Ambrosia beetles are wood-degrading insects that live in nutritional symbiosis with ambrosia fungi. Typically, ambrosia beetles are considered beneficial because they accelerate the decay of dead trees, which is important for nutrient cycling in healthy forests. The redbay ambrosia beetle, Xyleborus glabratus Eichhoff and its fungal symbiont, Raffaelea sp., are new introductions into the southeastern United States. Xyleborus glabratus was first detected in 2002 and is one of the 10 ambrosia beetle species in the US (Haack 2003, 2006). The beetle transmits the causal pathogen of laurel wilt disease among plants in the Laurel family (Lauraceae), which is caused by one of its fungal symbionts, Raffaelea lauricola (Mayfield and Thomas 2006, Fraedrich et al. 2009). The X. glabratus and R. lauricola complex is considered a "very high risk" invasive disease pest complex having potential equal to that of Dutch elm disease or chestnut blight (Global Invasive Species Database 2010). Laurel wilt is a relatively new disease and much is still unknown about how it will impact the flora of North America.
Xyleborus glabratus is native to India, Japan, Myanmar, and Taiwan (Rabaglia 2008). In the US, X. glabratus was first detected in a survey trap near Port Wentworth, Georgia, in 2002 (Rabaglia 2008). In Florida, X. glabratus was first detected in 2005 at the Timucuan Ecological and Historic Preserve in northern Duval County (Mayfield and Thomas 2006). Currently, the redbay ambrosia beetle is an economically important pest in Florida, Georgia, and South Carolina. Recently, the beetle was detected in Jackson County, Mississippi (Riggins et al. 2010); Mobile County, Alabama (Alabama forestry commission 2010); and Bladen County, North Carolina (R. Trickel unpublished results). The pest continues to expand rapidly to new areas posing a threat to redbay and avocado trees in the US and in the countries of Central and South America.
The adult Xleborus glabratus is a small, elongate, cylindrical beetle about 2 mm in length. It is very similar in appearance to other ambrosia beetles (both native and exotic) already found in the U.S. The combination of its blackish coloration, nearly glabrous upper surface, V-shaped and pointed abdominal tip, and abrupt apical declivity distinguishes this species from other ambrosia beetles occurring in Florida (Mayfield and Thomas 2006). However, expert examination by a specialist is needed for positive identification and confirmation. Males are dwarfed, haploid, flightless, and rarely encountered (Rabaglia 2008).
The larva of Xyleborus glabratus is similar to other scolytid beetles. It is a white, c-shaped, legless grub with an amber-colored head capsule (Rabaglia 2008).
Currently, very little is known about the life cycle and biology of X. glabratus. However, it is presumed that its biology is similar to that of other species in the Xyleborini (Mayfield and Thomas 2006). Adult females bore into the wood just below the bark and construct galleries in the sapwood, inoculating the galleries with a fungus (Rabaglia 2008, Mayfield and Thomas 2006). Most of the life cycle, including mating, egg laying and larval development, is completed within these galleries. The adults and larvae feed on fungi and not on the wood of the damaged host plant (Rabaglia 2008). Adults are active throughout the year with peak activity in early September (Hanula et al. 2008). In Asia, the beetle has been reported to survive in temperatures ranging from -26°C to 15°C. The flight activity is greatest late afternoon or early evening and the beetles usually fly at or below 15 ft (G. Brar unpublished results).
Most native ambrosia beetles attack only dead and dying trees. However, X. glabratus initiates attacks on healthy redbay trees. The beetles drill through the bark and inoculate tree xylem with their symbiotic fungi, R. lauricola. The beetle carries the fungus in mandibular mycangia (specialized sacs above mandibles adapted for fungus transport). After becoming infected, redbays wilt within weeks to a few months. The dying tree is also colonized by numerous other ambrosia beetle species, including X. affinis, X. ferrugineus, Xyleborinus saxeseni, and Xylosandrus crassiusculus, which further inoculate the tree with their associated fungi (J. Foltz unpublished results). Beetles reproduce within their galleries and newly emerging females fly in search of new hosts. In the southeast, there could be multiple overlapping generations per year (Global Invasive Species Database 2010). The brood development occurs within 50–60 days (Hanula et al. 2008, G. Brar unpublished results).
Both haplo-diploidy and inbreeding are common in ambrosia beetles including X. glabratus. Females lay both diploid and haploid eggs. The more prevalent diploid eggs develop into females, whereas males hatch from unfertilized haploids eggs. Therefore, males are haploid and flightless clones of their mothers with 50% of her genetic material. The males spend most of their lives inside the natal gallery fertilizing their own sisters. Such sons allow single females to establish successful populations in non-native locations (Kirkendall 1983).
The host range of X. glabratus includes plants in the Dipterocarpaceae, Fagaceae, Fabaceae, and Lauraceae families in Southeast Asia (Rabaglia et al. 2006).
