By Crawford “Wood” Johnson, U.S. Forest Service
In 2002, an otherwise unremarkable little brown beetle was detected for the first time in North America in an insect-monitoring trap near Port Wentworth, Georgia. The southeast Asian species, eventually identified as Xyleborus glabratus, had presumably just hitchhiked across the Pacific Ocean in wood-packing material on one of the many commercial freighters that arrive each year in the U.S. Our subject would soon be added to the growing list of non-native beetles intercepted annually at U.S. ports of entry, but its name would not simply fade into obscurity.
One short year later, many in the area reported declining and dead redbay (Persea borbonia). Reports stated the leaves of seemingly healthy trees would wilt within days, turn brown, but then persist on branches for weeks or months, as if they meant to ensure the world didn’t forget them. Emerging redbay ambrosia beetles were repeatedly collected from dead and dying redbay, and the connection was soon realized between the widespread redbay mortality and the recent arrival of X. glabratus. Our subject was later christened with the benign-sounding common name “redbay ambrosia beetle,” but as its list of hosts increased, so would its notoriety.
The redbay ambrosia beetle is now considered the principal vector of laurel wilt disease. After locating her host, the foundress redbay ambrosia beetle oviposits eggs at the termini of multiple branching galleries within the sapwood layer of the xylem. During gallery construction, she deposits spores of the fungus Harringtonia lauricola (formerly Raffaelea lauricola) carried within specialized pouches located in her mandibles. (The ambrosia beetle group is named for its association with the symbiotic fungi these beetles carry). The mycelium of H. lauricola begin decomposing the xylem tissue surrounding the galleries, and in doing so, concentrate nutrients from the otherwise poor food source for the developing beetle larvae. In its native range where it coevolved with Asian Lauraceae plant hosts, the redbay ambrosia beetle exhibits a strong preference for and primarily colonizes dead and dying trees and shrubs. Outside of its native range, however, it locates and kills otherwise healthy Lauraceae hosts. The precise manner by which laurel wilt disease kills its host is thought to be caused by a rapid decrease in hydraulic conductivity due to the occlusion of xylem vessels by tyloses and gel formation in response to infection. This is almost like the exaggerated immune response expressed by people infected during the 1918 influenza pandemic.
Laurel wilt now occurs across the southeastern U.S, extending from Virginia west to Kentucky, south to Arkansas and Texas, and east to Florida. Mortality has been reported in additional Lauraceae hosts occurring in North America, including sassafras (Sassafras albidum), swampbay (Persea palustris), spicebush (Lindera benzoin), pondspice (Litsea aestivalis), camphor tree (Cinnamomum camphora) and the economically important avocado (Persea americana).
Sassafras is among the more desirable native horticultural and urban trees on this list, being tolerant of poor soils and having a restricted growth form and outstanding fall colors. It is widely distributed across the eastern U.S., occurring from Maine to Michigan and Texas to Florida (Griggs 1990), and is most abundant in the Ozark regions of Missouri and Arkansas, the Ohio River Valley and the Appalachian Mountains (Randolph 2017). In the eight years since laurel wilt was first detected in Union Parish, Louisiana, the disease has burned through swampbay and sassafras throughout most of the northwest and central parishes. It’s known to occur in East Feliciana Parish in southeast Louisiana and is likely more widespread in the Florida Parishes as well but has not yet been reported. Monitoring studies where laurel wilt is established in Georgia have found mortality rates among sassafras populations approaching 80%, and surveillance of sassafras in central Louisiana where laurel wilt is present has indicated similar rates of mortality. Near this author’s home in Grant Parish, the only remaining living sassafras are the seedlings and small saplings less preferred by the redbay ambrosia beetle, and even those seem to suffer eventually. And unfortunately, it is now the norm to see brown-topped swampbay trees in the bottoms that drain the reestablished longleaf savannas of the Kisatchie National Forest.
Research on the cold tolerance of redbay ambrosia beetles suggests that 48% of the U.S. sassafras population will not experience sufficient cold to restrict laurel wilt distribution, and urban areas and forests with high sassafras abundance will experience significant rates of mortality. The removal of sassafras from forests of the U.S. will be a significant economic, ecological and cultural loss. Sassafras has some commercial value in southern forests, is a host plant of multiple native pollinator larvae, and provides forage for wildlife. Sassafras also has a unique cultural history and value, as Native Americans and early settlers sought it out for its medicinal properties and as a food additive (think filé gumbo, repeated in the voice of Hank Williams, Sr.).
So, is there any recourse? Tree infusion and injection systems using fungicides are a relatively safe and effective means for preventive treatments while significantly reducing non-target risks. Simple, pressurized systems used to assist in infusing products, including propiconazole, through shallowly drilled ports will effectively prevent laurel wilt in sassafras and redbay for at least one year if trees are treated prior to expression of disease symptoms. But this is only economical for high-value trees, and retreatment annually is necessary. If one seeks to protect such a tree, seek professional advice from your local LSU Extension agent or a licensed professional arborist for more information. As the sun potentially sets on another American tree species, we can only hope that some may prove resistant or tolerant to this devastating disease, and we, or our grandchildren, will read about that one day.
Vascular staining caused by laurel wilt disease. Photo by Ronald F. Billings, Texas A&M Forest Service.
Adult redbay ambrosia beetle. Photo by University of Florida, Institute of Food and Agricultural Sciences.