Description of the Pest
The black vine weevil is primarily a pest in central coast vineyards. The adult is a hard-shelled black beetle about 0.5 inch long with small patches of white scales on the forewings. A long and broad snout, typical of weevils, projects from the front of the head. In coastal areas adult emergence generally starts in early April and continues through May. About 2 to 3 weeks after they emerge, females begin laying eggs and continue for 6 to 8 weeks. Eggs hatch into white grubs (larvae) that feed on roots. Larvae feed for about 10 months before pupating in the soil during late winter.
Adults are nocturnal, feeding on buds, foliage, flowers, and the cluster rachis. Significant bud damage can occur on late budding varieties. Foliar feeding is characterized by notching along leaf margins. Larvae feed underground on roots but do not appear to damage the vines.
Weevils move from under loose trunk bark and the soil up to the vine canopy and back, so management measures target the vine trunk and the soil surface surrounding the trunk. Among cover crops, creeping red fescue supports black vine weevil larvae populations while oats, vetch, and alfalfa do not.
Adult emergence is monitored with a corrugated cardboard trap. Strip loose bark from a vine and wrap an 8- to 10-inch wide corrugated cardboard "tree wrap" around a trunk, cinching it in the middle with a plastic tie to hold it in place. Weevils will hide in the wrap's corrugations during the day. From mid- to late March, inspect the corrugations twice weekly to detect for first emergence. Thereafter, inspect weekly. This technique has shown that generally adult activity between the soil and vine peaks in mid- to late May and is complete by early July.
Black vine weevil is generally not treated.