WSU Whatcom County Extension

Integrated Pest Management for Raspberries

Weevil

(Otiorhynchus species)

Insects and Invertebrates

 

Clay Weevil

 

Symptoms

A number of species of weevils are found in raspberry fields. The larval stage is the most damaging. Larvae feed on roots and can completely devour small rootlets and destroy the bark and cortex of larger roots. Plants will soon begin to wilt because they are no longer able to provide moisture for the leaves. Adult weevils are nocturnal and cause characteristic notching in the leaf margins resulting in minor injury. Newly planted fields are more susceptible to root weevil attack. Adults can be a harvest contaminate in machine-harvested fields.

 

Identification

Larvae are white or pink, legless, C-shaped grubs. They can be found in the soil during most of the winter and summer. Adults are flightless, hard-shelled beetles that feed at night in the canopy and hide around the crown of plants during the day.

 

Black Vine Weevil

Black Vine Weevil (Otiorhynchus sulcatus)

Adults are up to 1/3 inch long, black with a few small yellow or orange spots.

 

Strawberry Weevil

Strawberry Weevil (Otiorhynchus ovatus)

Adults are 1/5 inch long and range from black to brown in color.

 

Rough Strawberry Weevil

Rough Strawberry Weevil (Otiorhynchus rugosostriatus)

Adults are ¼ inch long and dark red brick colored.

 

Clay Colored Weevil

Clay Colored Weevil (Otiorhynchus singularus)

Adults are slightly smaller than the black vine weevil and are mottled light and darker brown or gray.

 

Weevil Larva and Weevil Pupa

 

Life History

Weevils overwinter as larvae and adults. Pupation occurs in earthen cells approximately 3-4 inches in the ground. Several species found in Whatcom County have growth stages that overlap; it is possible to find adults, larvae, and pupae in a single location at the same time. Adults emerge from the soil as follows:

• Black vine weevils emerge from the soil in late spring.
• Strawberry weevils emerge in late spring to early summer.
• Rough strawberry root weevils emerge in early to mid summer.
• Clay colored weevils emerge in early spring.

Adult weevils feed nocturnally on foliage causing characteristic notching of the leaf margins and hide (usually on/in the ground) during the day. Adults feed for 3-4 weeks before laying up to 500 eggs in the soil. Only female root weevils exist and are flightless.

Eggs hatch after 2-3 weeks and the small c-shaped larvae begin to feed on plant roots. Larvae continue to feed through summer, fall, and winter when temperatures allow. Larvae feed on the fine roots and root crowns. It is important to note the overwintering adults will be active much earlier the following spring.

 

Weevil Damage

 

Monitoring

In early spring, inspect roots of plants with low vigor for larvae. Plants damaged by larvae may show symptoms similar to water stress. Examine 5 buds and new laterals per hill for signs of damage and record the number of buds per shoot damaged by adult weevils. In late May, begin inspecting canes for signs of damage, appearing as notching of leaf margins. Temporary shelters, such as a roof shingle, can be laid on the ground and used to detect adults seeking shelter during the day. Developmental stage should be noted to help time effective treatments.

Adult weevils can be detected in the evening by placing a beating tray under the foliage and shaking the plant gently. Take 10 beating tray samples (spaced 3-5 hills apart) at each of 3-5 sites per field and record the total number of adults found. If a beating tray is not available, placing a white sheet on the ground and shake the canopy above. Record weevil numbers for each inspection. Resample soon after treatment.

 

Weevil Feeding

 

Thresholds and Management

Fields can rapidly become infested in a short time, therefore controls should be applied to even lightly infested fields to prevent an increase in infestation. A pre-harvest spray is advisable if beating tray sampling results in 1-2 weevils per ten samples. Scout field soon after treatment to verify effectiveness.

Insecticides should target emerging adults; timing will depend on the species present. Treat the following spring if problems are discovered after adult emergence. Adult weevils are nocturnal feeders and become active after sundown. Applying foliar sprays at night will usually improve efficacy of the insecticide by escalating the chance weevils are directly hit by the spray or increasing the likelihood they will walk through a fresh spray deposit. Manage weeds in the infested area to eliminate alternate food sources for adults and larvae.

 

Weevil damage

 

Resources

University of California, IPM Online: Caneberries, Root Weevils
http://www.ipm.ucdavis.edu/PMG/r71300411.html

WSU Extension, Bulletin 0965, Root Weevils on Berry Crops
http://cru.cahe.wsu.edu/CEPublications/eb0965/eb0965.html

WSU Extension, Western Washington Field Guide to Common Small Fruit Root Weevils
http://cru.cahe.wsu.edu/CEPublications/eb1990/eb1990.pdf

Washington State University, Root Weevil Egg Laying Studies
http://whatcom.wsu.edu/ag/comhort/RWEGGS99.htm

WSU Whatcom County Extension, Study of the Clay Colored Weevil on Meeker Raspberries
http://whatcom.wsu.edu/ag/comhort/CCWREP1.htm

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WSU Whatcom County Extension 1000 N. Forest St., Suite 201, Bellingham, WA 98225 (360) 778-5800 whatcom@wsu.edu