This week, I was weeding my garden and noted considerable notching caused by root weevils on the leaves of various plants.
Root weevils are a common pest in area home gardens, but the adult weevils are seldom brought to the Extension office for identification. That is because they are nocturnal, feeding late at night and hiding during daylight on the undersides of leaves or beneath debris on the ground.
However, you do not have to see one of the critters to know that they are at fault. Root weevil feeding causes characteristic scalloping or small semi-circular irregular notches along the leaf margins.
While some insects feed only on certain species of plants, roots weevils are not selective. In home gardens, they tend to show a preference for rhododendron, lilac, forsythia, peony, rose, euonymus, Japanese holly, blueberries, and strawberries, but they will chow down on more than 100 other species of plants. Along with distinctive notching, another sign of its presence are black fecal deposits found on the undersides of notched leaves.
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Because there are at least 16 species of root weevils found in Washington, adult weevils vary in size from 1/8- to a half-inch long, and in color, from black to brown or gray. Based on research by Sharon Collman, Washington State University Extension entomologist, black vine, obscure and strawberry root weevils are common in Western Washington, but the lilac root weevil is dominant in Eastern Washington landscapes and gardens.
So what makes a weevil a weevil? Weevils are a specialized type of beetle. They are somewhat oblong in shape and have hard, crunchy outer wings.
What sets them apart from beetles is the elbowed antennae and specialized mouth parts that look like an elongated snout. Also, the inner wings of most weevils are not well developed, leaving them unable to fly.
So what makes a root weevil a “root” weevil? The creamy-white C-shaped legless grubs, or larvae, of root weevils eat plant roots. They start by feeding on fine roots and then may move onto larger roots, and even the crown or base of plants. Leaf notching caused by adults is primarily just cosmetic damage, but heavy feeding on the roots by larvae can kill plants.
One control strategy is to avoid introducing root weevils into your garden and landscape by inspecting plants before you buy, and avoiding any with the characteristic notching. Root weevils cannot fly. To get from here to there they have to walk or hitch a ride on infested plants, soil or plant litter.
Once root weevils get started in a yard or garden, it is hard to get rid of them. There are some home garden insecticides available for control of the adults, but generally these materials are not effective. If you do try chemical control, apply sprays at night between nightfall and midnight when the adults are feeding.
WSU Extension recommends gardeners manage root weevils by hunting them after dark. No, I am not kidding. Place sheets or box tops under your infested plants and then go out late at night and shake the branches. The weevils will drop onto the sheet, where you can collect and dispose of them. Do this on successive nights until you are not getting any adults dropping from plants. (You may want to warn your neighbors about this so they do not call the police.)
Another method of control is the application of beneficial nematodes to the soil when the larvae are present. The nematodes are applied as a drench to moist soil when the soil temperature is above 55 degrees. Because of climatic conditions in our regions it may be difficult to effectively control root weevils with nematodes, but some local gardeners indicate they have worked for them.
For more information on the biological, cultural and chemical control of root weevils, go to bit.ly/root_weevil.
Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.