(file photo/farmnewsNOW Staff)

By Alice McFarlane

Bertha Army Worm

Scouting for bertha armyworm

Aug 17, 2020 | 3:17 PM

MELFORT, SASK. – If you grow canola you should be scouting each and every field for bertha armyworm according to a local entomologist.

The bertha armyworm monitoring program said there is a low to moderate risk for most of the Prairies with a few higher counts near Humboldt, Saskatchewan and southern Alberta.

Saskatchewan entomologist James Tansey said even if trap accumulations indicate low risk, it is important to note local hot spots can flare up which is why each farm should make its own assessment on a field by field basis.

“An important consideration is that when we’re not monitoring every field, we can miss some populations that could be on the rise so there is no replacement for growers getting out and monitoring,” Tansey said.

Bertha army worms can be brown, black, or green with a yellow-orange stripe along each side. Mature larvae can be .5 inches long or 40 millimeters. Like other members of the cutworm family, they will curl up when agitated.

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The greatest damage to the crop comes when mature larvae start feeding on canola.

Canola Council of Canada agronomy specialist Keith Gabert said checking every field is a necessary practice.

“I’ve literally stood in my father in law’s field and told him he didn’t have to spray, and a spray plane was flying next door,” he said.

Thresholds are based on counts per square metre, but a square metre is a large area especially since scouting requires a few counts per field. It is recommended farmers use a 50 by 50-centimeter square which works out to one quarter of a square metre and multiply the results by four.

Insect survey technologist with Alberta Agriculture and Forestry Shelley Barkley said she starts scouting by shaking the plants.

“Then I drop my quarter metre square onto the ground and then carefully start picking the leaves and trash off that area making sure you turn those leaves over because the worms can hang on,” Barkley said. “They are very well camouflaged, so you need to be patient and hang out in the field.”

Thresholds will be about 15 to 20 bertha larvae per square metre in most situations. If thresholds support a decision to spray, farmers should use a product with a preharvest interval that fits within the expected timeline between spraying and cutting the crop. It is also recommended to check fields every four days.

The interval calculator at spraytoswath.ca can be used to compare preharvest intervals by product.

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alice.mcfarlane@jpbg.ca

On Twitter: @AliceMcF