The complete host range of X. glabraus in the US is unknown. However, all investigated American members of the Lauraceae family have been found susceptible to the disease (Ploetz and Peña 2007). The hosts of X. glabartus and laurel wilt in the US include:
avocado, Persea americana
California bay laurel, Umbellularia californica
northern spicebush, Lindera benzoin
redbay, Persea borbonia
sassafras, Sassafras albidum
swampbay, Persea palustris
(Rabaglia et al. 2006; Smith et al. 2009).
The laurel wilt fungus has also been found on two critically endangered shrubs in Florida:
pondberry, Lindera melissifolia
pondspice, Litsea aestivalis
(Fraedrich et al. 2007).
The beetle also feeds on camphor tree, Cinnamomum camphora, which is listed as a Category I invasive species by the Florida Exotic Pest Plant Council. However, the camphor tree is not on the Federal or State noxious weed list (Florida Exotic Pest Plant Council 2011).
Damage and Sysmptoms
Xyleborus glabratus is the only known vector of R. lauricola (Crane et al. 2008, Fraedrich et al. 2008). Trees attacked by X. glabratus exhibit few external symptoms initially. Small strings of compacted sawdust protrude from the bark at the point of attack; however, these strings disintegrate easily and are not always readily apparent. Removal of bark at the point of attack reveals shot-holes from which a dark stain extends into the surrounding xylem. Initial attacks introduce spores of the fungus into the xylem of the host tree. The fungal infection is visible by stained sapwood upon removal of bark or in cross sections of the stem. The stain is the tree's response to fungal infection that gradually spreads through the outer sapwood.
Trees infested with laurel wilt are characterized by a dark discoloration in the outer sapwood. Affected trees exhibit wilted foliage with a reddish or purplish discoloration. Foliar discoloration may occur within a section of the crown or the entire crown. The foliage eventually turns brown and tends to remain on the branches (Fraedrich et al. 2008). In redbay, the fungus moves rapidly through the xylem, plugging the flow of water and causing the trees to die within few weeks or months.
Xleborus glabratus is presumed to have been introduced into the U.S. through solid wood packing materials (Fraedrich et al. 2007). The cryptic nature of ambrosia beetles makes them difficult to detect in shipments of trees or wood products (Oliver and Mannion 2001, Rabaglia et al. 2006, Koch and Smith 2008). Local spread of beetles is potentially caused by the transport of fuel wood, tree trimmings, and other infested wood products (Rabaglia 2008). Barbecue smoke-wood may also serve as a potential vehicle for moving the beetle and the fungus into new areas. The natural rate of movement of the X. glabratus-laurel wilt complex within forests is estimated up to 34 miles per year (Koch and Smith, 2008). The actual spread could be even faster, especially when aided by human transport. The beetle is attracted to volatiles naturally emitted by living trees, severed limbs, tree stumps, and pruned trees of avocado, Persea americana, and redbay, Persea borbonia (Hanula et al. 2008). Female beetles are believed to be capable of flying 2–3 km in search of a host (Rabaglia 2008).
Currently, there are no methods for preventing further infestation of susceptible trees where redbay ambrosia beetle is established. In Florida trees with signs of rapid wilting, dieback, and insect boring in redbay and other host trees should be reported to the Florida Department of Agriculture and Consumer Services Division of Plant Industry or Division of Forestry.
To avoid spreading the beetle and pathogen to new areas, redbay and other hosts of redbay ambrosia beetle should not be moved or sold as firewood, tree trimmings, or barbecue smoke-wood. Wood from infested trees should not be transported out of the local area where the infested trees are detected. Dead redbay or other Lauraceous tree species cut in residential areas should be chipped and left onsite as mulch, or disposed of as locally as possible. The pathogen does not survive in the mulched wood chips (Spence et al. 2013).
Manuka oil, the essential oil extracted from Leptosperum scoparium and Phoebe oil, an extract of Brazilian walnut (Phoebe porosa) are potent attractants of X. glabratus. Both oils are effective baits for monitoring X. glabratus populations (Hanula and Sullivan 2008).
Chemical control of X. glabratus through aerial sprays is complicated and impractical because adult beetles must be in the immediate area that is treated. Once adult beetles bore into trees, contact insecticides are ineffective. For management of the vector and pathogen in avocado groves, growers should maintain tree health by preventing plant stress caused by abiotic and biotic factors. Ambrosia beetles are known to attack trees suffering from some type of environmental or cultural stress (drought, flooding, freezing, nutrient deficiencies, etc.). Avocado trees with confirmed disease symptoms should be removed to prevent spread of the vector and pathogen (Cameron et al. 2008, Hanula et al. 2008). The infested wood should be burnt within the infested grove without transportation (Crane 2009). Avocado trees not showing symptoms but present adjacent to trees with confirmed symptoms of disease may be treated with contact insecticides to kill flying beetles (Crane et al. 2011). Damaged or pruned avocado wood is more attractive to X. glabratus than non-damaged wood for up to three weeks. Therefore, groves should be pruned during the late fall or winter when X. glabratus activity is low. Groves should be pruned early in the morning and the cut surfaces should be treated with contact insecticides with residual activity (Crane et al. 2011).
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