Hi and thanks for your great picture and question about the chew marks on your rhododendon leaves. This is classic rhododendron root weevil work. You probably won't see the as they live in the soil and climb up and feed on the leaves at night.

You will probably want to look for a product to apply to the soil that contains one or more of the following ingredients:
carbaryl
clothianidin S
imidacloprid S
pyrethroids P
pyrethrins O
spinosad O
The ones with O are organic, the ones with S treat the whole system of the plant and the P means it is very broad and will affect other insects. There are specific rhododendron and azalea products for root weevils, so try to find one of those.

It's a beautiful plant!