Twolined chestnut borer (Agrilus bilineatus) is a native beetle that attacks weakened oaks of every species in Minnesota. It lives in and feeds on the inner bark and cambium, a layer of cells between the bark and the wood. When trees are not stressed, twolined chestnut borer confines its attack to unhealthy trees or broken branches. Oaks become susceptible to attack when tree vigor is reduced from drought, construction, defoliation, or disease. Infested oaks usually die after 2-3 years of infestation.
The drought in 2021 caused widespread outbreaks of twolined chestnut borer in 2022, resulting in the death of thousands of oaks across Minnesota. Outbreaks are expected to continue in 2023, particularly around the Twin Cities and to the southwest.
Other oak problems can easily be confused with twolined chestnut borer, and the presence of twolined chestnut borer doesn't necessarily mean it is the primary problem. Read about diagnosis in the Identification section.