Answer: There is an insect called the giant hornet (Vespa crabro germana) that will occasionally drill into the branches of lilacs, rhododendrons, and boxwoods, however I did not find it listed as a holly pest. The drought would also cause the symptoms you are describing, so there may be a combination of forces at work here since plants will die back and/or drop foliage as a protective measure in drought.
The hornets can be deterred by applying carbaryl (for example, Sevin) to the actual branches deep in the shrub where they are drilling (it is not particularly helpful to spray the foliage since that is not where they are going). This is best done in July, however since they are still active it is probably worth a try now to try to limit their presence next year.
If they have caused serious damage the branches may eventually die off and need to be removed neatly. On a more encouraging note, these shrubs are able to regrow remarkably quickly due to their large established root systems and any dieback will help offset the effects of drought.
I hope it rains soon.
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