Black Vine Weevil Control
Description: A pest throughout most of the United States, black vine weevils (Otiorhynchus sulcatus) attack over 100 different kinds of ornamental plants, including rhododendrons, azaleas, yews and hemlocks. When weevils enter greenhouses or indoor gardens they are damaging to begonias, ferns, and other common container plants. They are particularly damaging to cyclamens and are often called the “cyclamen grub.” However, there are many preventative measures for black vine weevil control.
Adult black vine weevils (3/4 inch long) are large slate-gray to black insects that cannot fly. They have short, broad snouts, bent or “elbowed” antennae, and patches of short hairs on their wings. Adults feed at night, damaging plants as they chew small notches in the edges of leaves. During the day, they hide in soil cracks, garden debris and mulch.
Larvae cause the greatest level of injury to plants. They are small (1/2 inch long), white, C-shaped grubs that tunnel through roots as they feed. Leaves will often wilt (even when properly watered) and plants may be stunted or die. Larvae may also girdle the main stem just below the soil line.
Life Cycle: Black vine weevils overwinter as nearly grown larvae in the soil around the roots of host plants. In spring they change to pupae and begin emerging as adults. In two or more weeks (depending on temperature) they begin depositing eggs near the crowns of the host plants. Hatching occurs in about 10 days, and the tiny larvae burrow into the soil and begin feeding. One generation per year.
Control: Remove mulch and other pest hiding places from around plants and water only when necessary (adults and larvae prefer moist soil). As non-flying insects, weevils travel from plant to plant by walking. It stands to reason then, that barriers like Tanglefoot and diatomaceous earth should form the first line of defense. Black vine weevils are particularly vulnerable to attack by beneficial nematodes, especially in potted plants. Don’t Bug Me and other crack and crevice sprays can be used around windows, doors, and vents to prevent adult weevils from entering structures. As a last resort, spray plants in the evening with a botanical insecticide.
Tip: Place burlap at the bases of trees and shrubs to trap weevils that hide under it during the day.