Weevil
(Otiorhynchus species)
Insects & Invertebrates
Symptoms
Root weevils are sporadic pests in blueberries. The larval stage is the most damaging. Weevil larvae, or grubs, found in the soil can girdle root crowns and trunks resulting in bush death; new plantings are especially vulnerable. Larvae will feed on the root structure of established plants and large populations can significantly reduce crop vigor and yield. 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.
Identification:
Black Vine Weevil (Otiorhynchus sulcatus)
Adults are up to 1/3 inch long, black with a few small yellow or orange spots.
Strawberry Weevil (Otiorhynchus ovatus)
Adults are 1/5 inch long and range from black to brown in color.
Rough Strawberry Weevil (Otiorhynchus rugosostriatus)
Adults are ¼ inch long and dark red brick colored.
Clay Colored Weevil (Otiorhynchus singularus)
Adults are slightly smaller than the black vine weevil and are mottled light and darker brown or gray.
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. When mulch is topped over the root crown, larvae can girdle the trunk of blueberry bushes. It is important to note the overwintering adults will be active much earlier the following spring.
Monitoring
Monitor regularly throughout the growing season for plants with reduced vigor. Inspect plants with low vigor that show signs of water stress such as yellowing or reddening of leaves. In spring, inspect roots for larvae feeding on roots. After May, look for notching on lower leaves and leaves on new shoots. Look for weevil adults feeding at night or in debris and mulch during daylight. Weevil infestations in blueberries will appear in patches.
Management
Consider insecticide management if plants show signs of low vigor and if weevils have been found. Insecticides should target emerging adults; timing will depend on the species. Treat the following spring if problems are discovered after adult emergence. Manage weeds in the infested area to eliminate alternate food sources for adults and larvae.
The use of the nematode Heterorhabditis marelatus has shown to reduce populations of black vine weevil larvae in strawberries. This must be applied during late spring or early summer, when soil temperatures are adequate for nematode survival.
Web Resources
Washington State University, Root Weevil Egg Laying Studies
http://whatcom.wsu.edu/ag/comhort/RWEGGS99.htm
WSU Extension, Bulletin 0965, Root Weevils on Berry Crops
http://cru.cahe.wsu.edu/CEPublications/eb0965/eb0965.html
WSU Whatcom County Extension, Study of the Clay Colored Weevil on Meeker Raspberries
http://whatcom.wsu.edu/ag/comhort/CCWREP1.htm
WSU Vancouver Research and Extension Unit, Control of Rough Strawberry Root Weevil in Strawberry
http://vancouverreu.wsu.edu/research/entomology/RSRWstrawberry.htm
Washington State University Extension, Western Washington Field Guide to Common Small Fruit Root Weevils
http://cru.cahe.wsu.edu/CEPublications/eb1990/eb1990.pdf
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