Canola School: Bare patches could signal cutworm feeding

Cutworms are a common pest in several crops, including canola, across the Prairies. These below-ground dwellers cause damage by clipping or severing stems of seedlings, so unlike other pest damage, cutworm feeding kills young plants.

In this episode of RealAgriculture’s Canola School, Kara Oosterhuis sits down with Dr. Vincent Hervet, research scientist with Agriculture and Agri-Food Canada at Winnipeg, Man., to discuss biocontrol options for cutworms, specifically a species of parasitoid wasp that lays eggs inside the cutworm.

When the eggs hatch and develop into larvae, they feed on the internal organs of the cutworm, killing it, he says. One parasitoid species that’s found in Ontario has the potential to be relocated across Canada as a predator of cutworms.

Injury to plants can be seen in the field from the last week of May to the last week of June, when canola is just starting to grow. Cutworms prefer dry conditions, and eggs are laid in the field the year before and overwinter as either eggs or larvae. Some species overwinter as eggs, will hatch in the spring, then start to feed. Other species will overwinter as larvae and begin feeding the previous fall, then resume when temperatures warm up in the spring.

Hervet says that you can scout for cutworms at any time of the day but you will have to dig to find them since they will hide in soil. To start, look for missing, dead, or dying plants that have created gaps within rows or bare patches in a field. Cutworms will feed on a plant, then move to the nearest plant.

“Usually these bare patches happen on the hilltops, especially if it’s sandy, or on south facing slopes because they prefer these conditions. That’s where the eggs are laid,” Hervet says.

To ensure if the damage is in fact caused by cutworms, “examine the top two or three inches of soil, in a one foot by one foot square,” says Hervet. Count the number of larvae in this one foot square area at about ten sites along the edges of an affected area, as cutworms will spread from the initial bare patch to more green plants to feed on.

Average the of larvae at the ten sites, then multiply by 10. This will give you the number of larvae per square meter and be an indicator of an economic threshold.

The most common species of cutworms that affect canola are: redback cutworm, pale western cutworm, dingy cutworm, and sometimes the dark-sided cutworm. Economic thresholds vary from four to five larvae per square meter, for the redback cutworm to 25 to 30 per cent of  stand reduction for the dingy cutworm.

Knowing what species is present will be important for those thresholds but also for insecticide effectiveness. Pale western cutworm will remain underground and are more difficult to control with insecticide. The other three most common species will come above ground to feed on plants at dusk, so insecticide is more effectively sprayed then.

“If you see young cutworm in a field in the fall, then you should be careful.” The eggs are laid at the end of the summer or in early fall, while their adult form, the moth is looking for uncultivated fields with volunteer weeds. When cutworms are about an inch long they are ready to pupate and will stop feeding.

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