C. fagisuga is regarded as a highly invasive species, especially in North America, where its potential area of spread was analyzed by Morin et al. (2004)...
C. fagisuga is regarded as a highly invasive species, especially in North America, where its potential area of spread was analyzed by Morin et al. (2004).
The specific name fagi was replaced by fagisuga because of homonymy. The species has been placed in various combinations, and the genus Cryptococcus was placed in a separate family, the Cryptococcidae, by Kosztarab (1968). However, the genus Cryptococcus is currently regarded as a member of the Eriococcidae (Miller and Gimpel, 2000).
All three developmental stages of beech scale secrete a covering of felted white wax filaments (Toronto Parks and Recreation, 2001). First-instar larvae (crawlers) are yellow, about 0.1-0.2 mm long, and possess legs and antennae; the second-instar and adult female lack legs and have antennae reduced to short stubs. The adult female is 0.5-1.0 mm long, yellow, with an ovoid to spherical body. Yellow eggs (each elongate-oval, about 0.1-0.15 mm long) are laid within the felted wax covering of the ovisac (CABI Bioscience, 2004). Males are unknown.
C. fagisuga is an Old World temperate species that may have originated in Europe or western Asia; however, its area of origin has not yet been located (CABI Bioscience, 2004). It was accidentally introduced to North America via Nova Scotia in 1890 (Ehrlich, 1934), and has spread steadily westward and southward through the natural forests of Canada and the United States (Houston, 1996). It reached Quebec, Maine and Massachusetts in the early 1930s (Houston, 1975); New England and New York in about 1960 (McCulloch, 2000); Pennsylvania in about 1975 (McCulloch, 2000); West Virginia in about 1980 (Houston and O'Brien, 1983); Michigan around 1990 (O'Brien et al., 2001); and North Carolina and Tennessee in 1993 (Kennard, 2001).
The potential area of spread of C. fagisuga in eastern North America was analyzed by Morin et al. (2004). The scale insect has not been recorded from South America, Africa, Australia or the Pacific islands.
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.
Dispersal of sessile adults and eggs over long distances occurs through human transport of infested planting material of Fagus species. C. fagisuga is regarded as a highly invasive species. It appears as a regulated pest on New Zealand's unwanted organisms register (MAF, 2004).
C. fagisuga feeds in cracks and under flakes of the bark of its host. It is therefore not found on young saplings, and is most commonly found on trees 11-31 cm d.b.h, whose bark is sufficiently thick to contain cracks (Fernandez and Boyer, 1988; Lunderstadt, 1998).
C. fagisuga is host-specific to trees belonging the genus Fagus. In Europe the host is the European beech, Fagus sylvatica, which is replaced by F. orientalis in the Balkans and western Asia. In North America the host is the American beech, F. grandifolia (CABI Bioscience, 2004). There is a single record of it occurring on Pinus sylvestris in Hungary (Kosztarab and Kozár, 1988) but the identity of this sample is uncertain.
Initial infestation by C. fagisuga appears as small patches of felted, white wax filaments in cracks in the bark and under bark flakes on the trunk. Heavy infestations appear as coatings of felted, white wax filaments on the rough bark of the trunk, branches and exposed roots (Kosztarab and Kozár, 1988; McCulloch, 2000).
If the scale insect infestation leads to infection of the tree by Nectria fungus, the tree develops beech bark disease, with cankers beneath the bark on the trunk and large branches. The scale insects fall off after the fungus kills the underlying tissues (Toronto Parks and Recreation, 2001). The dead bark may crack further and brown slime flux may form tarry spots on the outer surface. Leaves on dying trees do not reach their full size; they turn yellow and later brown, remaining on the tree through autumn. Branches die back (Kosztarab, 1996), and on some infected trees, may break off in high winds - a condition known as "beech snap" (McCulloch, 2000). The fungus may produce small, reddish fruiting bodies on the surface of the bark in autumn (ODNR, 2004).
C. fagisuga is a parthenogenetic, oviparous species (Kosztarab, 1996); males have never been found (Miller and Miller, 1993). Each female matures in May or June and lays 6-60 eggs in a felted wax ovisac, between late June and early August. The eggs hatch after 4-6 weeks (Kosztarab and Kozár, 1988). There is one generation per year, and winter is spent as first-instar larvae hidden in cracks in the bark (Miller and Gimpel, 2000), or occasionally as eggs; second-instars are found only during a short period in spring (Kosztarab, 1996).
