<This article was originally submitted to the Cherokee Co. Master Gardeners for their summer newsletter in 2023>
“Sawdust hairs”, “worm-like, dust fringes”, “toothpicks” - all phrases homeowners have used to describe ambrosia beetle damage. Gardeners are clever describing the protrusions they’ve seen along the trunks of crape myrtles, figs, maples, and even cherry trees. This is something to fear. These are signs of ambrosia beetle attacks. Though this is a telltale sign, other symptoms might be the wilting of emerging spring foliage, leakage of sap, and stained bark. And this time of year [late winter], scout out susceptible plants for fine, pinholes along the trunk and limbs.
Ambrosia beetle adults can overwinter in wood or in leaf litter. Once they emerge in late winter & early spring, ambrosia beetles are attracted to trees that are weakened, stressed, or those that sport a thinner bark. This is how they target their host in order to reproduce. They bore inches deep into a tree’s trunk to lay their eggs. What’s left behind are the frass tubes seen protruding from trunks and limbs. If a cross section of damaged wood is taken, their elaborate galleries can be seen. Unfortunately, it is their galleries that kill a tree over time.
By the time ambrosia beetle damage is spotted, a homeowner is about two months behind when the adult first began its reproductive cycle. Male beetles are without wings and stay in the galleries the rest of their lifecycle. A new generation of females however leave affected trees around late spring, and the destructive process begins all over again.
Ambrosia beetle damage was heavy on folks’ radars last summer because the deep freeze that much of Georgia experienced during Christmas 2022, plus at least two more late freezes the following March. Those events caused stress to many trees and ornamentals in the landscape. Those plants send out stress chemicals that these opportunistic insects take advantage of. Arborists can consult with homeowners, but once ambrosia beetle damage is observed, there are very limited options to save the tree. Some arborists may recommend the application of a barrier spray around the trunk, but this needs to be repeated and does not guarantee zero infestations. The spray employed would be a pyrethroid insecticide, and traps are generally set in the middle of winter to detect where activity is likely to occur.
During a consultation, arborists could also assess other trees, identifying what could potentially make them vulnerable as well. In addition to freeze damage, another environmental stressor is drought. Also, trees planted too deeply have roots that are unable to ‘breathe’, and this causes decline. Finally, trees where the bark and trunk have been wounded become susceptible to the introduction of fungus, moisture retention and also to ambrosia beetles.
What can be done? Heavily infested wood needs to be cut, removed, and destroyed. Burning it would be the easiest option for eliminating this pest. In a best-case scenario, the cut to a tree can be made below the last visible signs of holes bored, and perhaps the tree can grow back from this heavy, rejuvenative-type pruning. With crape myrtles, suckers will emerge once the tree has been cut, and one or two strong water sprouts can be chosen to become the new trunk.
Proper tree planning, care and best management practices when maintaining ornamental trees are all key to a tree’s survival. The stronger the tree, the more easily it can ward off disease and attacks by insects. Responsibility lies with the property owner to first ensure the right tree was chosen for the given environment. Considerations of hardiness, light and access to moisture should all factor in. “Right tree, right place” is an iteration of a phrase commonly used by horticulturalists, and it couldn’t be more true.
Soil type needs to be considered, and upon installation, a proper sized hole dug, and mulch added around, but not right up to, the tree’s trunk. Mulch is most resourceful for keeping out weeds and maintaining moisture when put down 2-3 inches thick. Watering, feeding and pruning are all perpetual tasks in maintaining the health of a tree.