Are you noticing a lot of notches on the edges of your rhododendrons? Have you looked on and under the foliage trying to see just who is dining on your prized plant, only to find nothing?
More than likely, root weevils are the culprits. The adults are nocturnal feeders, and spend the day in the loose soil and mulch beneath the plant. They crawl up from the ground to feed at night.
There are several varieties that attack rhododendrons and azaleas, a couple of the most prevalent would be the obscure root weevil and the black vine weevil.
Although adult weevils cause a very noticeable characteristic notching in rhodie leaves, this damage is relatively minor compared to damage done by larvae to the plant roots. Larvae of all root weevil species are quite similar in appearance and habits. They are C shaped, legless and white with tan heads, up to half an inch in size. The larvae feed throughout the winter and spring on root systems resulting in plants that are stunted, grow poorly and may die.
In April through June, the larvae pupate and adults emerge. Adult weevils cannot fly, but walk or are carried from one location to another. Also, all adults are females capable of laying eggs. Adults are slow moving and should not be confused with swifter, predacious ground beetles.
Eggs are laid in clusters in or on the soil from June to September. The eggs hatch, and the larvae immediately wriggle though the soil to begin feeding.
Because weevils will continue to feed beyond the effective period of our treatments, we recommend that homeowners treat through September (ask a local nursery for recommended products).
Weevils can be picked from the leaves at night, if one has the patience and is willing to spend the time well after dark with a flashlight. Sticky barriers such as Tanglefoot, Tangletrap, Tack Trap or StickEm* will prevent adult weevils form traveling up the trunk to the leaves. Apply at the base of the trunk, and make sure no branches are touching the ground to prevent them from climbing up.
The easiest and most recommended defense against root weevil damage is to replace susceptible rhododendron varieties with some of the many resistant varieties now available, thus eliminating the problem altogether.