The scale insects feed on parenchyma tissue of the bark (Dubeler et al., 1997). In France, Malphettes (1977) found that infestation was greater on the east-facing side of the trunk and higher up the tree, due to the insects being washed off the side of the tree by rain carried in the prevailing wind.
In Europe, the nutrient quality of the host tree has been found to be an important factor affecting population fluctuations of C. fagisuga (Petercord, 2000). Air pollution and certain soil nutrient levels may make beech trees more susceptible to bark lesions due in part to frost damage and subsequent attack by C. fagisuga; algae cover (an indicator of high nitrogen) and beech scale occurrence were also found to be positively correlated (Jönsson, 1996, 1998).
Some lichens with loose, straggly growth on beech bark form spatial habitats that favour the establishment of C. fagisuga. In North America, these growths can help the scale colonize trees that would otherwise be too small to be at risk of attack (Houston, 1996).
Periodic population declines in North America suggest that some abiotic or biotic factor may help control beech scale outbreaks, but the cause(s) are not known (Houston, 1996).
The only extensive exploration for natural enemies of beech scale was done in Germany and northern France, where only generalist predators were found; these areas may not even fall within the true centre of origin of C. fagisuga (CABI Bioscience, 2004). Of the natural enemies recorded attacking C. fagisuga, none of those found so far are sufficiently efficient to significantly reduce the scale population. The role of predators in controlling beech scale remains unclear (NRC, 2004). The most effective predator of C. fagisuga recorded in North America is Chilocorus stigma, but while these coccinellids may have a significant impact on scale insects on individual trees, their overall effectiveness is limited (Houston, 1996).
Periodic population declines in North America suggest that some abiotic or biotic factor may help control beech scale outbreaks, but the cause(s) are not known. Similar declines in other forest insect populations have been associated with attacks by microbial pathogens (Houston, 1996). In England, the entomogenous fungus Verticillium lecanii was common where heavy beech scale infestations occurred; it was absent from light beech scale infestations because it spreads from one colony to another by hyphal growth rather than by aerially dispersed spores (Lonsdale, 1983a).
The first-instar crawlers are the only active dispersal stage; their dispersal is influenced by precipitation, temperature and light (Augustin, 1986). They may walk up to a few metres to new areas of the host tree, or from one tree to another if the branches are touching. Mortality due to abiotic factors is high during crawler dispersal.
Vector transmission
Crawlers are dispersed over longer distances by wind (Augustin, 1986; Kosztarab, 1996; Toronto Parks and Recreation, 2001) and therefore disperse mainly down the dominant wind direction. Dispersal of sessile adults and eggs occurs through human transport of infested plant material.
Movement in trade
Dispersal of sessile adults and eggs of scale insects over long distances occurs through human transport of infested plant material (Greathead, 1990).
C. fagisuga is the first of two causal agents of beech bark disease, a devastating disease of beech trees (Houston, 1994a; Toronto Parks and Recreation, 2001). It feeds on parenchyma cell contents in the bark (Dubeler et al., 1997), using long, fine mouthparts (stylets). Attack by beech scale on its own does not seriously damage beech trees; however, the feeding punctures it makes in the bark enable access to the cambium and phloem tissues by one or more species of Nectria fungus, the second causative agent of the complex (Houston, 1994a; Kosztarab, 1996). Beech bark disease is caused by both agents acting together; neither agent can produce beech bark disease symptoms on its own (Perrin, 1977).
Nectria spores are carried by the wind (McCulloch, 2000) but these are unable to infect beech trees unless there is a wound present in the bark (Kunca and Leontovic, 2000). It is the fungus that is responsible for the serious damage to beech trees associated with beech bark disease. Nectria fungus kills the cambial tissues, and blocks the tree's vascular tissues, sometimes ring-barking the tree (Kosztarab, 1996), with cumulative effects on tree vigour (Gavin and Peart, 1993). In North America, mortality may occur 2 to 5 years after initial scale infestation (Stimmel, 1993). In Europe, N. coccinea var. fagiata is the main native fungus involved (although N. ditissima also occurs in Germany (Lunderstadt, 2002)); in North America, the native N. galligena and the exotic N. coccinea var. fagiata are both present.
