Weevils

Weevils, or snout beetles (family Curculioninae), as adults have an elongated head, elbowed antennae, and hard-surfaced body.  At least 600 weevil species occur in California. They commonly are flightless, nocturnal, and mostly dark colored, black, brown, or grayish. The grublike larvae and pupae are pale yellow to whitish.

Black vine weevil adults and mature larvae are 1/2 inch long. Adults are blackish with pale flecks (scales) on the forewings. Adult females lay eggs in or on soil. Larvae and pupae occur in soil; larval feeding on roots causes the primary, economic damage.

Fuller rose beetle adults are brown to grayish weevils about 3/8 inch long. Fuller rose beetle's oblong, yellowish eggs are laid in groups above ground in protected plant parts or in or on soil. Larvae chew on roots but do not seriously damage most hosts. Adult chewing and feeding on above-ground parts is the primary damage from Fuller rose beetle.

Adult weevils feed on foliage and flowers at night. The edges of blossoms and leaves appear notched or ragged or plant parts may be clipped off. Unless adult populations are high, this damage does not usually threaten the health of established plants, but can make nursery crops unmarketable. Black vine weevil larvae chewing roots and the root crown of container plants and field-grown trees and shrubs can cause serious damage, sometimes girdling plants around the soil line or severely wounding crowns of herbaceous perennials and killing the host.

Periodic monitoring of host plants, growing weevil-resistant plant species and varieties, and applying entomopathogenic nematodes and insecticides are the main methods for managing weevils.

Where plants are well established and relatively large and can tolerate some foliar damage, pesticide application can be delayed until about 3 weeks after first detecting adult weevils or damage, typically during late winter through spring. Weevils need to feed for about this period before they are able to lay eggs. Weevils are susceptible to most broad-spectrum, persistent insecticides (e.g., carbamates, organophosphates, pyrethroids) labeled for control of foliage-chewing insects.

To target larvae, beginning about 6 weeks after adult weevils or their chewing damage are first observed that growing season, soil can be drenched with nematodes or systemic insecticide. Drenching with nematodes can be repeated as long as soil or growing medium is well drained (e.g., high in organic matter, sandy), moist, and warm.

Weevils are susceptible to several insect pathogens, parasites, and predators. For example, Fuller rose beetle eggs are commonly parasitized by a tiny blackish wasp, Fidiobia citri. However, resident natural enemies are usually insufficient to satisfactorily control weevils in nursery crops.

Consider growing plant varieties that are more resistant to weevil damage. See table Hybrid Rhododendrons Resistant To Feeding Injury By Adult Root Weevils for suggestions. Where soil has been infested with weevil larvae and pupae, steam treat planting beds or otherwise pasteurize growing media immediately after removing any infested crop to kill any weevils in the soil before replanting beds. Steam heat is difficult to use in field soils but can be applied using a plowlike steam rake to raise the topsoil temperature to levels sufficient to kill most pests.

Biological and cultural controls are organically acceptable management methods. Botanical pyrethrins without piperonyl butoxide (PyGanic) and entomopathogenic nematodes are organically acceptable.

Regularly inspect the buds, flowers, and foliage of hosts for feeding damage and pests. Adults can be monitored 1 to 2 hours after dark by sweeping foliage with a net or by beat or shake sampling of branches onto a light-colored sheet or tray. Adults can be monitored using pitfall traps or trap boards on the soil or by burlap bands or corrugated cardboard wrapped around trunks; inspect these for adults that sought shelter during the day. Sift through loose soil at the base of plants where the adults hide during the day. Temporarily remove plants from their containers to inspect the root ball and crown, which may reveal the presence of larvae and their chewing damage.

Target adult weevils when applying insecticide. Persistent, broad-spectrum insecticides that kill on contact or through ingestion will control the night-feeding adults if thoroughly sprayed on foliage. If some chewing damage to aboveground parts can be tolerated, delay application until 3 weeks after adults or their damage are first observed. Egg laying lasts about 6 to 8 weeks, so repeated application can be warranted depending on the insecticide applied and if adults are still present.

Application of a systemic insecticide to control root feeding larvae is of uncertain effectiveness. Drenching containers and around the base of small trees and shrubs in the field with entomopathogenic nematodes (Heterorhabditis and Steinernema spp.) when larvae or pupae are present can be done in well-drained soils and growing media that is warm and kept moist but not soggy.

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