New leaves are coming in healthy, so not to worry, even if it is a bit unsightly.

Yes! I did spray during the day! Thanks for your replies & advice! :)

Just curious.. did the borers drill thru the stems of the hydrangea?

I have a tree that these pests drilled through (and yes, I had a hydrangea besides the tree) and, based on the resulting damage to the tree, I would have preferred that they drill the hydrangea and not the tree. Sniff, sniff. Oh well.....

Yes... they drilled down through the stems and in the wooden sticks down at the bottom of the plant... in the part inside that looks kind of like foam. On the leaves I noticed what looked like little piles of sawdust -- then I figured out it was coming from the stems/sticks and was falling down on the leaves. When I started really looking -- the holes were everywhere!! :(

Right, the sawdust is hard to see at first and, once you notice the sawdust, they have caused a lot of damage. And that is assuming there is only one borer.