Beech bark disease has become a serious threat to the timber industry in Europe and in North America (Perrin, 1977). In Europe, C. fagisuga is an occasional pest (Kosztarab and Kozár, 1988) because the native beech trees have co-evolved with the scale, so some are resistant to it. In Normandy, France, Malphettes and Perrin (1974) estimated that beech bark disease was responsible for the loss of 25% of beech timber. In Europe, Malphettes (1977) recommended that beech trees heavily infested with C. fagisuga should be felled and removed without delay because infested timber quickly loses its value.
In eastern North America the American beech, F. grandifolia, did not co-evolve with beech scale and the level of resistance to beech bark disease is much lower than in Europe; resultant mortality frequently approaches 90-100% in individual stands (Morris et al., 2002). It is anticipated that perhaps 50% of large American beech trees in North America will be killed by beech bark disease over the next 20 years; a further 25% will be damaged by the disease, while the remaining 25% should escape scale infestation or Nectria infection during the first wave of the disease (McCulloch, 2000). Evidence suggests that N. galligena will eventually be replaced by the exotic N. coccinea var. faginata as the dominant pathogen in North America (Houston, 1994b).
In aftermath forests (after the first wave of infection has passed), the causal agents of beech bark disease are established on root sprouts and seedlings, which often develop into dense stands. Most of the new and surviving trees become cankered and highly defective (Houston, 1996). Mortality levels can be influenced by the species composition of the forest, with higher levels of mortality being recorded in stands dominated by Tsuga canadensis (Twery and Patterson, 1984).
A sawmill study of beech bark disease-affected lumber in Vermont, USA, showed that most bark defects were removed with the slab and that, where the lumber was affected, the defects reduced grade rather than volume. When the cambium was damaged, however, defects led to losses in lumber yield or quality (Burns and Houston, 1987). In Canada, losses due to beach bark disease varied from region to region; in 1969 1.4 million m³ of timber were lost in the Maritime Provinces (Lavallee, 1976).
C. fagisuga is the first of two causal agents of beech bark disease, a devastating disease of beech trees (Houston, 1994a; Toronto Parks and Recreation, 2001). It feeds on parenchyma cell contents in the bark (Dubeler et al., 1997), using long, fine mouthparts (stylets). Attack by beech scale on its own does not seriously damage beech trees; however, the feeding punctures it makes in the bark enable access to the cambium and phloem tissues by one or more species of Nectria fungus, the second causative agent of the complex (Houston, 1994a; Kosztarab, 1996). Beech bark disease is caused by both agents acting together; neither agent can produce beech bark disease symptoms on its own (Perrin, 1977).
Nectria spores are carried by the wind (McCulloch, 2000) but these are unable to infect beech trees unless there is a wound present in the bark (Kunca and Leontovic, 2000). It is the fungus that is responsible for the serious damage to beech trees associated with beech bark disease. Nectria fungus kills the cambial tissues, and blocks the tree's vascular tissues, sometimes ring-barking the tree (Kosztarab, 1996), with cumulative effects on tree vigour (Gavin and Peart, 1993). In North America, mortality may occur 2 to 5 years after initial scale infestation (Stimmel, 1993). In Europe, N. coccinea var. fagiata is the main native fungus involved (although N. ditissima also occurs in Germany (Lunderstadt, 2002)); in North America, the native N. galligena and the exotic N. coccinea var. fagiata are both present.
Beech bark disease has become a serious threat to the timber industry in Europe and in North America (Perrin, 1977). In Europe, C. fagisuga is an occasional pest (Kosztarab and Kozár, 1988) because the native beech trees have co-evolved with the scale, so some are resistant to it. In Normandy, France, Malphettes and Perrin (1974) estimated that beech bark disease was responsible for the loss of 25% of beech timber. In Europe, Malphettes (1977) recommended that beech trees heavily infested with C. fagisuga should be felled and removed without delay because infested timber quickly loses its value.
