Black vine weevil—Otiorhynchus sulcatus
The black vine weevil feeds on many garden and landscape plants. Hosts include azalea, caneberries, rhododendron, euonymus, grape, liquidambar, and strawberry.
Adult weevils are hard-bodied, flightless, snout beetles (Curculionidae) about 1/2 inch long. They are black with small patches of white scales on the forewings. Adults have elbowed (sharply bent), clubbed antennae and their head and mouthparts are elongated. Adults spend the day hidden beneath loose bark, and in litter on the ground and topsoil. Adults emerge at night to chew and feed on foliage. Note that several other common weevils resemble black vine weevil and they have similar biology.
Eggs are round and pale orangish. They occur in soil around the base of plants.
Larvae are oblong and whitish to pale yellow with a brown head. They grow to about 1/2 inch long and occur hidden in soil, feeding on roots and the basal stem. Larvae are legless and when disturbed or exposed commonly have a C-shaped posture.
Black vine weevil develops through four life stages: egg, larva, pupa, and adult. Overwintering is primarily as late instars (older larvae) or prepupae. A few individuals may overwinter as adults. Weevils overwintering as late instars pupate in late winter to spring.
Adult weevils emerge in spring and chew and feed on the foliage of host plants, mostly at night. Adults do not fly, but may walk to new plantings from nearby infested garden plants, ornamentals, and unmanaged vegetation. Peak adult populations and leaf chewing occur in summer and early fall.
Female weevils feed for several weeks before laying eggs. About 4 weeks after adults emerge and begin chewing foliage, eggs are laid in soil near the base of plants. A single female can lay as many as 500 eggs. Eggs hatch into larvae that develop through 6, increasingly larger instars as they feed underground over a period of 8 to 10 months. There is generally 1 generation per year.
Adults chew and feed on foliage. Host leaves or flowers can become notched or ragged, and leaves or needles may be clipped from twigs. Where bud break coincides with adult emergence, a high percentage of primary buds and new shoot tips may be consumed.
The most serious plant damage is done by larvae, which chew on basal crowns and roots. Larval feeding can kill or weaken certain hosts, especially azalea and rhododendron. Feeding by larvae can also increase damage from root diseases such as Phytophthora root rot.
If planting azalea or rhododendron, select cultivars that are less susceptible to weevils. Check roots of new plants before planting to make sure they are free from larvae. Provide a good growing environment and proper cultural care to keep plants vigorous and better able to tolerate insect feeding.
Handpick and destroy adults beginning in spring to reduce the later abundance of larvae and their feeding damage. For example, where leaf notching is observed, after nightfall shake or branch beat plant parts over a sheet or other collecting surface; gather and crush or dispose of any dislodged weevils.
Trapping can reduce adult abundance. One strategy is to wrap trunks overnight with burlap or corrugated cardboard, then during the day remove and inspect wraps and dispose of any weevils that sought shelter there. Pitfall traps such as cups buried in soil near the base of plants can also be used.
Trim branches that provide a bridge to other plants or the ground and apply a 6-inch band of sticky material (e.g., Tanglefoot) to trunks to prevent the flightless beetles from climbing trunks to feed on foliage. First wrap the trunk several inches wide with landscape fabric or plastic, then apply the sticky material on top the wrap to avoid potential bark injury from the sticky material. Periodically replace wraps to prevent growth-restriction damage to trunks.
Parasitic (entomopathogenic) nematodes (e.g., Heterorhabditis or Steinernema spp.) can be drenched onto soil beneath infested plants to reduce larval abundance if soil is well drained (e.g., high in organic matter or sandy) and not compacted. Apply nematodes when weevil larvae are expected to be present in midsummer to fall. During hot weather, apply nematodes in the early morning or evening. Soil must be warm (at least 60°F) and moist (well irrigated) but not soggy before applying nematodes and for 2 weeks after application. If warranted, irrigate every 2 to 3 days after applying nematodes to soil.
If damage has been intolerable, monitor plants in spring. About 1 week after adult chewing is first observed, spinosad can be sprayed on leaves in the evening; because of its short persistence, repeat application 7 to 10 days later may be warranted. If plants are especially susceptible to damage, a more persistent insecticide (e.g., carbaryl) can be applied. If weevil emergence is prolonged and new leaf chewing continues to appear, a second foliar spray may be made about 3 to 4 weeks after the first application. Applying a systemic insecticide such as imidacloprid to soil or trunks can control both adults and larvae.
For more information and photographs see Black Vine Weevil-Controlling a Major Nursery and Landscape Pest (PDF) from the University of Maryland and Grape-Black Vine Weevil from Pacific Northwest Extension.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).