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Get Ready to Scout for Cutworms (05/21/20)

As newly planted crops start to emerge, it is time to start routine scouting for cutworms.
As newly planted crops start to emerge, it is time to start routine scouting for cutworms.

Approximately 32 cutworm species are economic pest species of field crops in North Dakota! Cutworms damage plants in the larval stage (caterpillar) and cause plant injury by cutting stems near the soil line, chewing on the foliage and reducing plant stands. Cutworms often will move down a row as they continue to feed on plants. Cutworms can do significant feeding injury during the early growth stages (seedling through 4-6 leaf stage) of field crops.

There are three types of cutworms in North Dakota based on their biology:

 

1) Overwinter as a partially grown larva, usually one of the first cutworms to cause problems during crop emergence from early to mid-May. (e.g., dingy cutworm). The larval stage lasts for 8-9 months for these species since overwinter as young larvae.

2) Overwinter as eggs, which hatch in mid to late May (e.g., red-backed cutworm and darksided cutworm). Larval feeding injury by these cutworms normally occurs in late May to mid-June, and can last three to four weeks depending on temperatures.

3) Migrate as adult moths called ‘miller moths’ into North Dakota from southern states (e.g., black cutworm).

 

Routine scouting for cutworm larvae is best in the evening, since they feed at night and hide underneath clumps of soil and debris during the day. If you find cut off plants, dig around these plants about two or more inches deep, and search for cutworms. When disturbed, cutworms curl up into a ‘C-shape.’ Row crops, such as soybean, canola, lentils, field peas and sunflowers, are more susceptible to cutworm damage than small grains, because cut plants do not grow back (grains compensate by tillering).

All cutworm species are lumped together for the action thresholds. If cutworms are at or above the action thresholds listed below for different field crops, then a ‘rescue’ foliar insecticide application is warranted. An evening application is recommended to target the peak feeding of cutworms at night, but be sure to monitor for temperature inversion and do not spray during an inversion.

  • Alfalfa – 4 or more larvae per square foot (new stands – only 2 larvae per square foot)
  • Canola – 1 larva per square foot
  • Corn - 3-6% of the plants are cut and small larvae (<3/4 inch) present
  • Peas / Lentils – 2 to 3 larvae per square meter
  • Small grain – 4 to 5 larvae per square foot
  • Soybean - 1 larva per 3 feet of row or 20% of plants are cut
  • Sugarbeet - 4-5% cutting of seedlings or 3-5 larvae per square foot
  • Sunflower - 1 larva per square foot or 25-30% of plants cut

You can often find both small and large larvae of the same or different cutworm species while scouting. If the majority of the larvae are small, <¾ inch, they still have a lot of crop feeding to do before maturity, so an insecticide treatment will be necessary when you are at or above the action threshold. If you are finding a mixture of some small cutworms, many large cutworms and some pupae, it may be too late for a foliar insecticide application since the majority of the larvae are mature (done feeding) and/or pupating (a non-feeding life stage). Remember most cutworms are difficult to kill, so they require the mid- to high labeled rates of an insecticide for effective control. A low rate of insecticide may not provide the kill needed and you may need to respray a field with high densities of cutworms. Another advantage of using the mid- to high labeled rates of insecticides is that you get a longer residual of 7-10 days for most pyrethroid insecticides.

For insecticides registered for cutworm control by field crops, please consult the 2020 North Dakota Field Crop Insect Management Guide E1143.

 

 

Janet J. Knodel

Extension Entomologist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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