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Cabbage Seedpod Weevil

By Cory Jacob, PAg, Provincial Specialist, Oilseed Crops, Regina

July 2021

Adult cabbage seedpod weevil
Adult cabbage seedpod weevil

The cabbage seedpod weevil (Ceutorhynchus obstrictus) is a major pest of canola and brown mustard in Alberta and Saskatchewan. The insect was first detected in canola fields in southern Alberta in 1995 and has since spread north and east across Alberta and Saskatchewan. All host plants of this insect belong to the mustard family. True host plants are those with large seedpods that the larvae can develop in such as canola, brown mustard and wild mustard. Food and true hosts both provide food, particularly pollen for adult feeding. Food hosts include weeds such as stinkweed and flixweed.

The majority of crop damage from cabbage seedpod weevil is from the larvae feeding inside developing pods, each larva can consume about five seeds during its development. This predisposes the pod to premature shattering, which can cause a total yield loss of that pod. Larvae emerge from pods through exit holes, which can provide an entry point for fungal pathogens, causing further yield loss. Adult weevils will feed on developing flower buds. Typically, the plant can compensate for this except for dry years when moisture is limited. Pods with larvae inside may become misshapen, due to some seeds being consumed by the larvae and undamaged seeds within the pod are maturing.

cabbage seedpod weevil larvae damage on canola pods
Normal canola pod (top) and
pods misshapen by internal
feeding damage from cabbage
seedpod weevil larvae.

The adult is the over-wintering stage of the cabbage seedpod weevil. They over-winter under leaf litter in shelterbelts or roadside ditches. In the fall they will select sites and burrow beneath the soil surface, protecting them from low temperatures. Their survival during this time can be reduced if the winter is severely cold and has little snow cover. In the spring, the adults will emerge from these sites and seek out hosts.

Scouting for cabbage seedpod weevil should begin when the crop is at the bud stage and continue through flowering. Select 10 locations within each field and using a sweep net count the number of weevils in 10, 180 degree sweeps. For an accurate estimate of weevil numbers in a field, sweeping should occur both in the field perimeter and interior.

Cabbage seedpod weevil survey map
Cabbage seedpod weevil
2020 survey results

The economic threshold is when an average of 25 to 40 weevils are collected in the 10 sweeps. If an insecticide application is warranted, the best time to spray is when crops are in 10 to 20 per cent flower. This will prevent eggs from being laid in newly formed pods. Spraying should be done late in the day to minimize impacts to beneficial insects such as bees. There are insecticides registered for control of cabbage seedpod weevil in canola and mustard. Please refer to The Guide to Crop Protection for registered products.

In conjunction with Agriculture and Agri-Food Canada, the Ministry of Agriculture monitors cabbage seedpod weevil populations throughout Saskatchewan, a map is produced advising of local pressures. These numbers on the map for the 2020 survey (the 2021 survey is underway) represent the number of adult weevils per 25 sweeps as a surveyor walks into and out of the field.

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