How to Manage Pests

Pests in Gardens and Landscapes

Weevils

Adults are flightless and dull gray, blackish, or brown, 0.13 to 0.38 inch long, with curved snouts and elbow-shaped antennae. Because they feed at night and often hide in litter during the day, adult beetles are often not observed. Larvae are whitish, pinkish, or green grubs that live in soil and feed on roots or crowns. Mature larvae are up to 0.38 inch long, C-shaped, and legless.

Identification

There are more than 1,000 species of weevils or snout beetles in California. The most common pest weevil species in California is the black vine weevil, Otiorhynchus sulcatus. Other important species include the cribrate weevil, Otiorhynchus cribricollis, fuller rose beetle, Asynonychus godmoni, obscure root weevil, Sciopithes obscurus, strawberry root weevil, Otiorhynchus ovatus, vegetable weevil, Listroderes costirostris, the woods weevil, Nemocestes incomptus, the rough strawberry root weevil, O. rugosotriatus, and the Douglas-fir twig weevil, Cylindrocopturus furnissi.

Life cycle

Peak adult weevil populations occur in summer and early fall, and some may overwinter. Female weevils may feed for an extended period before laying eggs, and many species produce viable eggs without mating. Eggs are laid in soil near the base of plants about 4 to 6 weeks after adults have emerged. A single female may lay as many as 500 eggs. Eggs hatch into legless, white grubs that feed on roots. The larvae develop in soil through 6 instars over a period of 2 to 8 months. They are whitish grubs with a brown head and commonly have a C-shaped posture.

Black vine weevil overwinters primarily as a late-instar larva. A few individuals of this and other species can overwinter as adults. Weevils overwintering as late instars form pupae in spring. Adult weevils emerge in late spring or summer and feed on the foliage of host plants at night. Adults do not fly, but may crawl into new plantings from nearby native plants, ornamentals, blackberries, or second-year strawberries

Damage

Adults generally feed on foliage. Leaves appear notched or ragged and may be clipped from twigs. The most serious damage is done by larvae, which feed on roots and can destroy root hairs and chew the bark and cortex off larger roots. Some may tunnel into the crown and destroy crown tissue. In herbaceous plants, damage is usually first seen as wilting and dying plants in a localized area of the garden.

Solutions

Destroy adults to prevent more serious damage. Grow species or cultivars that are less susceptible to weevil damage, and avoid replanting susceptible crops at infested sites. Grow older plants that are more likely to be infested away from younger plants susceptible to weevils. Provide cultural care to keep plants vigorous and better able to tolerate damage. Check roots before planting to make sure they are free from larvae. On trees, trim branches that provide a bridge to other plants or the ground and apply a 6-inch band of sticky material to trunks to prevent flightless beetles from feeding on foliage. On strawberries, destroy all affected plants, then move outward in a circular pattern, removing plants (even if they are healthy looking) that are infested with larvae and pupae until you no longer find weevils. Replant if necessary after working the soil well and solarizing or treating with commercially available parasitic nematodes when larvae or pupae are expected to be present (in midsummer to fall). Soil must be warm and moist. Trapping adults may help or you can apply a barrier around the bed made of tarpaper and coated with Stickem or Tanglefoot about 18 inches high and buried 2 to 3 inches into the soil to keep adults out. No effective insecticides are available for larvae.

Black vine weevil adult
Black vine weevil adult

Root weevil pupa (left) and larva (right)
Root weevil pupa (left) and larva (right)

Feeding damage on viburnum
Feeding damage on viburnum by adult weevils

Root weevil damage to strawberries
Root weevil damage to strawberries


Statewide IPM Program, Agriculture and Natural Resources, University of California
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