Control of cabbage seed weevil on oilseed rape

Cabbage seed weevil can be a problem for oilseed rape growers, particularly in the North of the UK. Larvae feed within the pods and their exit holes can be used by the brassica pod midge to lay their eggs in summer, causing further damage to the crop. 

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Risk factors in oilseed rape

  • Higher risk in the north of the UK 

Weevil identification

Scientific nameCeutorhynchus obstrictus 

Adults are small (2–3 mm) lead-grey to black weevils with a long snout.  

Eggs are laid singly in young pods.  

Larvae are plump, white, with a definite brown head capsule and no legs. 

Cabbage seed weevil life cycle and crop damage

JanMarAdults overwinter in woods and hedgerows.  

AprJun: Adults migrate into crops during flowering and lay eggs in pods. A brown scar, usually resulting in a kink in the pod, indicates where the pod has been punctured for egg laying.  

JunAug: Larvae feed within the pod, consuming around 25% of the total seeds before burrowing out, leaving a neat circular hole. They fall to the soil to pupate.  

AugDec: Adults emerge in August. If brassica crops are present, they may continue to feed before overwintering. 

Non-chemical and chemical control

Non-chemical control 

There are several parasitoid species that attack the egg, larval and adult stages of this pest. Encouraging their presence can help control. 

How to encourage natural enemies of field crop pests

Monitoring 

Check the crop during flowering for the presence of the weevil. They are readily visible on flowering racemes on sunny days. 

Thresholds 

Northern UK: one every two plants. 

Elsewhere: one per plant. 

Insecticide resistance 

None known. 

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Cabbage seed weevil on oilseed rape flower
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