In eastern North America the American beech, F. grandifolia, did not co-evolve with beech scale and the level of resistance to beech bark disease is much lower than in Europe; resultant mortality frequently approaches 90-100% in individual stands (Morris et al., 2002). It is anticipated that perhaps 50% of large American beech trees in North America will be killed by beech bark disease over the next 20 years; a further 25% will be damaged by the disease, while the remaining 25% should escape scale infestation or Nectria infection during the first wave of the disease (McCulloch, 2000). Evidence suggests that N. galligena will eventually be replaced by the exotic N. coccinea var. faginata as the dominant pathogen in North America (Houston, 1994b).
In aftermath forests (after the first wave of infection has passed), the causal agents of beech bark disease are established on root sprouts and seedlings, which often develop into dense stands. Most of the new and surviving trees become cankered and highly defective (Houston, 1996). Mortality levels can be influenced by the species composition of the forest, with higher levels of mortality being recorded in stands dominated by Tsuga canadensis (Twery and Patterson, 1984).
A sawmill study of beech bark disease-affected lumber in Vermont, USA, showed that most bark defects were removed with the slab and that, where the lumber was affected, the defects reduced grade rather than volume. When the cambium was damaged, however, defects led to losses in lumber yield or quality (Burns and Houston, 1987). In Canada, losses due to beach bark disease varied from region to region; in 1969 1.4 million m³ of timber were lost in the Maritime Provinces (Lavallee, 1976).
In northeastern USA forests, canopy gaps are the main mode of forest disturbance and thus regulate future forest structure and species composition (di Gregorio et al., 1999). In stands of American beech in North America, mortality due to beech bark disease frequently approaches 90-100% (Morris et al., 2002); beech bark disease is therefore creating conditions likely to result in a change in species composition of affected forests.
In beech bark disease-affected hardwood forests in Maine, a change in forest composition to dominance by American beech, Fagus grandifolia, is apparently in progress (Chokkalingham and White, 2001). In Allagaheny hardwood forests in central New York, USA, the proportion of canopy gaps made by falling, dead American beech trees changed from 53% in 1990 to 76% in 1996, whereas the proportion of gaps caused by oak deaths did not change in this period; beech was predominant in the subcanopy and regeneration layers, with implications for the succession (Krasny and di Gregorio, 2001). In contrast, di Gregorio et al. (1999) found that beech bark disease-induced canopy decline in Allegheny northern broad-leaved forest stands in central New York appeared to be promoting the growth of gap subcanopy sugar maples, Acer saccharum, which exhibited a radial growth rate 30% higher than before the disturbance; however, maple trees not adjacent to gaps showed no change in growth rates.
The changing balance of dominant tree species in the broadleaf hardwood forests of eastern North America is likely to alter the ecological balance of the forests and hence their native floral and faunal species composition.
Reduction in beech timber yield, and income due to poor timber quality, may impact employment and industry in some areas of the USA. Infected and damaged amenity trees are unsightly and potentially dangerous and may have to be felled, changing the appearance of parks, streets and possibly botanic gardens.
Microscopic examination of slide-mounted adult females is required for authoritative identification to species. Kosztarab and Hale (1968) provided an identification key to all the described species of Cryptococcus. A suitable method of preparing microscope slide mounts of Cryptococcus is described by Watson and Chandler (2000).
Closely examine cracks and rough patches of bark on the trunk of species of Fagus; good light and a hand lens may be necessary to find scattered individuals, each the size of a pinhead and covered by a small patch of white wax fibres. In heavy infestations, patches of white felted wax filaments may occur on the surface of the bark (CABI Bioscience, 2004).
Microscopic examination of slide-mounted adult females is required for authoritative identification of scale insects to species level. C. fagisuga is the only member of the genus known to feed on Fagaceae; a total of six species of Cryptococcus have been described, but the other five feed on trees belonging to other plant families (Miller and Gimpel, 2000).
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.
Cultural control
Unfavourable site conditions such as shallow soil, steep slopes and south to east exposures were found to have some influence on the occurrence of beech bark disease in Slovakia (Kunca et al., 2000).
Regular thinning measures have been found to be very important in the control of beech bark disease in Germany (Lunderstadt, 2002). Removal of susceptible trees heavily infested by beech scale reduces the spread of beech bark disease considerably, especially if done in the early stages of the outbreak. Heavily infested trees should be cut in winter, when the crawlers are inactive, to avoid spreading the infestation further (NRC, 2004).
