What is a common sign of wood boring insect damage in trees?

A common sign of wood-boring insect damage in trees is the presence of visible holes in the bark. These holes are created when adult insects bore into the wood to lay eggs or feed. The holes vary in size depending on the species of the insect, and they can indicate active infestations. This damage not only compromises the structural integrity of the tree but also can provide entry points for pathogens and secondary pests, leading to further decline.

The other options do not accurately represent symptoms primarily associated with wood-boring insects. Swollen branches may indicate issues like resource competition or environmental stress, whereas excessive leaf drop can be attributed to a variety of problems, including drought stress or disease, rather than wood-boring insects. Root rot is related to fungal infections more than to pest damage, specifically involving the root system rather than the wood of the tree itself.