Root Weevils

Family: Circulionidae

Identification: The group of beetles that weevils are classified in is the most diverse family of any living organism. One entomologist suggested that if every living animal was lined up in a row, every sixth one would be a weevil. Suffice to say, weevils are a very difficult group of insect to deal with individually. However, 'weevils' as a family, can be easy to identify. The mouth parts of weevils are situated at the end of an elongated snout or beak with elbowed antennae placed between the eyes and mouth parts. Root weevils (RW) are very hard, robust beetles with a 'bulbous' shape. The larvae are legless white grubs that have a characteristic 'C' shape. There are at least six species of root weevils that you will encounter in your landscape: black vine RW (Otiorhyncus sulcatus), clay-colored RW (O.singularis), strawberry RW (O. ovatus), rough strawberry RW (O. rugosotriatus), woods RW (Nemocestes incomptus), and obscure RW (Sciopithes obscurus). The adult black vine RW is the largest (1/3") weevil with spots of orange or yellow. The clay-colored RW has brick-red colored legs and mottled colors of tan, gray and dark brown on the back. The strawberry RW's are dark brown to dark red in appearance with the strawberry RW (1/5") much smaller than the rough strawberry RW. Woods RW is colored light to dark brown with gray spots on the back. The obscure RW is gray to tan colored with a wavy line crossing the rear end.

Life Histories: There are many similarities in life histories of all root weevils discussed here. All root weevil adults are wingless (unable to fly) and no males of some species have ever been found. This group of weevils is realizing that they have no need for males anymore. Black vine females are essentially cloning themselves every year and are quite successful at it. Interestingly, eggs will not be matured as quickly unless there is a ritualistic mating (pseudocopulation) between females; this action seems to stimulate the development of ovaries. There is a period of 3-5 weeks that the females need to develop their eggs and feed on foliage. Most weevil adults are only active at night. They spend the day hiding in the litter below the plant and move up the stems at night to feed on the fresh foliage. Once egg-laying begins, a female may be responsible for laying up to 200 eggs in her life time. Most females lay their eggs on the soil surface, but obscure RW's lay eggs on the foliage of the plant. When the eggs hatch, the larvae migrate through the soil to feed on the developing roots. This is where they stay until the following spring. Most weevils pupate by April and emerge in June as adults. The clay-colored weevil is active as early as late February to early March. In species such as the black vine RW, a prolonged emergence through August may produce females that can overwinter as adults instead of larvae. These adults are then ready to lay eggs in the early spring. So you really need to be looking for adults a few times of the year.

Damage: Root weevils have a huge host plant range. One researcher screened a couple hundred plant species and found only two that the black vine RW didn't like. Adult weevil damage is hard to miss. Look for circular notches taken from the margins of the leaf. Damage can look quite severe, especially on evergreen shrubs like rhododendrons. The preferred hosts seem to be in the family Rosaceae. Ornamentals like rhododendrons and azaleas are very susceptible to black vine, woods, obscure, and clay-colored root weevils. Berry plants like strawberry, raspberry and blueberry make good hosts for black vine, strawberry, rough strawberry, woods and clay-colored root weevils. Clay-colored root weevils can cause quite severe damage due to their early emergence. Plants such as raspberries can suffer from leaf flagging and bud feeding.

The larvae are responsible for the most damage and usually go unnoticed until its too late. Larvae feed on developing root shoots and move to the larger stems as they mature. A single larva can girdle a root stem or more importantly a root crown. The result of heavy root feeding is a nutrient and water-stressed plant. This root feeding can be mistaken for root diseases such as root rot. Minor root feeding can also leave a plant susceptible to such root diseases.

Control: Pesticides are a difficult and undesirable measure of control. Larvae are protected in the soil from most pesticides. There are some nematodes that offer relief from larval feeding. However, most nematodes on the market do not do well under the cool conditions that we have here. Look for brands that offer control at lower temperatures. Applications should be made in the fall and early spring. New strains of nematodes and entomopathic fungi are currently being developed to target the cooler spring and fall months of the Northwest. Adults are easiest to knock down with chemical pesticides but timing can be hard due to the prolonged emergence of the adults. The best method of control is to physically remove weevils when possible and if practical. If you have a tall shrub or plant that is infested, place a white cloth underneath the plant. At about 10 p.m., go out and shake the plant vigorously; this will dislodge the feeding adults. Discard and kill the adults to make sure that they won't re-infest any neighboring plants. In addition to this method, trapping the adults in 'stick-em' works well too. Bands of 'stick-em' can be painted around the trunk of the plant to prevent movement or trap the adults as they move up the plant at night. This may harm the plant tissue if used for a long time so it may be good to wrap wax-paper tightly around the trunk before applying the glue. This method of control will only be affective if there are no other routes for the weevils to get to the foliage.