Twolined Chestnut Borer

A native insect threatening already-stressed trees in Connecticut. 

Adult twolined chestnut borer.

The common name of this insect is given in various ways, including as the two lined chestnut borer, twolined chestnut borer, two-lined chestnut borer, or TLCB. This beetle is a close relative of the emerald ash borer (Agrilus planipennis), with a similar biology. The key difference, however, is that the TLCB is a native insect, while the EAB is not.

The twolined chestnut borer was first named because it was a principal pest on native chestnut trees. Oaks, and occasionally beech and hornbeam, are also hosts to this insect. Healthy oak and other native trees are usually able to resist this insect. In most years, the twolined chestnut borer is not a problem in the forest.

The pattern of twolined chestnut borer outbreaks is well known in Connecticut and has corresponded with spongy moth outbreaks. TLCB outbreaks tend follow peak years of spongy moth mortality. For example, many oak trees that survived the spongy moth infestations and the drought over the years of 2015 to 2017 were weakened and subsequently killed by the twolined chestnut borer through the summer of 2019.

It should be noted that other harmful organisms also take advantage of compromised oak trees. Various pathogens, most notably the armillaria fungus, as well as insects like the red oak borer, may also be attacking weakened trees, often alongside the twolined chestnut borer.

Oak infested with twolined chestnut borer (center). Oak to the right was previously killed by spongy moths.

Trees that are stressed by the twolined chestnut borer usually show browning of leaves near the top of the tree during summer. This is a sign that the larvae are feeding under the bark of branches in the upper crown. As summer moves on, symptoms start to show further down the tree, as the beetles begin laying their eggs on branches in the middle and then lower crown.

When a TLCB attack is in full swing, it is not unusual to see trees whose tops are full of brown, dead leaves; whose middle crown shows leaves turning red and yellow as they wilt; and whose lower crown has green leaves.

The Connecticut Agricultural Experiment Station provides information on the treatment of oak trees that have been affected by TLCB.

US Department of Agriculture Fact Sheet on TLCB

Content last updated August 2025.