It sounds as though your rhododendron is being munched by black vine weevils. This blackish-brown insect is about 1-2 inch long. It feeds on more than a hundred different kinds of garden plants, including its favorites, rhododendron, hemlock (Tsuga), and yew (Taxus) It chews half-moon notches in the edges of the leaves. Adult weevils feed at night. They don't fly, but can crawl from plant to plant. Larvae feed on plant roots and occasionally on stems. If stems are girdled, the plant may be killed. This insect is difficult to control because adults are active at night, and larvae develop underground, protected from insecticides. No insecticides are labeled for controlling larvae, but adults may be managed by spraying plants with an insecticide in late spring. You may also be able to reduce larval populations by cutting back on irrigation to keep the soil drier.