First signs of diamondback moth larvae are larvae hanging from threads and feeding on leaves. Feeding damage is usually a mining of the leaf, giving the appearance of a “window pane.” If the larvae are in the canopy, they will often move up to pods as leaves start drying off. Larvae feed on the surface of pods, which dries the pods causing premature shelling. The photo below shows canola at the advanced pod stage, a key time for scouting and control.
You may find diamondback larvae when taking sweeps for lygus or cabbage seedpod weevil. If you find them in sweep nets, then it’s worth doing a proper scout.
—Mark off 1/10 of a square metre — about one square foot. Pull plants from this area, knock them on the hood of the truck, and then count the larvae that come loose.
—Action threshold at the podding stage is 20 to 30 larvae per 1/10 of a square metre.
—Hint: If you have 100 plants per square metre, this threshold works out to 2 to 3 larvae per plant.
There is no established economic threshold for diamondback control at the flowering stages, but the threshold may be slightly lower at early flowering. Consider application at 10 to 15 larvae per 1/10 of a square metre at that stage.
Click here for a PDF on diamondback ID, scouting tips and thresholds. Click here to read a summary of the recent AAFC survey of insects in the Peace region.