The Q&A Archives: Rhododendron weevils (?)

Question: Several of my rhododendrons have weevils (or whatever it is that makes the holes in the leaves). How can I get rid of them? (I can't pick or spray them off with water because of the location --- steep hillside) Is there a spray or insecticide that would work?

Answer: Root weevils are crawling insects that notch the leaves of rhododendrons and other ornamental plants. While this causes an unsightly appearance, more damage is done by the larval stage of the weevil, while they are underground feeding on roots. The larvae hatch into adults in the summer. They then feed and lay eggs and become larvae again. Going untended, root weevil larvae can eventually cause enough damage to the roots to cause the plant to die.

There are over a dozen different types of root weevils native to our part of the world, which has different life cycles. The most common are the black vine or strawberry root weevil, which hatch one or two times a year. They live in the forest munching on salal, cedar, berries, rhododendrons and other native plants.
You can use beneficial nematodes to control the larvae of root weevils. Beneficial nematodes are microscopic predators that feed on the larvae of many insects: weevils, fleas, cutworms, sawflies, etc.

The soil and weather conditions must be right for beneficial nematodes to be effective. Briefly: the soil must be 50-52 degrees F (depending on what strain of beneficial nematode you use). This usually occurs at Meerkerk Gardens anywhere from late April to early June. Nematodes can be applied again in the autumn, while the soil is still warm and moisture is present, via rain or watering.

Or, you can use sticky barriers such as Tanglefoot, Tangletrap, Tack Trap or StickEm to prevent adult weevils from traveling up the trunk to the leaves. These barriers must be applied to the trunk so there are no unprotected avenues of travel up the stalk. The weevils either will choose not to travel up the stem or will become stuck in the barrier and thus cannot move into the canopy to feed. There are some indications that prolonged use of these materials can be somewhat damaging to the bark and stem, so a strip of polyethylene, waterproof tape or thin plastic can be fitted tightly to the stem and the sticky material applied to it. Plastic tape, etc., must be removed before stem growth is restricted.

Teflon barrier tape such as SureFire Insect Barrier Tape can be used in place of sticky barriers. The tape must be wrapped snugly around the trunk with the sticky side adhering to the bark leaving no avenue for the weevils to crawl under it. Make sure that you do not constrict plant growth. Another disadvantage of these products is the fact that the tape is white and so is visible on the plant.

Traps can be made from burlap or coiled corrugated paper and placed under plants. Take burlap and make folds in it as you place it at the base of the plant. The weevils will hide in the burlap or paper during the day and can thus be trapped. Before the weevils move into the plants in the evening, the traps can be removed and the weevils destroyed.

Hope these suggestions are helpful.

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