Which of the following as an adult beetle emerges from D-shaped holes in the bark of low-vigor oak trees?

The two-lined chestnut borer is known to create distinct D-shaped exit holes in the bark of trees where the adults emerge. This beetle typically targets low-vigor oak trees, particularly those that are stressed or weakened, making them more susceptible to infestation. The D-shaped holes are key indicators of the presence of this pest, as they are characteristic of the adult beetles' emergence after they have developed in the tree.

The other options listed do not produce D-shaped exit holes or do not primarily infest low-vigor oak trees. For instance, the Ips bark beetle primarily affects conifer species and produces different exit hole shapes. The pine sawfly is a larval stage pest that feeds on foliage but does not create exit holes like beetles. The fall webworm, while it can impact tree health, is associated with leaves rather than bark and does not emerge from holes in the same manner as the two-lined chestnut borer. Thus, recognizing the specific characteristics and behaviors of forest pests is crucial for accurate identification and effective management.