Hi Sylvia,

It can be difficult to determine what is eating your plant without seeing an insect. Whatever is eating your plant is eating at the edge of the leaf. Rhododendron are noted as a host plant for the Columbia silk moth – so that is a possibility. You could look on the undersides of the leaves for caterpillars. 

Your rhododendron looks stressed. It’s color is very light, which could be chlorosis. This is an iron deficiency caused by plant being unable to take up enough iron from the soil due to the pH.

Testing your soil will let you know your pH level and help to know how to treat for the overall health of your plant. You can order a test here.

Overall, the soils in Southeast Michigan aren’t conducive to rhododendrons. This article discusses soil acidity and rhododendrons.