Root weevils are among the most important pests of strawberries, raspberries, blueberries, and cranberries. Five species are most commonly found on these crops in Washington.
Strawberry root weevil, Otiorhynchus ovatus, rough strawberry root weevil, Otiorhynchus rugosotriatus, and black vine weevil, Otiorhynchus sulcatus, are the most important species economically. Woods weevil, Nemocestes incomptus , has in the past, been a problem mainly in northwestern Washington. Obscure root weevil, Sciopithes obscurus , occurs sporadically on berry crops.Appearance
Adult strawberry root, rough strawberry root, and black vine weevils are about 1/5, 1/4, and 1/3 inch long, respectively. They are usually black but some may be light to dark brown. The black vine weevil usually has a few yellow or orange blotches on its back.
Strawberry root weevil,
rough strawberry root weevil,
and black vine weevil.
Obscure root weevil. Woods weevil is about 1/4 inch long. It is light to dark brown with gray spots on its back. Obscure weevil is mainly gray with a wavy brown line across the back near the rear.
Larvae of all species of root weevils are superficially similar in appearance except for size. They are legless grubs with white bodies and brown heads. They characteristically bend their bodies in the shape of the letter "C."
The pupa is about the same size as the adult and is white. It is very soft and has the outlines of the parts of the adult weevil.Life History
The life history is similar for all species. Winter is spent as a larva in the soil feeding on plant roots. In April or May it changes to the pupa, which does not feed. Further transformation to the adult stage occurs in late May and early June. Adults feed on plant foliage and begin to lay eggs 4-6 weeks after emergence. Otiorhynchus weevils lay their eggs on the soil, but the obscure root weevil lays its eggs on the leaves. Larvae which emerge from the eggs burrow into the soil to feed on the roots. Many adult weevils survive winter and begin laying eggs in late April or early May.
Woods weevil has a life history very similar to the obscure root weevil except that the different stages of growth overlap. It is possible to find adults, larvae, and pupae all at one time in one location.
These weevils feed and develop on a wide variety of plants. Adjacent infested berry fields, woodland, fencerows, etc., all serve as sources of infestation for new plantings. These weevils cannot fly, so 2 to 3 years may be required for them to spread entirely across a large field.Injury
Most injury results from the larvae feeding on plant roots. Feeding on the foliage by the adults does the plants little harm but the characteristic notching of the leaves is a useful tool for diagnosing a weevil infestation. These weevils usually feed at night and return to the ground during the day. Adult root weevils sometimes remain high on raspberry or blueberry canes during the day and become a contaminant of the berries. This is especially true if berries are harvested by machine.
- Damage to foliage by root weevil adults.
Strawberry root weevil larvae. Control
Weevil control depends almost entirely upon chemicals. However, an insect-parasitic nematode is available commercially for biological control of weevil larvae on cranberries. Plowing of heavily infested strawberry plantings immediately after harvest will help protect nearby berries and plantings to be made the following spring.
Larvae may use weeds as hosts in raspberry or blueberry fields and the adults become contaminants of berries the following season. Therefore, control of weeds, especially perennials, is important.
It is very important to know if weevils have invaded a planting and, if so, which species, because some insecticides will not control all the species. A field free of weevils needs no treatment. This saves the grower money and reduces the possibility of secondary problems arising from the killing of beneficial predators and parasites by the weevil control chemical.
A field should be inspected every week or two during July and August for evidence of weevils, especially around the edges of the field where invasion would first be noticed. Look for the characteristic notches chewed in the leaf margins (see photo). If notching is seen, examine the foliage for adults at night with the help of a flashlight. If adult weevils are found, take them to your county extension agent for identification.
Appropriate controls should be applied to even lightly infested fields to prevent egg deposition and build-up of weevils, which can be rapid.
When foliar sprays are applied, doing the work at night will usually improve efficacy of the insecticide. The adult weevils are nocturnal feeders and become active after sundown. Spraying then improves the chances of the weevils being hit directly by the spray or of them walking through a fresh wet spray deposit. This usually results in better control than when they contact dried spray deposits.Home Gardeners
No chemical control is registered for Otiorhynchus weevils in home berry plantings. Strawberries are especially susceptible and heavily damaged plantings should be destroyed, as described above. Consult EB1015, Small Fruits & Berries-Insect/Disease Control for Home Gardens, for current information on chemical control of the obscure root weevil.
Commercial Fields
Blueberries, Cranberries, Raspberries, and Strawberries. Chemical control of root weevils differs greatly with the crop. Consult the following extension publications for current recommendations: EB0845, Cranberry Insect and Disease Control Program; EB1017, Furadan for Root Weevil Control in Commercial Strawberries; and EB1491, Pest Management Guide for Commercial Small Fruits.
By
Carl H. Shanks, Jr., Ph.D., Washington State University Entomologist and Station Manager, WSU Vancouver Southwestern Washington Research and Extension Unit.
Use pesticides with care. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.
College of Agriculture and Home Economics, Pullman, Washington
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Revised September 1991. Subject code 232. A
EB0965
