This feeding pattern on the leaves is characteristic of caterpillars and sawfly larvae. Black vine weevil leaves more rounded semicircles at the leaf edge and does not work its way in toward the mid rib as shown on two of the leaves. A pesticide containing spinosad is effective and safer to use than many. Bonide sells something called Captain Jack's Bug Killer that seems to be quite widely available.

The correct name for the Bonide product is Captain Jack's Deadbug Brew (ah, the genius of marketing departments). Monterey also has a spinosad based product. Many of the things that cause this sort of leaf damage are nocturnal feeders so it's rare to see them. This inability to find the culprits on particularly damaged rhododendrons drove me slightly crazy last year. I sprayed this year as leaves began to unfurl and the problem has been greatly reduced.

Thanks. I have a spinosad based slug and ground insect deterrent, but the pellets obviously wouldn't stick in this case. Time for a trip to the hardware store.

BTW the odd thing is this is mostly happening on my rhododendrons that are away from the core planting area for them. I wonder if some biological control could be naturally operating in that area and doesn't "know" about the other places to find these pests. Caterpillars, in particular, I would wonder if they could be eaten by some birds. (probably why they are nocturnal, to avoid that)

Funny enough I have a cultivar called Captain Jack...and it doesn't have this problem yet.

Black vine weevil leaves more rounded semicircles at the leaf edges ...

That isn't my experience with weevils, I find nothing rounded about their damage. Jagged and irregular notches is how I would describe their feeding pattern. Rounded semi circles and damage to mid rib on my own plants would be caterpillar.

Examples weevil: https://www.google.com/search?q=weevil+damage+rhododendron&rls=com.microsoft:en-US:IE-Address&source=lnms&tbm=isch&sa=X&ei=FHl0Ve7CJYvIogT3-ILwDA&ved=0CAcQ_AUoAQ&biw=1366&bih=607

morz8: Rounded may have been a misleading term. How about jagged semicircles as shown in the photos you linked to?

davidrt28 : Are the effected rhododendrons under trees by any chance? I've found the caterpillar/sawfly larva problem to be most common in plants growing under larger trees.

It seems to no longer be occurring, without any intervention.