Tree Insects

Tree Pests: Insects

Laurel wilt disease spread by an invasive ambrosia beetle is killing redbay trees in Georgia and Florida

From an article by Chuck Lippi printed in the St. Augustine Record on November 1, 2008. Laurel wilt is an example of a disease carried and spread by an insect.

Laurel wilt causes the leaves to die and turn a rusty red color.

Laurel wilt causes the leaves to die and turn a rusty red color.

You may have noticed some medium and small trees that have recently died in your neighborhood. What you are seeing is the result of the invasion of a foreign pest that is devastating our native redbay trees (Persea borbonia). There are actually two pests involved. One is an Oriental ambrosia beetle (Xyleborus glabratus) that carries the disease. The second is the disease fungus (Raffaelea sp.) that is carried by the beetle.

The beetle bores into the trunk of the redbay and deposits the fungus
which then quickly spreads throughout the vascular system plugging the flow of water and causing the leaves to wilt. The beetle and its brood feed on this fungus, not on the wood. The dark green redbay leaves turn reddish brown and stay attached to the tree. Soon the tree dies. The beetle was first detected in the United States in 2002, near
Savannah, Ga.

The pest is believed to have arrived in wooden crating material used in
shipments from the ambrosia beetles native range in southern Asia. The disease has been spreading rapidly southward from Savannah reaching northern Duval County in 2005 and western St. Johns County by 2006. I saw my first case in St. Augustine in April this year in Vilano Beach. Now I see it throughout St. Augustine. So far, redbay wilt has not been reported in Flagler County, but it is only a matter of time as the pest moves southward.

The beetle is tiny, about the size of Lincoln’s nose on a penny. When it bores into a redbay, it pushes out a string of compacted sawdust that is sometimes, but not always visible. Movement of the beetle is enhanced by the movement of infested redbay firewood logs.

Laurel wilt causes the leaves to die and remain attached to the tree

Laurel wilt causes the leaves to die and remain attached to the tree

It appears that the disease has almost 100 percent mortality. The insect and the disease organism are selective and only attack redbay
trees, which are the most numerous in our area, and a few other tree
species — sassafras, camphor and avocado trees that aren’t as plentiful in our area. The disease is a big concern to avocado growers in south Florida, where the avocado is a large commercial crop.

The Florida Division of Forestry is conducting some experiments with
an injected fungicide (propiconazole), with limited success on trees that are not showing wilt symptoms. The propiconazole injections have delayed or halted the infection when treated redbays have subsequently been inoculated with the fungus. But the tests are limited. We still don’t know how long the treatment will provide protection. And the fungicide treatment is expensive.

The redbay is found throughout Florida in scrub and hammocks. The redbay’s high salt tolerance made it an ideal tree growing near the dunes. The glossy, leathery leaves are fragrant when crushed and can be used as an herb in cooking, much like the leaf of the bay laurel (Laurus nobilis), which also grows here as a non-native but, as far as we know, is not susceptible to the fungus. Redbays provide fruit for birds, and are also the primary hosts for the larvae of the palamedes swallowtail butterfly.

When the redbay dies, it should be cut down. The logs or chips should be left on site. Moving the logs as firewood will spread the disease by moving the insect vector along with the logs.

When the redbay dies, it should be cut down. The logs or chips should be left on site. Moving the logs as firewood will spread the disease by moving the insect vector along with the logs.

With nearly 100 percent kill, the ambrosia beetle may eventually run out of redbay victims and die out. That process may take a number of years, but letting the insect and disease take their course could be our only alternative. Then we can look for resistant genetic material that has survived and eventually replant with that new genetic material.

The latest publication on the disease can be found at the Florida
Division of Forestry Web site at http://www.fs.fed.us/r8/foresthealth /laurelwilt/.