentration needs to be kept high for effective monitoring.

IPM Programmes

No detailed IPM programmes have been developed for X. germanus. However, general recommendations (Katovich, 2004) would include monitoring for the presence of the pest, and reducing stress on recently-planted, nursery, or plantation trees. Heavily attacked branches or trees should be removed and destroyed to prevent infestations of nearby stressed trees. Insecticides appropriately labelled as bark treatments may be employed against new attacks, but systemic insecticides are not effective.