Consider night attacks on controlling root weevils

KENNEWICK -- I was talking with a Master Gardener the other day about the root weevil problem in his little blueberry patch. Root weevils are a group of specialized brown or black snout-nosed beetles that attack a variety of ornamental plants.

Root weevil damage is characterized by notching of plant leaf edges. (They don't eat holes in leaves.)

Generally, this notching is a cosmetic concern and seldom does significant injury to a plant, but when their numbers are extremely large they can damage plants.

The weevil's white C-shaped, legless larvae develop in the soil, feeding on the plant roots, root hairs, and the root crown at the base of the stems. The larvae can cause serious damage in potted nursery plants and in very sandy soil.

Most gardeners only note the notching damage and seldom see the actual weevil adults. That's because during the day the adult weevils hide out in the soil and debris at the base of plants. After dark they come out and climb up the plants to feed on the leaves.

With the demise of many harsh garden pesticides, root weevil control has been particularly difficult to achieve, especially on food producing plants like the blueberries. That's why the Master Gardener has been trying to collect them off his plants on a nightly basis to reduce their numbers.

Because adult root weevils will drop from the plant and "play dead" if disturbed at night when feeding, he was able to spread a cloth under his plants to collect the adults. After dark he went out at midnight and shook the plants. However, he encountered a problem. No weevils dropped off at that time. However, when he went out at 2:30 a.m. he was able to collect 15 per plant. He did this for several nights in a row, collecting about 15 weevils each time. He stopped when the number dropped to just one or two.

Curious about root weevil behavior, I called WSU Extension's resident root weevil expert, Sharon J. Collman. She has done extensive research on these creatures for her PhD thesis.

Collman found 14 species of root weevils in the Northwest during her 450 nocturnal collections of weevils. The black vine weevil (Otiorhynchus sulcatus), the obscure root weevil (Sciopithes obscurus) and the strawberry root weevil (Otiorhynchus ovatus) are the most commonly known root weevils that she found. She also found that the lilac or privet weevil (Otiorhynchus meridionalis) was the most common and often dominant species in Eastern Washington gardens and landscapes.

Collman notes that research on the effectiveness of specific insecticides for control of root weevil adults is limited, with 60 percent of the studies researching the control of black vine weevil and only a limited number of studies (0 percent to 20 percent) looking at the other 13 species found in our region. Some studies indicated that pesticide efficacy differed from species to species. This points out that what works in one region, may not work in another.

WSU recommends using acephate, cyfluthrin, neem oil, befenthrin and azadiractin to control root weevils on ornamental plants such as lilac and rhododendron. They recommend malathion for control on berries, such as blueberries.

However, depending on the actual type of root weevil doing the damage, you may find these are not extremely effective. It might come down to setting the alarm clock and going out in the dead of night to shake the weevils from your plants.

* Marianne C. Ophardt is a horticulturist for the Washington State University Benton County Extension.

This story was originally published July 30, 2011 at 3:00 AM with the headline "Consider night attacks on controlling root weevils ."