Answer: Rhododendrons have such thick, leathery leaves that most insect pests don't bother them. The one exception is a weevil. When weevils feed on rhodie leaves they do so at the margins of the leaves and the leaves look notched. If the new leaves, those at the ends of the branches are healthy but the older leaves have holes in them, it is likely there was a fungal disease. Older leaves have numerous fungi that attack them once they get old. Often these fungi cause spots in the leaf. When these spots die and fall out they leave holes. The fungi will not attack the younger leaves. If you'd like some one to physically diagnose the problem with your rhodies take a few of the affected leaves to your local Cooperative Extension office. (You did not indicate your locale so I can't direct you to the nearest office - sorry.)
Best wishes with your rhododendrons!
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