KENNEWICK -- Tomato hornworms gorge themselves on tomatoes.
Colorado potato beetles devour potatoes.
The corn earworm feasts on corn.
Root weevils nosh on more than 100 plant species including lilac, strawberries, peony, rose, raspberry, phlox, rhododendron, azalea, cotoneaster, Euonymus, clematis, yew and more.
The numerous small (no more than one-eighth inch wide) notches are characteristic of the feeding damage caused by these snout-nosed beetles, commonly called root weevils. They feed only on the edges of leaves because their snouts gets in the way, making it impossible for them to eat holes out of leaves.
Root weevils are nocturnal feeders, so these culprits are usually not found when sought during the day. If "seeing is believing," you can go out on a summer evening after dark with a flashlight to look for them. During the day the root weevils hide under mulch, plant litter or in crevices in the soil.
There are numerous different types of root weevils, but the two most commonly found in our area are the black vine weevil (BVW) and the strawberry root weevil. The adults of both have a shiny brownish-black to black oblong body. Their head elongates into a snout and their abdomen is covered by hard wing covers with parallel ridges. The BVW is up to a third-inch in length, but the strawberry root weevil is only about a quarter-inch or less in length.
The notching caused by adult root weevils is often considered only cosmetic, not causing significant harm to the plant. However, a large population of root weevil larvae do injure and kill young woody plants by feeding on the roots and the bark of stems close to the soil.
If you want to manage a root weevil problem without traditional chemical pesticides, you can use several approaches:
-- Go out after dark and hand pick the adults off the plants and drop them in a container of soapy water or try placing a white cloth beneath each plant and gently shaking the branches. The root weevils will drop onto the cloth.
-- Trap root weevils by tying corrugated plastic or cardboard (with the grooves facing inward) around stakes and laying them in the garden. They will hide in the corrugation groves during the day. Open up the traps and dispose of the root weevils regularly.
-- In some areas, parasitic nematodes applied as a drench can be used to manage a root weevil problem, but they work best when applied to very wet soils in late summer or early fall. Experts are doubtful that success can be achieved with these nematodes in our region.
Earth-friendly bio-insecticides include Beauveria bassiana (an insect killing fungus), azadirachtin (an extract from the neem tree that disrupts insect feeding), and spinosad (a soil bacteria byproduct). Chemical pesticides that will work include acephate, cyfluthrin, bifenthrin and malathion. If applying materials to food crops such as strawberries, be sure the crop is listed on the pesticide label. Not all materials mentioned here are safe and labeled for use on food crops.
To be effective, bio-insecticides and chemical insecticides should be applied as a spray when new foliar notching is noticed when the adults first emerge in early summer.
The weevils feed for three or four weeks after they emerge before they start laying eggs. Each BVW female can lay 200 to 500 eggs and all BVWs are female! Applying controls early in the season will help reduce a potentially burgeoning population.
* Marianne C. Ophardt is a horticulturist for the Washington State University Benton County Extension Office.