The twolined chestnut borer (TCB), (Agrilus bilineatus), is a boring beetle. Like the bronze birch borer and the emerald ash borer it is a bupestrid beetle, also commonly known as a metallic wood boring beetle, or “flat head” borer. The TCB gets its name from its distinctive markings and for one of its historically preferred hosts, American chestnut.
What does it look like?
The adult beetle is approximately 1/4 to 1/2 inch in length, dark brown/black in color, with its namesake two gold colored lines running down the back. The larva reaches about 1 inch in length when mature and looks like a thin, white, segmented worm. One characteristic distinguishing TCB larva from other flat head borer larva is the two pincer-like spines that protrude from the abdomen or hind end of the larva.
Host material and range
TCB is found primarily in the eastern half of the United States and southern Canada. Its range extends as far as Nova Scotia in the northeast, lower Manitoba to the northwest, Florida to the south, and as far west as Texas. Since the American chestnut has become rare due to the chestnut blight, the primary host of the TCB is oak. Oak species most commonly attacked include, but are not limited to, white, black, red, scarlet, pin and live oak. TCB is a secondary pest, typically attacking trees that are already under stress or in decline from drought, soil compaction, age, injury or damage done by another insect.
The adult beetle emerges from its host tree by making a distinctive D-shaped exit hole in the bark. Emergence typically occurs from May to June. Immediately after emergence, the adult will fly to the crown of a stressed oak tree for a period of feeding on the selected host tree’s foliage, after which it will move inward to the branches to mate and lay eggs. The female TCB lays eggs in small groups in bark crevices. Eggs are usually hatched within a week or two, and immediately upon hatching the first instar larvae bore through the bark to begin feeding in the cambium, constructing intricate, winding “galleries” and destroying the vascular tissue that transports vital water and nutrients within the tree. The TCB larvae will go through four instars as they continue to feed and develop within the cambial area. The larvae are usually full grown by late summer/early fall and will overwinter inside the tree as mature larvae. The larvae pupate into adults the following spring to begin the cycle again.
The first sign of infestation in a host oak will be wilting, browning foliage in the crown of the tree where the beetle infestation typically begins. Though wilted and brown, the leaves tend to stay attached to the tree which will differentiate TCB from oak wilt symptoms, in which case the leaves tend to fall off not long after turning brown. Limbs infested with TCB will die and not produce leaves the following spring. Over successive years of infestation, the upper crown will be completely dead and browning, wilting foliage will progress downward through the tree. By the time the distinctive D-shaped exit holes are apparent in the lower trunk, the tree is already in an advanced stage of infestation. In most cases, death of the tree will occur within 2 to 3 years of successive TCB infestation, although in some cases death can occur in as little as one year.
Since TCB attacks stressed trees, controlling stressors and maintaining tree health and vigor are essential preventative measures. During drought conditions, susceptible oaks should be adequately watered and fertilized, and compacted soils should be broken up and aerated. Controlling outbreaks of other oak pests such as gypsy moth will also decrease the likelihood of a TCB infestation.
If TCB infestation has already occurred, what, if any, treatment measure is undertaken should be determined by the degree of infestation. Treatments will be most effective in the early stages of infestation. Pruning out infested or dying limbs will improve overall tree health and may inhibit the spread of TCB. Trees with advanced canopy death or exhibiting exit holes in the trunk may be better candidates for removal than treatment. For most boring insects like TCB, systemic injection is the most effective option. Applying imidacloprid in a soil application can also be effective, although it can take months to be absorbed into the tree and it is never certain how much material will be absorbed.
What you can do?
If you have or care for oaks in an area prone to drought or other serious oak pests, take adequate measures to reduce stress and maintain the vigor of the trees. Utilize appropriate watering and fertilization standards and take appropriate measures to keep the trees free from other pests. Be alert for early symptoms of TCB and — in the event you determine a tree has been infested with TCB — the sooner a treatment is applied, the better the chances of saving the tree.
Information compiled by Sean Facey of Arborjet, Inc. using information from the following sources: Forest Insect & Disease Leaflet 168, USDA, Robert A. Haack and Robert E. Acciavatti; “The Two Lined Chestnut Borer” Professor of Entomology Dr. Allen, SUNY-ESF; and “Oak Disorder: Twolined chestnut borer,” C.F. Koval, M.F. Heiman, University of Wisconsin Cooperative Extension. For more information, visit www.arborjet.com