Bertha armyworm got its name because the worms will march like an army in search of food. As canola crops dry down and are swathed, bertha armyworms that have not pupated will keep moving in search of lush green plant material.
Late canola fields could be in the crosshairs of this army.
However, do not panic and automatically spray late fields. Check for presence of the worms. Bertha armyworms are always hit and miss. Even within a hotspot area, some fields will be at thresholds while others will not. Don’t spray just because your neighbours are spraying. Click here for more on how to scout.
If numbers are still below the threshold of 20 per square metre based on current canola prices and typical insecticide application costs, hold off on spraying for now but check late fields again a few days later to make sure advancing armies do not arrive unexpected. If bertha armyworms are starving for green plant material, they’ll keep moving until they run out of reserves and die of starvation.
Late spraying. Crops that will be ready to swath in less than a week have only one insecticide option — Coragen. Keep in mind that the high rate of Coragen required to control large bertha armyworms is priced higher than the highest price on the thresholds chart. The thresholds should be fairly linear, so for example, a product that costs $20 per acre should have a threshold roughly double a product that costs $10 per acre. Another consideration before spraying in the week before swathing is to examine what bertha armyworms are eating this late in the season. If the crop is within a couple days of swathing, bertha armyworms may be eating only the greenest pods which may not produce any viable seeds and not contribute significantly to yield.
Dead and dying berthas. Various fungal and viral diseases and insect parasites attack bertha armyworms. These beneficials are what bring bertha armyworm outbreak cycles to an end. However, seeing dead bertha armyworms hanging off plants does not necessarily mean the immediate threat is over. If bertha armyworms are at or above economic thresholds, spraying will be necessary. Beneficials may not kill enough bertha armyworms fast enough to prevent economic losses to this year’s crop.