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Early-Season Scouting of Cutworms

By Sara Tetland, AAg, Crops Extension Specialist, Moose Jaw

Redbacked cutworm
Redbacked – photo John Gavloski

Cutworms are early-season pests that can affect a variety of crops in Western Canada. There are about 20 different species of cutworm pests in Canada, but, fortunately, only a few are capable of causing economic damage in Saskatchewan.

Cutworm is the common name given to the larvae stage of numerous moth species. Only the larvae stage of these species cause damage to crops, with either underground, above-ground or climbing feeding behaviours.

Signs of cutworms in the field can include notches or holes in foliage; plants that are wilting, falling over, or completely clipped off; and thin or bare patches. It is important to confirm that the symptoms are caused by cutworms and not by poor emergence or other issues such as damping-off disease, which may cause bare patches or wilting.

When scouting, it is also important to identify which species of cutworm is in your crop. Cutworms have diverse, species-dependent habitats and feeding behaviours that can affect your crop in a variety of ways. The lifecycle timing and length also varies between species, with some larval earlier or later in the growing season than others.

Scouting for cutworms should be done regularly starting early in the spring after plant emergence until approximately mid-July. Dingy and army cutworms both overwinter as larvae and cause most of their damage earlier in the season, whereas pale western and red-backed cutworms overwinter as eggs and need to advance to the larvae stage to cause crop damage. Cutworms tend to cause damage at hilltops, south-facing slopes and drier parts of the field, but they may be found in other areas, as well.

To scout, carefully search the top one to two inches of soil around the base of damaged plants. Cutworms will move from damaged plants to healthy plants to continue feeding, so also search for them at the base of healthy plants within or around the outside of the affected patches. The larvae tend to feed during the night and stay underground during the day. Therefore, even if the pest is not found on plants that show aboveground feeding symptoms, cutworms may be found on or beneath the soil surface. Sample multiple sites, a square meter around affected plants. The threshold is typically four to five larvae per square meter, but varies by species and crop type. There are various management practices that can be used to control cutworms. Natural enemies of cutworms, including parasitoids and predatory insects, are particularly important for ending multiple year outbreaks. Refer to Agriculture and Agri-Food Canada's Cutworm Pests of Crops on the Canadian Prairies for more information.

Cultural control options vary depending on the species of cutworm present. Weed-free, uncultivated fields in late summer are less attractive for egg laying for overwintering. Seeding later in the spring or cultivating the soil and keeping it black before seeding can starve cutworms that overwinter; however, the risk of soil erosion has to be taken into account when considering this control option.

If the level of cutworms is above the economic threshold for a specific crop, there are foliar insecticides available to control the pest. Apply pesticides in the late evening when larvae begin feeding so that they come in contact with the spray. For more information on insecticides available for specific crops, please refer to the 2020 Guide to Crop Protection.

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