Two-lined Chestnut Borer Prevention

We have recently seen a higher-than-normal level of two-lined chestnut borer (TLCB) activity in and around Lake Forest.  TLCB is a common insect that can cause oak mortality over several years if the infestation level is high.  The insect is attracted to stressed and weakened oak trees.  Last year’s drought, which stressed many trees, could be the cause of this year’s increased TLCB activity.

What is a Two-lined Chestnut Borer?
  • Two-lined chestnut borers can be identified by their elongated shape and greenish-black metallic color. They are about 1/5 to 1/2 inch in length.
  • It is a beetle that attacks weakened oak trees of every species in Illinois.
  • May through June, adult TLCB emerge from “D” shaped exit holes near the lower portion of a tree’s trunk and feed on foliage.
  • Females will lay eggs in the crevices in the bark.  When the eggs hatch, they will bore through the bark back into the trunk of the tree cutting off the flow of water and nutrients from the roots to the leaves.
Identifying Two-lined Chestnut Borers in Oak Trees
  • Leaf browning and twig/branch dieback.
  • Branches in the upper canopy have a wilted look while the lower canopy is still green
  • “D” shaped exit holes in the trunk of the tree (close to the base).

TLCB News story cover TLCB exit hole.jpgTLCB insect

             Oak Trees                     "D" Shaped Exit Holes         TLCB Insect

Preventing and Treatment of Two-lined Chestnut Borer
The good news is that the two-lined chestnut borer is both treatable and preventable.  Because healthy trees are less susceptible to borer attacks, the best protection is standard tree maintenance and care. This includes providing supplemental watering during drought conditions and increasing vigor by fertilizing if your soil analysis indicates a deficiency.  Regular pruning and pruning out infested limbs can also help decrease the impact of TLCB.  Insecticide treatments may be recommended.

City Management
We highly recommend contacting a certified arborist for treatment options or reaching out to the Forestry section for additional information. Your certified arborist will give you multiple treatment strategies for battling TLCB. A prophylactic approach to treatment may be an option if there are symptomatic trees in your area. If signs and symptoms of TLCB are present on your trees, a soil drench/injection insecticide may be needed.

The City will continue to monitor the trees under our care for two-lined chestnut borer and other insects and diseases.  We have recently begun treating symptomatic parkway trees for TLCB and may prophylactically treat trees in areas that we identify as high risk.

If you have any questions, please feel free to contact the Forestry Section at 847-810-3564.

Stay Informed!