Two lined chestnut borer (agrilus bilinaetus) is a native borer that attacks oak, beech, hornbeam, and chestnut trees. It is often referred to as a secondary pest or invader because it ONLY feeds on stressed trees. The drought experienced in 2012 has provided a buffet of stressed oak trees for this bug. Stress can also come in the form of root loss or soil compaction due to construction, the build up of road salt in the root zone, or even extensive storm damage (wind, snow, lighting).
Symptoms
The first symptom of these little (1/5-1/2" long) greenish black metallic pests with two yellow lines is branch dieback (dead braches) in the top of the tree. The larvea feed on the conductive tissue of the upper canopy first; once this tissue is destroyed, the branch above where they fed dies. The leaves on that branch or branches where the bugs are turn brown and usually stay on the branch. Moving down the infected branch, often you can notice leaves are small or sparse and not very green, sometimes even red or brown. Moving further down the branch towards the trunk the leaves of the infected branches are usually fine, normal size and green.
Defense
The first defense against two lined chestnut borer is to keep your oaks stress-free. Watering a mature oak is a great idea during extended periods of dry weather or drought. Putting the hose at or near the base of the tree for 45-60 minutes on trickle will help to keep the tree from reaching the point of drought stress.
Another major defense component is installing sufficiently large mulch rings to retain the soil moisture and keep soil temps a bit cooler.
The last comment on tree stress is construction-related stressors. If you have a tree in your yard you are concerned about or cherish, call an arborist before beginning extensive landscaping or construction. Changing grades, removing roots, and removing large branches all have consequences which are often not seen immediately, but five to ten years down the road.
Treatment
Treatment against TLCB is very effective and if done soon enough can often give the tree a fighting chance to survive. Doing nothing means the tree will eventually die a slow painful death, one branch at a time. Insecticides used to treat for TLCB are injected into the trunk and usually prove very effective. Treatments can usually stop once the infestation is under control.
Pruning is also recommended in order to remove the dead branches with the infestations. Removing the dead branches also provides a good visual marker to better assess the infestation level in subsequent years, which is necessary in order to decide if treatments are still needed.