My holly bush has holes bored into the thick, main trunks close to the ground. Although it continues to have new growth, the bush has been declining in health over the past few years, despite my pruning and removal of dead leaves and branches. There are no visible signs of bugs or webs. The one next to it looks healthy and green (same soil, sun, water, etc). I am not sure what is causing the holes, nor if there is some way to help the plant other than cutting it back to the base.
Holes in the trunk or branches generally indicate a borer insect.
Here are some links with more information.