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Cutworm

Cutworm is the common name given to the larvae stage of numerous moth species that affect crops grown in Saskatchewan including cereals, oilseeds, pulses and forages. It is very important to scout for cutworms in the spring when the young seedlings are most vulnerable and the most damage can occur. It is not possible to predict or forecast cutworm populations for the upcoming crop season so, in order to avoid considerable crop loss, producers must be vigilant and prepared for any potential infestations.

There are around 20 different species of cutworms in Canada but only a few major species cause concern in Saskatchewan.

Pale western cutworm

pale western cutworm
Photo courtesy of John Capinera

First, there is the pale western cutworm; it prefers drier regions and is primarily found in southern Saskatchewan. It is a subterranean feeder, meaning it feeds on plant material at or below the soil surface. Younger larvae will feed on immature leaves underground and the damage is visible once the leaves emerge. Older larvae will feed on the stems just below the soil surface and have the ability to completely sever the stem, killing the plant.
Economic threshold:

  • 3-4 larvae/m2 in cereals
  • 4-5 larvae/m2 in flax and canola
  • 2-3 larvae/m2 in peas

Red-backed cutworm

Red-backed cutworm
Photo courtesy Courtesy of John Gavloski,
Manitoba Agriculture

The red-backed cutworm is less accustomed to drier regions and is more likely to be found in the northern and parkland areas of the province. These cutworms are above-ground feeders. Young larvae will feed on freshly emerged shoots and leaves while older larvae cut off leaves and sever entire plants at the soil surface.
Economic Threshold:

  • 5-6 larvae/m2 in cereals and grain corn
  • 4-5 larvae/m2 in flax and canola
  • 2-3 larvae/m2 in peas

Dingy cutworm

dingy cutworm
Photo courtesy of John Gavloski,
Manitoba Agriculture

Dingy cutworm are mainly found in eastern Saskatchewan. They are above-ground climbing feeders. They feed on leaves and rarely feed on stems above ground. Crops are at the greatest risk early in the spring but larvae will also feed on plants later in the fall before overwintering.

Economic Threshold:

  • 3-4 larvae/m2 in cereals
  • 25-30% standard reduction in oilseeds
  • 2-3 larvae/m2 in top 7 cm of soil in peas

Army cutworm

Army cutworm
Photo courtesy of Whitney Cranshaw,
bugwood.org

Lastly, the army cutworm like arid regions, especially western Saskatchewan and feed on above-ground plant material. Young larvae feed on leaves leaving holes and notches on the leaf margins while older larvae consume entire leaves.

Economic Threshold:

  • 5-6 larvae/m2 in cereals
  • No thresholds established in canola

Typical signs of cutworm damage in the field can include notching or holes in the crop foliage or plants that are wilting, falling over or completely cut off resulting in thin or bare patches throughout a field. It is important to confirm the symptoms a field is experiencing are being caused by cutworms and not poor germination or other pests like disease.

While scouting, it is important to identify the specific species of cutworm that is causing damage to a crop since their habitats, feeding behaviors and life cycles are very different. Scouting for cutworms should be done as often as possible, starting in early spring after plant emergence, until around the middle of July. Army and dingy cutworms overwinter as larvae and will cause the majority of their damage in the early spring while pale western and red-backed cutworms overwinter as eggs and need to grow larger before they are able to cause damage early to mid-summer. You will see cutworm damage typically on hilltops, south facing slopes and drier parts of the field but they can be found throughout other areas as well.

Carefully search the top one to two inches of soil around the base of any damaged plants. Be sure to also check the bases of healthy plants just outside the affected area as cutworms will move to healthy plants to continue feeding. Larvae largely feed at night and retreat underground during the day. If you see above-ground damage on the plant but do not see any cutworms, it is crucial to check beneath the soil surface to confirm the damage is truly from cutworms.

There are several ways to control cutworms in Western Canada. Cultural controls such as weed-free uncultivated fields in the late summer will be less attractive for overwinter egg laying. Seeding later in the spring and avoiding the larval stage of cutworms altogether is another option. If the level of cutworms is above the economic threshold for the specific crop in the field, there are foliar insecticides that are registered for the control of cutworms. It is recommended to spray in the late evening when the majority of cutworms will be feeding for the application to have the best effect. For more information on insecticides available for specific crops please refer to the Guide to Crop Protection.

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