Master Gardener - Black twig borers are tiny but damaging
Published: Friday, July 12, 2013 at 9:31 a.m.
Last Modified: Friday, July 12, 2013 at 9:31 a.m.
What's small, brown, and has the potential to damage ornamental trees and shrubs?
Why, the black twig borer of course. But have you ever actually seen one? Unless you happen to be extremely observant and own a microscope, the answer is most likely, "No." Even so, the black twig borer is fast becoming a common pest in nurseries and home landscapes throughout our region.
Black twig borers are a type of ambrosia beetle, a family of wood-boring insects that attack many varieties of hardwood trees and shrubs. A non-native pest, the black twig borer was first introduced to Florida from Asia in 1941 and has since become a familiar resident in the coastal Carolinas.
Most types of ambrosia beetles attack dead or dying hardwood trees, but the black twig borer has the ability to infest healthy plants as well. The black twig borer is known to attack more than 220 different plants, including magnolia, dogwood, redbud, laurel oak, pecan and red maple, to name a few. Luckily, the damage they cause is not severe enough to kill established plants, but can be more serious for young or recently transplanted trees and shrubs.
The damage caused by the black twig borer is much easier to see than the insects themselves. The ends of branches infested with black twig borer wilt and turn yellow or brown, a symptom known as "flagging." If trees or shrubs in your yard suddenly develop many dying branches, take a closer look. If these symptoms are caused by black twig borer, inspection of the flagged limbs will reveal small pinholes on the underside of stems. The pinholes are tiny, measuring about 1/32 of an inch in diameter, so you might consider using magnification while inspecting limbs. (Don't worry about your neighbors thinking you have finally gone off the deep end when they see you inspecting your trees with a magnifying glass.)
If you find the pinholes that are evidence of black twig borer, the good news is this damage will not kill the entire tree or even the entire limb. Prune out flagged branches three to four inches below the pinhole. Within a few weeks, the tree or the shrub will begin to generate new growth. For good measure, place your pruned branches in a bag and throw them in a freezer for a few days to kill adults and larvae.
Though flagging becomes most noticeable in summer, the injury that causes flagging begins in early spring. This is when adult female black twig borers bore into the bark of trees and shrubs, chewing galleries into the wood into which they will lay eggs. Females generally prefer young, pencil-size stems, but can attack larger stems and branches.
Once eggs have been laid, females introduce a specific type of fungus into the galleries. By the time the eggs hatch, a fungal colony is growing, upon which the newly emerged larvae feed. About 28 days are required from the time eggs are laid to the time they becomes adult beetles, with highest populations occurring from June through September.
Adult females are solid black and, at their largest, measure just over 1/16 of an inch long. That's about half the size of a grain of rice. The male beetles are wingless and spend their entire lives inside the tree. Females can even lay eggs without mating with the male beetles, a phenomenon known as parthenogenesis.
There are no insecticide sprays that will control black twig borer once they are inside the stem. Spraying to prevent infestation is not recommended or necessary. Instead, continue monitoring for black twig borer through the summer and fall, pruning out any infested twigs you find.
Unless we get two weeks of below-freezing weather in the winter, we can expect the black twig borer to remain a widespread landscape pest in our area. The good news is that black twig borers will not kill your trees, and with a little vigilance, you can successfully control this pest.
For assistance identifying and managing plant problems, visit ces.NCSU.edu, where you can post your questions to be answered by an expert. Or, contact your local extension office. If you live in Pender County, call 259-1235. In New Hanover County, call 798-7660. In Brunswick County call 253-2610.
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