Many species of ambrosia beetles are known to infest fruit and nut trees and woody ornamentals causing significant damage in nurseries, orchards and home landscapes. However, in 2002, when the redbay ambrosia beetle (Xyleborus glabratus) was first detected in coastal Georgia, it was not considered to be a threat to healthy trees. The beetle was most likely introduced in a shipment of goods packed on untreated wooden pallets and in untreated wooden crates from Southeast Asia. However, fast forward to 2021. Statistics now show just how great the threat is – more than 300 million redbays have died as have other members of the laurel family attacked by the beetles, including avocado, sassafras, California bay laurel and spicebush.
Why should we care? Redbay (Persia borbonia) is an important native plant species of the southeast coastal plain. It is a small tree or large shrub of the maritime forest with three- to six-inch long evergreen lance-shaped leaves arranged alternately along the stem. The leaves have a distinct spicy odor when crushed. Redbay has long been used in boatbuilding, cabinet making and veneer work. Many species of birds feed on the small black fruit. Deer and bear feed on both fruit and foliage. The larva of the Palamedes swallowtail butterfly feed primarily on redbay leaves. As redbays continue to decline, threats to the lives of the animals that rely on them will increase. If you take a look at the surrounding coastal maritime forest and see a number of small trees with completely brown leaves hanging on, most likely you are looking at an infested dying redbay. The dead leaves stand out against the many other healthy evergreen leaves of other plant species found growing with the redbay.
The redbay ambrosia beetle is a small slender brown-black beetle, just 2mm long. Female adult beetles attack a tree and begin to burrow in, creating a feeding tunnel. While burrowing, they introduce the spores of a plant pathogen, Raffaelea lauricola (the laurel wilt fungus), that reside in sacs found above the beetles’ mandibles. The adult females feed on the growing fungus and lay eggs which hatch into white, legless larvae that also feed on the fungus. As the fungus spreads inside the plant host, it clogs the water and nutrient conducting systems of the plant. The plant starts to wilt within a few weeks. Over time, the beetle/fungus combination kills the plant. Infested trees are easily identifiable by the presences of sawdust straws that radiate from the trunks and branches. The straws are the frass, or excrement, from the beetles as they feed. Internal examination reveals feeding tunnels and a bluish stain caused by the laurel wilt fungus. Within 50-60 days after a tree is attacked, adult female beetles emerge from the infected tree and fly in search of another plant host. Wingless males will stay in the tunnels to mate with the females that hatched with them. Each female beetle can produce up to 300 young which translates to a very rapid increase in beetle population.
The beetle is already in Florida where research has demonstrated that the redbay ambrosia beetle and the infection by laurel wilt kill avocado. Florida has 56 avocado varieties growing on 6,000 acres. A grave risk is the spread to other areas including California and Mexico. Avocado is a major crop in California with more than 50,000 acres planted. In 2017, the U.S. avocado harvest produced 146,310 tons at a value of $392 million. However, the majority of avocados consumed in the U.S. are imported. Mexico is the world’s largest producer of fresh avocados. The economic impact brought on by large-scale plant mortality will be huge, as it will affect not only large farms but also the subsistence farmers and their families. Exotic wildlife, particularly birds such as the strikingly colored Quetzal that feed on the avocado, will also suffer. Left unchecked, the redbay ambrosia beetle and laurel wilt will continue to spread into the native and cultivated avocado trees that grow in Central and South America. This beetle/fungus is now considered to be as deadly and invasive as chestnut blight and Dutch elm disease that eradicated almost all the American chestnut trees and American elm species.
What can we as do to reduce the spread of the beetles? Avoid transporting infected redbay and other hosts of the redbay ambrosia beetle as firewood or barbeque smoke-wood from our coastal area into other areas. If you have infected redbay on your property, cut them down and destroy them. In the residential landscape, they can be burned or chipped on site and used as mulch as the beetle/fungus does not survive in mulched wood chips. In the commercial avocado orchards, diseased plants should be burned. Chemical control is challenging and at this time there is no reliable control.