Cabbage seedpod weevil review

Cabbage seedpod weevil continues to expand across the Prairies, contributing to yield losses of 5 to 30%. Insecticidal control remains the main management strategy despite decades of research into alternative biological and cultural methods of control.

The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), is native to Europe and was first reported in North America in the early 1930s near Vancouver, B.C. It has since spread to the interior of British Columbia, into the Prairies and most of continental United States. This summary covers a review of the distribution, biology, pest status, and management of cabbage seedpod weevil on spring-planted canola. It also highlight areas for future research to develop a comprehensive integrated pest management program.

Cabbage seedpod weevil was first detected on the Prairies near Lethbridge, AB in 1995 and remained mostly south of the TransCanada highway in Alberta.  Surveys have now found that it continues to expand north and east into Saskatchewan and Manitoba, and threatens the Peace River Region of AB and BC.

Cabbage seedpod weevil adults are approximately 3 to 4 mm long, have a distinct curved snout, and are ash-gray to black in color. Males and females are similar in appearance.

Sexually immature adults overwinter in the soil, primarily in tree shelterbelts and leaf litter. Overwinter survival decreases significantly at -5C. The average threshold lethal freezing temperature (supercooling point) has been estimated at -7.2C.

Adults emerge in the spring when soil temperatures reach 9 to 12C, and peak emergence temperatures occur with average soil temperatures of 15C. After emergence, they disperse to feed on Brassicaceae host plants such as stinkweed, wild mustard, flixweed and volunteer canola. Other Brassicaceae host plants include hoary cress, shepherd’s purse and radish.

Adults most often fly at temperatures of 22C and low windspeeds of less than 0.5 meters per second, although they can sustain flight at temperatures down to 12C.

Cabbage seedpod weevil adults then move to host crops at the bud and flowering stage that have large enough pods for larval development such as canola and mustard. With antixenotic and antibiotic properties, wild mustard is not a host. They feed on pollen, flower buds, flower parts and developing pods. During flowering, mating occurs and females then lay eggs through feeding punctures into canola pods at least 2 mm in diameter. Cage studies have found that a single female can lay up to 141 eggs over 37 days.

After oviposition, eggs hatch about 1 week later. Larvae develop inside the pod, are about 3 to 4 mm in length, and are yellowish-white. Larvae can consume 6 seeds each during their development over 2 to 4 weeks. At maturity, larvae exit the pod through holes chewed in the pod. They drop to the ground and pupate 1 to 2 cm below the soil surface. About 10 days later, adult weevils emerge and feed on green Brassicaceae plants before overwintering.

Management options

Crop loss is mainly caused by larval feeding on seeds within the pods, and are estimated at 5 to 30 per cent yield loss depending on seeding date, region and pest density. Fungal pathogens can also enter the pod through larval exit holes to cause further damage to developing seeds.

The Prairie Pest Monitoring Network (PPMN) monitors for the distribution and relative abundance of cabbage seedpod weevil, and distribution maps are published online at Farmers and agronomists can use these maps to identify their risk, and guides where scouting should be focussed.

The PPMN recommends scouting start when canola enters the bud stage and continue through flowering. Research has found that 180 degree sweep net samples at 4 locations in a field can be used for monitoring pest populations. One sample can be taken along the field border and 1 sample 50 meters into the crop. Another set of these samples are conducted at least 500 m away from the first set.

Research has calculated that the economic injury level is 20 weevils per 10 sweeps (2 weevils per sweep) at early flower  at 10 to 20% bloom. However, because sampling usually occurs along field edges where adults are concentrated, the economic threshold is recommended at 25 to 40 weevils per 10 sweeps (2.5–4 weevils per sweep). If beneficial insects are present, the economic threshold of 4 weevils per sweep is recommended.

Foliar insecticidal control remains the main management strategy. Currently in Canada, only Group 3 synthetic pyrethroids are registered for cabbage seedpod weevil control, which includes deltamethrin (i.e. Decis). This lack of diverse insecticide chemistries brings the risk of insecticide resistance, as has occurred in Germany to lambda-cyhalothrin.

Trap cropping has shown potential in research in Alberta in large commercial fields. A perimeter of canola is planted around the field 7 to 10 days earlier than the rest of the canola field. Weevils will move into the trap crop first, where they can be targeted with insecticides before they move into the rest of the field.

Early seeded canola is highly attractive to cabbage seedpod weevil, but delaying seeding to reduce infestations must be balanced with the agronomic benefits of early seeding.

Plant breeding research is working towards host-plant resistance based on resistance in white mustard. To date, no commercial varieties with resistance have been developed, but research is on-going.

An estimated 15 parasitoid species have extended their host range to attack cabbage seedpod weevil. These parasitoids are from five families of Hymenoptera. In Europe, parasitism rates have ranged from 52 to 90%, but in western Canada, parasitism rates are less than 15% and not sufficient to control the pest.

Further research on host plant resistance, biological and cultural control, trap cropping, and population migration is required, especially as climate change may allow further weevil expansion across the Prairies.

Financial support was provided by a NSERC Industrial Research Chair (545088) and partner organizations (Alberta Wheat Commission, Alberta Barley Commission, Alberta Canola Producers Commission, Alberta Pulse Growers Commission) as well as an NSERC Discovery Grant.

Altaf Hussain, Priyanka Mittapelly, Adam J Blake, Julian R Dupuis, Patrice Bouchard, Tristan D Skolrud, B Andrew Keddie, Meghan A Vankosky, Héctor A Cárcamo, Boyd A Mori, Biology and management of Ceutorhynchus obstrictus (Coleoptera: Curculionidae) in spring-planted canola on the Northern Great Plains, Journal of Integrated Pest Management, Volume 14, Issue 1, 2023, 17,

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Photo by Shelley Barkley

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