Two Lined Chestnut Borer
An insect threatening already-stressed trees in Connecticut
Adult Two Lined Chestnut Borer
Most damage is from the larvae feeding under the bark.
Oak Infested with Two Lined Chestnut Borer (center)
Oak to the right was previously killed by gypsy moths.
Oaks that survived the gypsy moth infestations and the drought that caused so much harm over the years of 2015 to 2017 are not necessarily out of the woods. During the summer of 2018, foresters and arborists in eastern Connecticut noticed the effects of another harmful insect, the two lined chestnut borer (Agrilus bilineatus). Connecticut continued to see oak deaths from this insect, as well as other secondary pathogens and the overall lingering effects of the drought, into the summer of 2019, and may continue to see these harmful effects on into the year 2020.
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 two lined chestnut borer was first named because of its attacks on native chestnut trees. It also attacks oaks and, occasionally, beech and hornbeam. As a result of co-evolution, unstressed native species of oak and other native trees are usually able to resist this insect. In most years, two lined chestnut borer is not a problem in the forest.
Following the recent onslaught of the gypsy moth and the drought, that dynamic has changed. Increasingly, oaks throughout the areas struck by gypsy moth have begun showing the symptoms of attack by this insect. These symptoms usually start with the browning of leaves near the top of the tree. This is from the larvae begin their feeding under the bark of branches in the upper crown.
This attack can be initiated at any point during the summer. As the summer moves on, the 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 the attack is in full-swing, it is not unusual to see trees whose tops are full of brown, dead leaves, whose middle crown show leaves turning red and yellow as they wilt and what appears to be healthy green leaves lower in the crown.
It should be noted that there are other harmful organisms that will take advantage of the compromised condition of the oak trees. Various pathogens, most notably the armillaria fungus, as well as insects such as the red oak borer, may also be attacking these trees, often alongside of the two lined chestnut borer.
The pattern of two lined chestnut borer outbreaks are well known from past experience with gypsy moths. The number of TLCB tend to reach a maximum following the peak year of gypsy mortality. After this peak, the number of oak deaths due to TLCB then declines as various parasitoids the move in on the TCLB.
Content last reviewed February 2020