Late Season Insects

From Amanda Homans, Decisive Farming Agronomist.

There are canola growers in the area that are spraying their fields for late season insects including lygus, bertha armyworm and diamondback moth. It is very important to keep an eye on your crops as insects can cause a large drop in yield if they break over their threshold levels. However, many fields are nearing the end of the susceptibility stage, which is when pods thicken and become leathery (about 2 weeks after flowering). When you are sweeping for insects, make sure that you are swinging the net from side-to-side, in a full 180-degree arch, doing one sweep per step.

Lygus:
There are many stages of development with Lygus bugs, and not all of these stages are detrimental to your canola crops. The tiny, bright green instars do not cause damage, as their mouth pieces are too small to puncture the pods. Even the oldest instars, the ones with the visible “V” on their backs, are still too immature to cause troubles. However, they are able to puncture the pod but not the seed. The mature adults are what you should be counting when you are doing your sweeping. If you are spraying your crops early, when there is still flowering going on and Lygus instars, you are actually doing more harm than good. Not only are you making an uneducated decision, but also spraying all the beneficial incest’s as well. According to the Canola Council, the threshold levels for Lygus are 20 adults per 10 sweeps at the pad stage.

Cabbage Seedpod Weevils:
The weevils are near threshold levels in parts of southern Alberta, but are down from previous years.

Root Maggots:
There are plants that are toppling over in some fields in central Alberta, growers are assuming disease, but once you take a closer look they find the roots full of maggots.

If you do decide to spray your crops for late season insects, you must pay close attention to the pre-harvest intervals. This is the number of days that must pass between the last application of a pesticide and cutting of the crop. Below is a chart that has some common insecticides and fungicides with their respectable pre-harvest intervals.

*chart from the Canola Council of Canada