Walking into a green, fully-podded canola field isn’t easy — it’s actually more like swimming if you’ve got a good stand and your plants are well knit together. But, it’s important to really get into the field to scout for bertha armyworms.
Before you wade in to your canola to look for this pest, Gregory Sekulic, agronomy specialist with Canola Council of Canada, recommends going to the Prairie Pest Monitoring Network blog and seeing what the environmental conditions and insect risks are, not just for bertha armyworm, but several other insects.
“There are Prairie-wide monitoring programs aimed at catching bertha armyworm moths throughout the summer, to try and predict where the hotspots will be and focus attention on where scouting is more critical,” says Sekulic.
If the risk map shows red in your area, you’ll want to make sure you get out and scout regularly. Bertha armyworms are in the cutworm family and have a huge preference for canola, says Sekulic. They’ll eat all of the above-ground material and getting into late August, the worms will go after pods. Evidence of the worm is stripping and aggressive bite-marks. Sekulic has seen a field so bad that the whole field looks white because the worms ate all of the green material, leaving only the white pithy plant material behind.
The economic threshold for bertha armyworm is well established — there’s good data behind the amount of damage each insect can do and behind the per metre square number. If the cost of applying an insecticide, including damage done by sprayer tires, is going to be somewhere around $10 to $15 per acre and at $10 to $12 per bushel value for canola, the threshold is between 15 to 18 worms per square metre to justify the application. The Canola Council of Canada archives have a good chart that outlines all of the thresholds.
A number of beneficial insects will attack bertha armyworm — carabid beetle, damselfly, and ladybird beetle, to name some. “Even more importantly though, the outbreaks are usually more controlled by other natural processes, like parasitism by a small handful of very specific parasitic wasps that help control their numbers, but also a virus that will spread through their population when there gets to be enough of them,” says Sekulic.
To scout for bertha armyworm get out into the field (maybe do some light stretching before hand, cause it’s exercise to get out far enough). Shake the canopy vigorously, peel the plants aside, get down to the ground, and count the number of worms in a quarter of a square metre, then multiply by four to get a count for the full square metre. Familiarize yourself with what bertha armyworm look like, since there are a number of other green worms out there that will live throughout the canopy. Bertha armyworm tend to curl up in your palm when prodded.
Without control, and if the worms are greater than 20 per square metre, they can do severe damage to the point of reducing yield to zero.