Annual root weevil assault is coming

One of the most widespread pests of ornamental shrubs and flowers is about to make its appearance in gardens throughout Oregon. Late May through early June is prime time for adult root weevils to begin their assault on more than 100 varieties of plants.

Among their favorites are rhododendrons, azaleas, raphiolepis, peonies, photinia and strawberries. Although several different species of weevil affect our plants, we can control them all using the same measures.

Perhaps the only good thing about dealing with root weevils is that the damage caused by the adult is very easy to recognize and, once seen, is unlikely to be attributed to any other pest. Unless you are out in the garden at night, it is unlikely you'll ever see these pests. They are nocturnal feeders, seldom venturing out of their daytime habitat in the leaf detritus at the base of plants except on dark, overcast days. Their feeding damage is confined to the edges of leaves, beginning with just a notch or two, coalescing into the entire outer edge if the infestation is severe. It can appear as if the leaf had been trimmed with a pair of pinking shears.

Although this damage is unsightly and may weaken the plant, the real damage is done underground by the larvae of this insect. Hatching from eggs laid at the base of plants, the young begin feasting on tender rootlets. In severe cases, stunting or plant death can result. Control measures should begin at the first signs of feeding on this year's foliage, which signals that adults have pupated and are beginning to lay eggs.

Limiting the number of egg-laying adults will result in fewer larvae attacking the roots. I do not recommend the spraying of harsh chemicals like Orthene to control them. It is a systemic insecticide that is very effective against root weevils. Unfortunately, it also is very effective against beneficial insects, too. Instead, try using Bacillus thuringensis var. San Diego (BTSD) to control the adult weevils. In addition, Neem, another organic-approved product, can offer protection against adults.

Adult weevils do not fly. Access to plants is gained by crawling up plant stems or leaves that touch the ground, other plants, or structures. By pruning foliage up off the ground, by keeping plants from touching each other or rubbing against fences or buildings, we limit availability of foliage for adults to eat. Lacking a food source, they will likely move on to find a meal elsewhere.

Adults can be trapped as they try to climb the trunks of trees and shrubs. Sticky barriers such as "Tanglefoot" 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. Do not apply directly to the plant or damage may occur with prolonged use. A strip of polyethylene or waterproof tape can be fitted to the stem and the sticky material applied to it. Make sure you do not constrict plant growth with any materials.

Traps can be made from burlap or corrugated cardboard 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 cardboard 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 in soapy water.

Beneficial nematodes are a biological control that is a relatively new pest control product. They are effective against a number of serious pests that live in the soil, including weevil larvae. The nematodes are in the genera Steinernema and Heterorhabditis. They are available locally at Lady Bug Indoor Gardens. Nematodes do require specific conditions to be effective. Those who have used nematodes stress the importance of following the directions supplied with the product, particularly concerning soil temperature and soil moisture levels.

Combining several methods of control always produces better results than relying on a single technique. Weevil control requires vigilance, proper timing and persistence. Now is the time to be on the lookout for their emergence. Good luck!

Stan Mapolski, aka The Rogue Gardener, can be heard from 9-10 a.m. Saturday mornings on KMED 1440 AM and seen in periodic gardening segments for KTVL Channel 10 News. Reach him at

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