Recovering from Laurel Wilt

Originally from Asia, the redbay ambrosia beetle and the fungus it carries in its jaws have found a new home in the southern United States. Eradication is impossible at this point, and the fungus causes laurel wilt, a highly destructive disease that affects redbay, swamp bay, sassafras, avocado, and pondberry – as well as every other member of the laurel family.

“Redbay is widespread in southern coastal forests, but millions of mature and sapling-sized trees have been killed by laurel wilt,” says U.S. Forest Service scientist Bud Mayfield. “Redbay is a food source for birds, mammals, and many other animals – including two butterfly species whose caterpillars either require or are strongly dependent on redbay leaves.” Mayfield recently coauthored a recovery plan for redbay and other forest species affected by laurel wilt. A separate recovery plan has been written for avocado.

The plan was published in Plant Health Progress, and outlines the laurel wilt disease cycle, the cultural and ecological impacts of losing redbay and other forest species, and offers strategies to help slow the spread of laurel wilt. Many university and federal scientists collaborated to write the recovery plan, which was led by Marc Hughes, a post-doctoral researcher at the University of Florida.

Redbay and swamp bay play central roles in the medicinal and cultural traditions of American Indian communities. “The Seminole and Miccosukee Tribes of Florida use redbay or swamp bay in over 90 percent of their traditional medicines,” says Mayfield. “Tribal foresters are looking into ways to maintain plants for cultural uses.”

Although redbay is not generally considered economically important, it is so abundant that its loss could have economic impacts, such as the cost of tree removal, lowered property values, and increased costs to state and local governments as they educate the public about laurel wilt and monitor its spread.

Because redbay logs can be infested with the fungus-carrying beetles, one of the most important control strategies is to not move firewood or unprocessed wood products. Ideally, firewood is burned within 10 miles of where it is harvested, and some areas such as Miami-Dade County in Florida, have banned all imported firewood unless certified. Moving wood has led to documented expansions of the disease in Florida, and may have initiated outbreaks in Alabama, Louisiana, Mississippi, and Texas.

To reduce population of the beetle, infected redbay trees can be cut down and removed – even the stumps, as they are known to support the ambrosia beetle. Chipping the logs and stumps reduces beetle emergence by 99 percent. However, because just a few females can start a new population, these wood chips should not be transported to new areas. Scientists recommend burying the wood chips or covering them with plastic. Efforts to stop the spread of the disease in forests and parks through sanitation removals alone has not been successful, and additional tools for managing laurel wilt are needed.

Redbay trees in neighborhoods and parks are sometimes treated with fungicides injected into the lower trunk. However, it is impractical to treat redbay in forests with fungicides because of high costs, the need to reapply fungicides every year, and environmental limitations. In some forests, removing redbay trees – even the healthy ones – has been suggested as a way to protect pondberry, a federally endangered shrub in the laurel family.

Forest Service and university scientists are collecting seeds and germplasm of vulnerable hosts, especially from rare species such as pondberry, pondspice, and gulf licaria. Scientists are also growing new trees from cuttings taken from the few redbay trees that appear to be naturally resistant to laurel wilt and will test them under natural conditions. Biological control may be a possibility as well. Three strains of fungus that attack the redbay ambrosia beetle have been identified, and scientists are working to determine whether the fungus can be used to protect trees from the disease.

Read the full text of the recovery plan.

For more information, email Bud Mayfield at amayfield02@fs.fed.us.