Earliest canola fields are just coming into flower in southern Alberta and southwest Saskatchewan, the highest-risk areas for cabbage seedpod weevil in Western Canada. Cabbage seedpod weevils tend to cluster in fields that are first to flower, so farmers with early fields will want to check them closely.
Scout at 10-20% flower (or approximately one week after first flower) using a sweep net. The economic threshold is 25-40 CSPW per 10 sweeps generally across the field. Use proper sweep net technique and make ten 180° sweeps at each location.
New analysis from AAFC indicates that a minimum of four samples (10 set of sweeps at four locations in a field) can provide a good assessment of weevil numbers. The AAFC recommendation is to do two sets of sweeps 50 metres apart at one location and another two sets of sweeps at the opposite end of the field or from a different field access point. Large differences in CSPW numbers between the four sample sets may indicate continued movement of CPSW into the field and the need for continued scouting.
Avoid spraying too soon as CSPW will continue to be attracted to fields coming into flower, and damage does not occur until the pods are large enough to support egg laying by the adult female. This would be when the oldest pod reaches 1 inch in length.
Cabbage seedpod weevils 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. Also, continue to scout after application if relying on perimeter sprays (keeping in mind safe field re-entry times) in case re-invasion occurs.
If you are in a field with variable maturity, sweep randomly and don’t just favour flowering patches. You could easily find elevated numbers in the area that has started to flower, but when the field begins flowering more uniformly, the average number of weevils per sweep will decrease.
Note the possible weather effect: On days above 12°C with low to moderate wind, weevils will be concentrated in the top area of the crop. In these conditions, they are easy to pick up with a sweep net.
Please participate in Alberta Agriculture’s CSPW survey. Report sweep net results here
For more on thresholds and when to spray, read this