Cabbage seedpod weevil: When to spray

June 15, 2016 - Issue 13

Cabbage seedpod weevils are attracted to the buds on early canola plants. While they will feed on these buds and will destroy some of them, spraying is rarely recommended before 10% bloom.

Why not? Three reasons

1. Economic loss from cabbage seedpod weevils is due almost entirely from weevils laying eggs in young pods. These eggs then hatch a couple weeks later, with larvae feeding on developing seeds. Bud feeding is not known to cause economic loss.

2. Weevils move very easily from field to field. Spraying weevils on buds on early canola fields will take out those particular weevils, but by early flowering — which is the critical control stage — a whole new group of weevils will be back. Not only could a second spray be required, but some insecticides cannot be sprayed twice in one season.

3. Low levels of bud feeding can cause a defence response in healthy plants, causing them to flower longer, This can actually lead to an increase in yield when weather conditions in mid to late summer allow for good compensatory growth. This has been shown with early lygus feeding and the thinking could be extended to weevil feeding as well.

Cabbage seedpod weevil on canola buds. Credit: Brooke Moon

Cabbage seedpod weevil on canola buds. Credit: Brooke Moon

When to spray?

At the right crop stage. The goal is to stop adults from laying eggs in newly formed pods. Cabbage seedpod weevils (CSPW) don’t lay eggs until pods are at least 3/4” long. First pods reach this size 4 to 5 days after first flower. This will be around 10% to 20% bloom. A few eggs laid in the first pods is better than spraying too early and not getting good control of high CSPW populations as they move into your fields.

When weevil numbers are at economic levels. The economic threshold is 20-40 CSPW per 10 sweeps generally across the field. Use a sweep net to sample at least 5 spots along the field edge. If more than 2 per sweep, sample another 5 spots about 150 feet into the field. CSPW will tend to be present in higher numbers at field edges and as they spread out, the number present per sweep will decrease. If you are in a field with variable maturity, sweep randomly and don’t just favor 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.

Avoid peak sun. When you spray an insecticide, avoid peak sunshine hours of the day when bees and other beneficials will be most active in the field. Note that some of the common CSPW registered products are synthetic pyrethroids that can lose efficacy at higher temps. This is another reason to spray later in the day.

Other considerations:

The seed production exception. Commercial seed producers may want to spray cabbage seedpod weevil at the bud stage. This is not ideal, but they don’t want to spray flowering crop because they’ve paid a lot of money for pollinating bees and they don’t want to jeopardize this investment.

Tank mixing insecticide with fungicide. If the economic threshold is reached for CSPW and conditions also warrant a fungicide for sclerotinia stem rot management, holding out CSPW control until 20% bloom provides an opportunity to tank mix insecticide with the fungicide application. This is a little later than the ideal timing for CSPW but the earliest stage for fungicide labels.

Lygus benefit. AAFC research in Alberta found that fields sprayed for CSPW at the appropriate time often do not have a lygus issue. It seems that the timing for CSPW also provides a management benefit for lygus bugs.

Further reading:

Alberta Insect Pest Monitoring Network includes a form to report weevils here.
What does 10%, 20% and 50% bloom look like?

Canola Watch