Chemical control
Control of C. fagisuga in forests by chemical sprays is impractical (NRC, 2004). Ornamental trees can be protected by spraying the bark with insecticide (NRC, 2004).
Ornamental trees can be protected from beech scale infestation by spraying the bark with insecticide, or by scrubbing the bark and branches with a strong solution of kitchen detergent in September or early spring, before the end of April. At that time, application of a lime sulphur spray is beneficial. If an infestation is noticed after mid-summer, spraying the bark with a registered insecticide is suggested; however, the foliage should not be wetted with strong sprays (NRC, 2004).
Host-plant resistance
In Central Europe, C. fagisuga is only an occasional pest (Kosztarab and Kozár, 1988) because the native beech trees have co-evolved with the scale, so some are resistant to it. Partial resistance to C. fagisuga is associated with the possession of smooth bark with a regular vertical pattern on its surface (Lonsdale, 1983b); this is a congenital condition (Malphettes, 1977). Resistant trees respond to feeding by the scale insect by multiple processes in which nutrient availability to the scale insects is reduced by concentration shifts and by the formation of inhibiting compounds (Krabel and Petercord, 2000). The chemical composition of beech bark tissues appears to affect scale population density (Suvak, 1999); in Germany, the most strongly infested beeches were found to have the highest concentrations of phenolic compounds and procynidin in their bark, with a transfer of compounds between the inner and outer bark - a defense reaction by the tree (Gora et al., 1996; Dubeler et al., 1997). In European beech in Germany, different isoenzyme genotypes were identified (involving 11 gene loci) in which high heterozygosity was associated with high levels of infestation by beech scale (Gora et al., 1994). Amino acids were also found to be shifted from the outer to the inner bark in response to scale feeding (Lunderstadt and Borgel, 1994).
In North America, about 1% of American beech trees appear to have natural resistance to C. fagisuga attack (McCulloch, 2000), so should be used in breeding programmes to increase resistance in American beech populations (Kennard, 2001). Resistance to scale attack offers the greatest long-term prospect for control (Houston, 1996).
Resistant trees often occur in groups, which are easier to recognize and protect in programmes against infected trees than individual trees would be. Isozyme genetic studies have shown that resistant trees originate from both root sprouts and seed (Houston, 1996). Tissue culture techniques have been used to multiply resistant genotypes but trials are still needed to find ways to transplant these trees into forests successfully (Houston, 1996).
Biological control
Control of beech bark disease is complex because of its dual organism etiology. Approaches focused on reducing the effects of the scale insect initiator hold the most promise for biological control (Houston, 1996).
No natural enemy capable of controlling populations of C. fagisuga has been identified to date, suggesting that Western Europe is not the area of origin of the scale insect. Surveys are currently under way in Central and Eastern Europe and in western Asia to locate the area of origin of beech scale, in the hope that natural enemies from that area may be suitable for use as biological control agents in North America (CABI Bioscience, 2004).
In Nova Scotia, some dense, crustose bark epiphytes (lichens) provide significant levels of protection against beech scale (Houston, 1983).
Periodic population declines in North America suggest that some abiotic or biotic factor may help control beech scale outbreaks, but the cause(s) are not known. Similar declines in other forest insect populations have been associated with attacks by microbial pathogens (Houston, 1996). In England, the entomogenous fungus Verticillium lecanii was common where heavy beech scale infestations occurred; it was absent from light Cryptococcus infestations because it spreads from one colony to another by hyphal growth rather than by aerially dispersed spores (Lonsdale, 1983a). These pathogens may provide another potential means of biological control of C. fagisuga.
The parasitic fungus Nematogonum ferrugineum has been reported to attack Nectria fungi, the causative agents of beech bark disease. High populations of this parasite sometimes occur after severe outbreaks of beech bark disease; however, more research is needed before this could be used to help control beech bark disease (Houston, 1996; Kennard, 2001).
Houston DR, 1996. Potential for biologically based control of beech bark disease in the Southern Appalachians. Southern Appalachian Biological Control Initiative Workshop, Asheville, NC, September 26-27, 1996; http://www.main.nc.us/SERAMBO/BControl/beechbark.html.
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