Scouting for cabbage seedpod weevil

Scouting for cabbage seedpod weevil

This canola insect pest has continued to spread on the Prairies and in Saskatchewan can be found south of the TransCanada Highway. Although adults feed on flower buds, most of the economic crop damage occurs when larvae feed within pods and destroy developing seeds.

Canola pods harbouring cabbage seedpod weevil (CSPW) larvae may appear distorted. When larvae consume some seeds within pods, the undamaged seeds enlarge and mature, often leaving misshapen pods. Larvae will emerge from pods via exit holes. Pods with exit holes shatter before harvest more frequently than non-infested pods, and exit holes may serve as entry points for fungal spores that can germinate to further reduce yield.

Early flowering increases the risk of infestation, as adult CSPWs are attracted to the first yellow canola flowers available, which indicate a suitable food source and egg laying location.

Scout for CSPW as the canola crop begins to flower. If numbers are near threshold, continue to scout through the early flowering period of the field and nearby fields. CSPWs enter a field from the side and work their way to the middle. Scouting borders of fields twice a week may allow you to catch their first arrival, possibly allowing adequate control with a perimeter spray of the outside edges of the field. Shelterbelts and fence lines are a good clue as to which side might be first, but check all four to be safe. Also, continue to scout after application if relying on perimeter sprays.

When scouting, select four locations within each field: two paired locations, with each pair of locations in a distinctly different portion of the field and each location (within a pair) separated by at least 50m. At each location count the number of weevils from 10 - 180 degree sweeps with a sweep net. Sample both the perimeter and interior of the field.

To get an accurate count, sweep between 11 am and 5 pm (when temps are above 20° C) when winds are calm and insects are active. 

The action threshold for applying insecticides is 25-40 weevils in 10 sweeps during canola flowering, varying slightly with canola price and cost of control. 

Best management practice: Consider using predictive CSPW population maps. Predictive maps forecast the risk of infestation for the following summer based on adult cabbage seedpod weevil populations from the preceding summer. 

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