Description of the Pest
Adult root weevils are beetles. They feed at night and hide around the crowns of plants during the day; they cannot fly. The adults, nearly all females, emerge in late spring or early summer, feed on foliage, and lay their eggs around the crowns about 1 month after emergence. After hatching, weevil larvae work their way into the soil and feed on the roots and crowns of canes. They have curved, white or pink bodies that are about 0.38 inch (9 mm) long when fully grown. They have distinct brown heads but no legs. Root weevils overwinter as larvae, and in spring they resume feeding and can cause extensive damage before they pupate. Root weevils have a single generation each year. The Fuller rose beetle can be distinguished from the other weevils by an oblique, white band on the side of each wing cover. In addition, their larvae have pale, almost white heads. The black vine weevil is the largest and has a distinct black color. The woods weevil is the smallest of the group.
Larvae feed on the roots of both blackberry and raspberry plants and can completely devour small rootlets and destroy the bark and cortex of larger roots. Soon after feeding begins, plants wilt because the roots can no longer provide moisture for leaves. It is not uncommon to find weevil larvae that have penetrated into the lower portion of the plant's crown. Adults feed on foliage and remove large scallops from the leaves. Such leaf damage is a good indication that weevils are present but is not economically damaging to the plants.
Destroy all infested plants and move outward in a circular pattern removing plants that appear healthy. Examine roots and crowns for larvae and pupae until you no longer find weevils. Replant, if necessary, after working the soil well. Sticky barriers can be used to prevent movement of adult weevils from infested second-year canes and host areas to replanted areas or newly fumigated plantings.
Diazinon is registered and can be effective as a drench application early in the season. Preplant soil fumigation for weed and disease control will destroy larvae and pupae in the soil. An alternative to the use of soil fumigants is the application of commercially available parasitic nematodes, Heterorhabditis bacteriophora or Steinernema carpocapsae. Apply the nematodes from mid-summer to fall or before adults start emerging in spring, about mid-March for most weevil species in northern California. In hot areas, apply nematodes in early morning or evening. Soil must be warm (at least 60°F) and moist but not soggy before application and for 2 weeks after; if necessary, irrigate every 2 to 3 days during this period.