Published on 05/06/99

Nectar for Tree Lovers: Ambrosia Beetles Strike

Thousands of small Georgia trees are sprouting spikes this spring. The thorny turn of events is a clear sign of trouble, say University of Georgia experts. It's the attack of Asian ambrosia beetles.

"TOOTHPICKS" SHOW BEETLES HAVE MOVED IN As Asian ambrosia beetles bore into tree limbs, they push the sawdust out. It stays put with help from 'webbing' from the beetle. If you see these, it's a sure sign the beetles have infested your tree.

Infestation in Trees

"The classic signal that the plant has been attacked by Asian ambrosia beetles is an inch-long toothpick emerging from the trunk," said Walter Reeves, a horticulture educator with the University of Georgia College of Agricultural and Environmental Sciences.

The female Asian ambrosia beetle emerges in spring and travels to a nearby shrub or tree. She prefers a small plant or limb 1 to 2 inches thick, boring into it at up to 1 inch per day.

"As the female bores, she pushes her sawdust out of the limb," Reeves said. "The particles, bound with webbing, may stick straight out of the bark for 1 inch or more, like a toothpick. Occasionally the toothpick collapses and the particles dangle four inches from the hole, swinging in the breeze."

The female lines her tunnel with a fungus, which grows in the moist environment. When her eggs hatch, they feed on the fungus "ambrosia." Almost the entire life cycle is spent inside the plant, making the beetles hard to control with insecticides.

Many Trees Susceptible

"The beetle can invade many species of otherwise healthy trees and shrubs," said Will Hudson, a UGA Extension Service entomologist. "So far, we have seen them on ornamental cherry, crape myrtle, goldenrain tree, redbud, hickory and Japanese maple."

Elm, oak, Bradford pear, apple and others can be affected, too. "It will attack almost any broadleaf tree or shrub of the right size," Hudson said.

Asian ambrosia beetles must be controlled. When and how is the question.

Few Treatments for Infested Trees

"In most landscape situations, there is little to do except keep your plants as healthy as possible," Hudson said. "Nurseries, however, may need to spray during the green-up period, but not other times."

"There is no good way to determine if it's better to remove the entire plant or treat it with insecticides," Reeves said.

Once a tree is attacked, insecticides won't help unless it's squirted into the tunnel.

"Even then," Hudson said, "the fungus is the problem, not the beetle. The unknown here is whether the beetle introduces a pathogen incidental to the tunneling. (The ambrosia fungus is not a pathogen.)"

Reeves offers these control recommendations:

  • If the trunk or limb has 25 to 100 holes, remove it completely. It probably won't survive so many attacks.
  • If the plant has only one to 15 widely scattered holes, keep a close watch on the plant this year. Make sure you water it properly, especially this summer.
  • If a tree or limb has completely wilted, remove it. It won't come back.
  • It isn't clear whether spraying nearby trees is helpful. Borer sprays such as chlorpyrifos (Dursban) and benzene hexachloride (Lindane) don't seem to protect them.

Faith Peppers is the director of public affairs with the University of Georgia College of Agricultural and Environmental Sciences.