MG Volunteer: “OSU Extension Master Gardener volunteer. How may I help you?”

Caller: The leaves of my rhododendrons are being eaten. We’re getting to the end of June, and now there are notches appearing on the leaves.
MGV: Where are the plants located in your yard? What is your soil type?
Caller: They are all in a shady or partly shaded areas that are close to the house. My soil is just Willamette Valley clay.
MGV: What method of irrigation do you use on the plants?
Caller: They get watered with the lawn.
MGV: Is there any damage on other parts of the plants? (Bark, roots or lower stems)
Caller: I do see some kind of chewing on the lower stems.
MGV: Have you seen any insects or mites on the plants?
Caller: I went out with a light one night and saw some black insects.

Diagnosis:

Weevils--strawberry root weevil, black vine weevil, obscure root weevil. Root weevil emerge as adults in mid-summer and begin to feed on plant foliage and begin to lay eggs 3-4 weeks later. As larvae emerge, they burrow into soil to feed on roots. More information can be found at this website

Recommendations:

Cultural: Add parasitic nematodes in soil below to limit larvae. Add when temps. are above 52 degrees. Crop rotation (if possible). Handpick with flashlight at night. Trim leaves touching ground to limit access.

Mechanical: Use sticky Tanglefoot-type product to trap weevils before they can crawl up to notch leaves.

Chemical: Refer to PNW Insect Management Handbook. Treatments are available only for adult insect with foliar applications.

(Thanks to the Benton County Master Gardeners for developing this scenario)