N.C. Forest Service collaborates with USDA Forest Service to monitor the spread of laurel wilt

by | Aug 19, 2020

Since 2002, nearly all redbay trees have been disappearing in areas south of North Carolina. First detected in Georgia, the nonnative redbay ambrosia beetle, Xyleborus glabratus, carried these redbay trees to their dooms by introducing a deadly fungus. While the redbay ambrosia beetle is only about 1/16 inch long, it packs quite a punch against trees in the laurel family, including redbay and sassafras. However, it couldn’t complete its attack without its accomplice: the laurel wilt fungus, Raffaelea lauricola.

Leaves that have turned reddish or purplish brown are telltale signs of laurel wilt, a fungus carried by the redbay ambrosia beetle. Image: R. Scott Cameron, Advanced Forest Protection, Inc., Bugwood.org

The redbay ambrosia beetle and laurel wilt fungus are native to Asia but hitched a ride to the United States through wood products. Like all ambrosia beetles, the redbay ambrosia beetle “farms” fungus to feed its young. The fungus is carried inside pouches in the mouth of the adult beetles. When the beetles bore into trees to lay eggs, they infect the tunnels with the laurel wilt fungus, which serves as a tasty snack for their young. While most other ambrosia beetle and fungus duos are not lethal, the redbay ambrosia beetle carries a pathogenic fungus, meaning it causes disease. In North Carolina, the disease has only been detected in redbay and sassafrass, but spicebush, pondberry, and pondspice are all possible victims. Pondberry and pondspice are especially significant; pondberry is federally endangered, and pondspice is of special concern in North Carolina.

Laurel wilt is very aggressive and spreads quickly. The fungus causes the trees it infects to wilt and die within a few weeks or months. Further, just one female redbay ambrosia beetle can cause tree death and create an entirely new population! The female flies to a host tree, where it reproduces and starts a new population without even mating. The offspring leave to attack new trees. While these beetles spread naturally through flight, they are also spread through infested wood, which is why it is important to avoid the movement of firewood and other wood products.

As the redbay ambrosia beetle bores into trees to lay its eggs, it introduces the laurel wilt fungus, which is carried inside the mouth of the beetle. The fungus can cause dark staining beneath the bark of infected trees. Image: R. Scott Cameron, Advanced Forest Protection, Inc., Bugwood.org
Frass “toothpicks” are tubes of beetle excrement and sawdust left behind by ambrosia beetles. These frass toothpicks are signs that the redbay ambrosia beetle has ventured into a tree. Image: Albert (Bud) Mayfield, USDA Forest Service, Bugwood.org

Laurel wilt has caused a significant reduction in redbay. In fact, it often kills more than 90% of redbay trees that are over 1 inch in diameter. Redbay is important because it provides food and shelter to many animals like turkeys, bears, deer and songbirds. Two species of swallowtail butterfly rely almost completely on redbay and spicebush. Signs and symptoms of the beetle and the disease it carries include dead, reddish or purplish brown leaves that persist on the tree for up to a year, dark staining below the bark of infected trees, tiny beetle exit holes in the bark , and small “toothpicks” or tubes of sawdust and beetle excrement extending from some of these holes.

Since it is so important to monitor the spread of laurel wilt, the N.C. Forest Service has been collaborating with Bud Mayfield at the USDA Forest Service Southern Research Station on a project to map the spread and impact of laurel wilt in sassafras. In the past, laurel wilt has only existed on the Gulf-Atlantic Coastal Plain, but it is now spreading to nearby Piedmont and Mountain regions (where redbay is not present) using sassafras. Laurel wilt is more difficult to detect in sassafras because these tree sheds it leaves shortly after they die. The goal of the USDA Forest Service’s project, which began in 2018, is to detect laurel wilt early at sites that are near “risk-of-entry” points for redbay ambrosia beetle. These “risk-of-entry” areas include existing infestations and campgrounds where firewood is likely to be transported. Ultimately, the project will not only detect and monitor the spread of laurel wilt but also provide locations of potentially disease-resistant sassafras to researchers.

Hopefully, research projects like this one will allow us to limit the spread of laurel wilt. There is no consistent way to treat laurel wilt because insecticides are ineffective, and fungicides are costly and must be reapplied annually. We must monitor the pest and try to limit its spread by educating others on the dangers of moving firewood and other wood material. It is important to keep infected trees where they are. Even moving dead material to a landfill can spread laurel wilt! Dead trees should be cut and left on site or burned following all state and local regulations. if you think your trees have been infected with  laurel wilt, contact your county forest ranger for assistance.