Black Twig Borers: Tiny insects pose a big threat to your trees and shrubs

Black twig borer

Black twig borer is a tiny species of beetle. This adult female is barely larger than the letters stamped on the head of a dime.

Photo by Sam Marshall
Published: Thursday, July 11, 2013 at 02:27 PM.

What’s small, brown, and has the potential to damage your 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. However, 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, 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; however, the black twig borer has the ability to infest healthy plants as well. Black twig borer is known to attack over 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 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. Your neighbors will think 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. Just for good measure, place your pruned branches in a bag and throw them in the 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.

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