Black vine weevil, also called Taxus weevil, is most effectively managed by controlling the adults soon after emergence. This occurs when Vanhoutte spirea finishes bloom, which is occurring in central Illinois and is about to occur in northern Illinois. Treatment should still be effective in southern Illinois.
Black vine weevil larvae feed on the roots of yew (Taxus), strawberry, hemlock, rhododendron, and other plants. They tunnel through larger roots and eat smaller ones. This root pruning results in a slow-growing nursery plant and may kill the plant in severe infestations. Slower-growing plants in a nursery are a liability because they delay realized income from sales and take up acreage that could be used for growing more plants. Black vine weevils are common in landscape situations but usually do not cause decline of the plant. Although declining or dead landscape plants usually have a reduced root system due to larval feeding, nearby plants in good health have similar root injury. Typically, declining plants are due to additional stresses, such as too much water or compacted soil, with these plants frequently being near a downspout or footpath.
Fully grown larvae are about 3/8 inch long, thick-bodied, white, and legless. They pupate underground close to the soil surface and emerge as charcoal black, 3/8-inch, hard-shelled beetles with scattered yellowish spots on the wing covers. The head is elongated into a broad muzzle. Adults hide in debris below the shrub during the day, emerging to feed on the leaves at night. Only females are known of this insect, but they have to feed for 2 to 3 weeks before they are able to lay eggs.
While feeding on euonymus, yew, and other plant leaves, they are susceptible to insecticide applications. Damage on the leaves appears as notches on leaf margins. In yew, most damaged leaves are those inside the shaded canopy rather than on the surface, so you have to look inside the canopy to detect damage. Insecticides recommended for the adults are acephate (Orthene), bifenthrin (Talstar), and cyfluthrin (Tempo). The foliage should be sprayed thoroughly, allowing a significant amount to fall to the ground beneath the plant, where additional control will be obtained on the hiding beetles.