Now is the time of year to implement control of black vine weevil, Otiorhynchus sulcatus, which is also referred to as the taxus weevil (I guess this is because it feeds on taxus!!). This is when the use of insecticides is most effective and reduces plant damage. In addition, this may reduce the larval population by killing adult females before they lay eggs. Although black vine weevil attacks landscape plants, it is primarily a pest of nursery stock.
Black vine weevil larvae are primarily responsible for plant damage as they feed on the roots of plants including arborvitae, euonymus, hemlock, strawberry, rhododendron, and yew. The larvae chew the bark of mature roots, feed on smaller roots, and may tunnel into large roots. This root pruning leads to plants either stunted or growing very slowly. Extensive larval infestations can result in plant death. Slower-growing plants may take longer to reach a salable size, thus increasing the costs associated with production in nurseries. In addition, heavily damaged roots due to larval feeding may cause plants to die after transplanting. Black vine weevil is common in landscapes, but they generally do not cause significant plant damage. However, small or newly planted trees or shrubs may be damaged or take longer to establish when fed upon by high larval populations. Although black vine weevil may not directly kill a plant, it may cause enough stress, thus increasing susceptibility to opportunistic wood-boring insects.
Female black vine weevils (only females are known of this species) lay eggs in the soil, which hatch into white, legless larvae that when mature are 3/8 in. long, with a brown head capsule. Larvae are typically found near plant roots and are active from midspring through fall. The larva then enters a pupal stage and locates near the soil surface. Adults, which are 3/8-inch long, and black with yellowish spots on the wing covers, emerge in the spring. The mouth is unique and is best described as a “snout,” which is used to feed on leaves. Adult black vine weevil damage is very distinct, as they tend to feed on the edge of leaves or leaf margins, creating crescent-shaped notches. Adults are active only at night (nocturnal), hiding during the day on plant stems, in mulches, or in debris below plantings. Females feed for 2 to 3 weeks before they lay eggs. Females are capable of laying up to 500 eggs over a 2-to-3-week period. Feeding damage to yew (Taxus sp.) typically occurs in the lower canopy or interior foliage, which is not noticeable until branches are spread apart. Black vine weevil adults cannot fly, as the elytra are fused together. Adults may also enter homes (and even greenhouses) and feed on houseplants or herbaceous annuals. The larvae are the overwintering stage, and there is one generation per year in Illinois.
When the adults are feeding on plant leaves, they are exposed, thus increasing their susceptibility to insecticide applications. Insecticides recommended for control of black vine weevil adults include acephate (Orthene), bifenthrin (Talstar), and cyfluthrin (Tempo). Thorough coverage of all plant parts is important for maximum control. In nurseries, soil drenches of bifenthrin (Talstar) and/or the entomopathogenic nematodes, Steinernema feltiae and Heterorhabditis bacteriophora, have been shown to be effective. The use of nematodes is more appropriate in nursery containers, as opposed to field-grown plants.
In established landscapes, it is generally not necessary to treat for black vine weevil. One simple way to avoid problems with black vine weevil is by installing a diversity of plants, that is, avoiding massive plantings that are susceptible to black vine weevil.