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Black Cutworm and True Armyworm Identification, Scouting and Management

Categories: PLANTING, CORN
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Spring weather encourages both black cutworm and true armyworm to come back to many fields throughout the Corn Belt. That’s why we’re reviewing identification, desired field environment, scouting and best management practices for controlling both black cutworm and true armyworm.

Identification and Desired Field Environment
  • Black cutworm identification: 
    • Rough, grainy skin ranging from light gray to black
    • Pale, indistinct, narrow stripe along the center of the back 
    • Curls into a "C" shape when disturbed 
    • Although black cutworm looks similar to dingy cutworm, make sure to not confuse the two since dingy cutworm doesn’t cause economic damage to corn
  • Black cutworm desired field environment: 
    • Prefers weedy fields to lay eggs 
    • Finds fields full of chickweed, shepherd’s purse, peppergrass and yellow rocket highly attractive
  • True armyworm identification: 
    • White-bordered, orange line down each side of the body
    • A large, single dark spot at the base of each fleshy, abdominal proleg
  • True armyworm desired field environment: 
    • Tends to prefer more grassy type environments 
    • Finds corn fields no-tilled into rye grass cover crop, wheat fields, pastures and grass waterways more attractive
Scouting Best Practices
  • Start with your high-risk fields, such as emerged cornfields that had heavy green growth over the last several months, including Cover crops or weeds
  • Determine 3 key field items
    • Percentage of cut or damaged plants
    • Corn growth stage
    • Average development stage of the larvae
  • Select 5 different areas of the field to get a good representation, and it  may be helpful to flag these areas so you can monitor developing populations
  • Randomly select a plant to inspect for leaf feeding or plant cutting at each location 
  • Repeat for the next consecutive 19 plants, for a total of 20 at each location, and record your results
  • Determine the corn plant growth stage of the 10th & 20th plant at each location
  • Scout for live larvae and collect around 10 total to determine the average stage of development
  • Scout early morning or near dusk, as both pests are more active during nighttime hours
Black Cutworm & True Armyworm Management with Insect Traits

It is important to know that there are many good corn insect traits on the market today to help manage grower risk for black cutworm and true armyworm, but corn insect traits vary in their control and effectiveness. Agrisure Viptera® is labeled for control of both black cutworm and true armyworm. 

Managing Black Cutworm without Insect Traits

Historically, 3% cut plants has been the trigger point to consider insecticide treatments, but farmers should consider cost of treatment, corn markets, expected yield and how many plants it takes to accomplish that yield target. Iowa State University has an Excel-based threshold calculator to help consider all these factors. 

It’s also key to factor in size of the larvae and how much longer they will feed. The smaller the average larvae size, the lower the threshold. Larvae greater than ¾” in size increases threshold levels. However, the Iowa State University calculator doesn’t factor larvae size into their threshold equation.



To find out more about black cutworm and true armyworm pressure near you, check out your local Extension site for more details on treatment thresholds: 

University Extension
Insect Economic Thresholds
Colorado State University

University of Illinois

Iowa State University

Kansas State University

Michigan State University
University of Minnesota
Missouri State University
University of Missouri
North Dakota State University
University of Nebraska-Lincoln
Ohio State University
Purdue University
South Dakota State University
University of Wisconsin

True Armyworm Economic Damage

Historically, it takes true armyworm quite a bit more feeding to cause economic damage because armyworms don’t often cut plants, but rather they prefer to feed on the leaf tissue. This results in quicker recovery and less dead plants, so thresholds for control treatments are closer to 25% of the plants being fed upon and larvae averaging 1” or less in size.


 
Insecticide Treatments 

Reach out to your local Extension service for a complete list of insecticides labeled to control black cutworm and true armyworm. Contact your Golden Harvest Seed Advisor with questions or for additional agronomic insights.

Photos are either the property of Syngenta or used under agreement. 

Syngenta hereby disclaims liability for third-party websites.


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