One of the most destructive and widespread root weevils in the northern U.S. and Canada, the black vine weevil, can cause damage to susceptible plants at both the adult and larval stages of its life cycle. Feeding on a wide range of herbaceous and woody ornamentals, the black vine weevil does the most damage to yews, and is often called the Taxus weevil. It will also attack strawberry, blackberry, blueberry, and cranberry plants, as well as azalea, rhododendron, hemlock, forsythia, and euonymus. Other plants prone to infestation include begonia, cyclamen, fuchsia, impatiens, primrose, and sedum.
Adults are black to brownish-black, hardshelled beetles, generally 3/8 inch long, with ill-defined yellowish spots. Larvae are C-shaped grubs, white with brownish heads, approximately 1/2 inch in length when fully developed. The weevil overwinters in the soil as full-grown larvae. The following spring, after a short feeding period, the larvae pupate into adult beetles. The adult black vine weevil is a nocturnal creature that appears in the Chicago region during June and early July. During the day, the weevil hides in dark places on the stems of dense plants or in the ground litter and mulch beneath the crown. At night, the weevil feeds on foliage, cutting a characteristic U-shaped notch in the leaf margins.
Eggs are laid, without fertilization, during July and August in the soil beneath the host plants on which adults have been feeding. There are no male weevils. As many as 500 eggs are laid over a 2-3 week period. Only one generation of weevil occurs per year. Once the eggs hatch in mid-summer, the larvae work their way through the soil at the base of the host plant and feed on the small roots and the bark of larger roots. This feeding takes place from the time the eggs hatch through the fall.
The damage produced by the adult weevil is usually not serious, and is most recognizable by notched or crescent-shaped scars along the edges of the leaves or needles. It is in the larval stage that the weevil is most destructive by consuming tender feeder roots. The foliage of infested plants will turn yellow or brown and can wither or otherwise show signs of poor growth. If left uncontrolled, a severe infestation of the black vine weevil may ultimately kill the host plant. On yews, notched needles are positive diagnostic symptoms of the black vine weevil since other weevils do not feed on yews. Examine the needles at the center of the plant near the main stem. If there are many notches on most of the plant foliage, it is likely that the roots are also severely damaged by larval feeding. The black vine weevil has fused wings and can only move from plant to plant by walking or by being carried in infested plant containers or root balls. Therefore, infestation is relatively slow to spread.
Management of the black vine weevil is possible if symptoms are recognized and treated early. However, as with many other garden pests, the black vine weevil has developed a degree of immunity to many pesticides that were once effective. Often physical or biological controls are the most efficient means available to the home gardener.
A surprisingly simple way to control black vine weevils is to knock them off the plant, collect, and destroy. This is best accomplished by putting a white cloth at the case of the plant (because the weevil’s protective coloring makes them hard to spot on the bare ground), then gently shake the plant to dislodge the weevils. Finally, transfer the weevils from the cloth to a jar of soapy water to kill them.
A second technique is to affix cardboard strips covered with a sticky substance, such as Tanglefoot™, to the stems and trunks of susceptible plants in early spring before adult weevils emerge.
Good garden sanitation is an effective control, as weevils live in dead plants and ground litter. Cut away the dead foliage of any infested plants to prevent overwintering sites. Discard infested plant material, including the soil around the root system (do not compost). Weevils also feed on various weeds such as cocklebur, Joe-pye weed, morning glory or bindweed, ragweed, and thistle. Eradicating these weeds from your lawn and garden will remove an important food source.
Encouraging animal and insect parasites is still another effective control. Animal predators include bluebirds, warblers and wrens. Insect parasites, such as beneficial nematodes, are effective against larvae that may be more resistant to pesticides. These can be released into the garden in early June for best results.
Chemical management be applied to foliage in mid-May to early June. Contact the Plant Clinic (630-719-2424 or firstname.lastname@example.org) for current recommendations. Use pesticides safely and wisely; read and follow label directions. The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.