Some canola fields have lygus bugs, cabbage seedpod weevils, bertha armyworms, diamondback moths and grasshoppers. Economic spray thresholds are calculated based on individual species. We don’t have scientific evidence to combine thresholds, but where an “additive” effect of multiple species can make sense is when more than one species are feeding on key yield-producing areas — flowers, buds and pods — at the same time.
For example, if you have lygus bug and cabbage seedpod weevil in the same field and if they are both at 50% to 60% of their economic spray thresholds, spraying may provide an economic benefit.
Cabbage seedpod weevil counts low but damage high. In southwest Saskatchewan, quite a bit of cabbage seedpod weevil damage has been reported even though sweeps have not shown high numbers. Why? Wind may have pushed the weevils down into the canopy during the day, throwing off the sweep counts. Maybe another insect was also at work. Or maybe the thresholds need to be adjusted for specific environmental or crop conditions. If you are in this situation, check out the sweep net techniques provided in this week’s Canola Watch to see if any differences with your technique may have affected your numbers. If counts versus damage still have you scratching your head, a second opinion may be warranted.