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Scouting for bertha armyworm

While there haven’t been major outbreaks of bertha armyworms in recent years, this might be the year you hav e to decide whether or not to spray them on your farm

The Prairies haven’t seen sweeping bertha armyworm outbreaks for a few years, but canola farmers need to watch for the voracious pests this growing season.

Right now it’s difficult to say what the bertha armyworm population will be like in 2012, says Sean Miller, integrated pest management agrologist with Saskatchewan Agriculture.

“It is a bit hard to tell. Last year in some of those areas, especially in the northwest, we weren’t picking up high counts in 2010. But, that said, there are those higher counts in the north central, northwest and east central regions (of Saskatchewan) that may be an indication they are on the increase,” says Miller.

A bertha armyworm map is updated weekly beginning in the first week of July until early August. Farmers can find the map on Saskatchewan Agriculture’s website. Manitoba and Alberta Agriculture also post moth count updates online during the growing season. (At,, or, search for “bertha”).

Scouting tips

Farmers need to scout fields regularly for the larvae once the moth population peaks, and continue scouting until harvesting begins or the larvae have been controlled. Miller suggests sampling five locations in each field, in a W, X or Z pattern. The locations should be at least 50 metres apart. At each site, farmers should mark a one square metre area and shake the plants to dislodge and count the larvae.

“Take your time while counting. Carefully search the soil and the leaf litter within the soil. The larvae are difficult to see and may be hidden underneath soil clumps or cracks, or even curled up within that leaf litter that’s dropped to the ground,” says Miller.

Bertha armyworms are fairly distinctive. Young larvae will be pale green with yellow stripes on the side. Mature larvae are black with a brown head and pale orange stripes. While young larvae don’t cause economic damage, mature larvae feed heavily on leaves and seedpods.

“Crop losses due to pod feeding will be the most severe,” says Miller.

Spraying suggestions

When deciding whether or not to spray, it’s important to take into account the forecasted value of your crop and the costs of spraying, as well as the number of larvae in the field. For instance, if canola prices are expected to be $12 per bushel and it costs $10 per acre to spray, the economic threshold would be 15 larvae per square metre.

Larvae should be half an inch long before chemicals are applied. Farmers should spray in the early morning or late evening when the larvae are feeding. Farmers need to use enough water to cover the crop adequately, and more water is needed for crops with dense canopies. Follow application rates on the label, and use the higher rates if the population warrants it.

If possible, spraying should be delayed until the crop is done blooming to avoid killing bees. If application can’t be delayed, farmers can use a synthetic pyrethoid, such as Matador or Decis, and spray in the evening.

There are several insecticides registered to control bertha armyworms in canola. They include Ripcord, Monitor, Matador/Silencer, Lannate, Decis and the chlorpyrifos group. Check provincial crop protection guides for more insecticide information. †

About the author

Field Editor

Lisa Guenther

Lisa Guenther is field editor for Grainews based at Livelong, Sask. You can follow her on Twitter @LtoG.



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