Weevil Hits Early Canola Hardest

Alberta Agriculture suggests that farmers seed a border of earlier blooming canola around the perimeter of the field as a trap crop.

Cabbage seedpod weevil is a relatively new pest on the Prairies. Though it was introduced to North America from Europe over 70 years ago, it wasn’t seen in significant numbers in Alberta until 1997. Matthew Stanford, a Lethbridge-based agronomy specialist with the Canola Council of Canada, says that now, “pretty much any farmer in southern Alberta has had issues with this insect.” CSW continues to spread. A 2008 survey by Saskatchewan Agriculture and Food shows that it has traveled as far north as Saskatoon and as far east as Regina, though it’s concentrated in the Swift Current area.

The insect is ashy in colour, three to four mm in length, with a pronounced snout. It attacks plants from the mustard family, which includes canola. Stanford calls CSW a “terrible pest” because it causes damage in so many different ways. First of all, adults feed on the buds and flowers, which causes blossom blast (buds fail to open, dry out, or die) and pod abortion (poor podding and seed set). Second, the adults lay eggs in the pods. When the eggs hatch, the larvae feed directly on the developing seeds. Third, once the larvae finish feeding, they chew an exit hole in the pod, which allows moisture and disease to enter. Affected pods also mature earlier and are more prone to shatter in the fall. Finally, the second generation of adults feeds on the pods. CSW causes significantly more damage than pests who simply attack leaves or other vegetative parts of the plants.

Lloyd Dosdall, associate professor in Agricultural, Food and Nutritional Science at the University of Alberta, encourages farmers in southern Alberta and Saskatchewan to monitor their crops carefully, especially during the bud to early flowering stages when they are most vulnerable to invasion by CSW. He advises farmers to take a set of standard, 180 sweep net samples through the canola in at least five locations in each field. If the weevil populations exceed 30 per ten 180 sweep samples when the crop is in early flowering, then he recommends that farmers apply a registered pesticide.

You have several pesticides to choose from. Apply under calm conditions, preferably late in the day after pollinators and other beneficial insects have abandoned the fields, Dosdall says. Stanford notes that this threshold assumes $10 per bushel canola, so it’s not set in stone. If canola increases or decreases in price, the threshold must be adjusted accordingly.

PREVENTION MEASURES

Jim Broatch, a Lacombe-based pest management specialist with Alberta Agriculture and Rural Development, says “seeding date has a lot to do with the CSW populations. Early blooming fields generally attract the largest populations.”

Dosdall emphasizes that the most practical, effective, and cost-effective step that a farmer can take is simply to plant canola later rather than earlier. “Weevils emerge from overwintering sites rather early in the season and feed on brassicaceous weeds before invading crops in the bud to early flowering stages,” he says. “Crops seeded early are the ones that suffer the greatest damage.”

Stanford concurs: Later flowering fields tend to have lower numbers overall. If growers can curb the urge to be the first to get the crop in the ground, they’ll be doing themselves a favour.

Other biological and cultural controls exist, but none is foolproof. Because CSW is found in large concentrations on the edges of the fields before invading it, AARD suggests that farmers seed a border of earlier blooming canola around the perimeter of the field as a trap crop. The border will attract the CSW and then can be sprayed before they spread to the main crop in the interior of the field.

Several parasitoids help keep the populations down by feeding on the larvae, including two species of wasps. “Parasitism of weevils can reach 50 per cent in some fields, although more commonly this is near 15 per cent,” Dosdall says. This is another reason why those farmers who opt to spray, should do so carefully to avoid wiping out these beneficial insects.

Important developments are on the horizon in the control of CSW. Together with Dr. L. Kott of the University of Guelph, Lloyd Dosdall has been working on what he calls “the most promising CSW control” so far: a weevil-resistant germplasm. They are on the brink of releasing this germplasm to seed companies for incorporation into their breeding programs. Because the material confers some resistance but not complete resistance, the team advises incorporating the new seed (when it’s available) into an integrated management program that involves other effective control strategies such as late seeding and trap crops.

The battle with CSW may be about to get easier, but it’s still going to be a battle for quite awhile.

Patty Milligan lives on a farm near Bon Accord, Alta.

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Patty Milligan is a freelance writer based at Bon Accord, Alta.

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