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Washington State University trained Master Gardeners receive more questions about rhododendrons than any other garden ornamental. People commonly have related problems. The plants are too dry in the summer, the sun is too strong and the leaves scorch or unpruned plants grow out of control and become scraggly and unattractive.
Other than these frequent problems, insects chewing on the leaves disturb gardeners most often. A group of root weevils of various types will chew on leaves, producing a characteristic notching that looks a little as if the plant has been attacked with pinking shear.
As I've suggested before, if possible, choose plants that have some natural protection from root weevil damage. Some that show good resistance are P.J.M. (pink), Dora Amateis (white) and many of the Yakushimanum types (pinks and white). Check with your nursery for others.
Many beautiful rhododendrons stay "clean" and resist root weevil chewing. The root weevils don't like the taste of some leaves, which helps the plant resist chewing damage. Some other types of rhododendrons have a thicker leaf edge, invisible to human observers but obvious to the root weevil. The thick leaf doesn't fit in the root weevil's jaw.
A few techniques can also help to deal with problem of root weevil chewing. The adult weevils (there are several species in this area) emerge from the ground after winter and feed on new leaves. Once the leaf has been damaged, it won't outgrow the notching. When they lay eggs, in May and June, the larvae hatch and enter the soil to feed on roots. The larvae may damage roots or chew bark at the soil level and can be particularly troublesome on plants in containers.
Handpicking the adults off the plants is one way to reduce the population. They feed at night. Look after dusk with a flashlight, and drop the small brown adults into soapy water. Any adults that are killed now will reduce the later populations. Some people spread sheets under the plants and shake hard, the dispose of the fallen weevils. The adult weevils will climb up to the new spring growth, gnawing on it.
Wrapping the trunk of the plant with nontoxic sticky substance such as Tanglefoot will sometimes help to catch the critters as they emerge from the soil and start up the plant for their leaf dinner. Wrap some clear plastic cling wrap around the trunk of the shrub, then spread the Tanglefoot over the wrap so it isn't directly on the bark. The wrap should be tight enough so beetles can't crawl under it. Other gardeners have also tried teflon tape wraps, about 4 inches wide, which tend to be so slipper that the beetle falls to the ground. These two methods work only if the plants are not touching the ground or other bushes, which provide alternate routes up.
A biological control, the beneficial nematode, has been tried in the last five years or so with some success in some locations. The nematodes are microscopic roundworms that infest and kill the weevil larvae in the ground. Make applications now that the soil has warmed. The product works only when the soil is both moist and above 55° F, sot his means applications in late spring and all the way up to mid-fall.
The nematode moves on a film of water. If you choose this method of root weevil control, do not allow the soil to dry. Keep it evenly moist. These are living organism, sold under several different trade naes, including ScanMask. Mix with water that is cool or lukewarm, under 95° F (they are killed by both cold and too much hear). Apply when they wont be exposed to direct sunlight or low humidity (early morning or late evening is best).
Nematodes are not harmful o people or plants when used properly and are effective against the root weevil larvae in the soil. They don't effect the adults.
Hortsense: Managing plant problems with Integrated Pest Management
