Pests

Identification, Scouting and Management of Black Cutworm and True Armyworm

April 27, 2022
 by NK Canada Agronomy Team
Every year, there are a number of weather events that help to bring moth flights for both black cutworm and true armyworm to the Great Lakes geography. Links to articles for both pests are contained in this article, along with general information on Identification & Desired Field Environment, Scouting Best Practices and Management. 
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Black Cutworm and True Armyworm

Identification & Desired Field Environment:

  • Black Cutworm Identification1: Rough, grainy skin ranging from light gray to black; Pale, indistinct, narrow stripe along the center of the back; Curl into a "C" shape when disturbed. Note: BCW can be confused with Dingy cutworm. This is important because Dingy cutworm does not cause economic damage in corn.
  • Black Cutworm Desired Field Environment: Prefer weedy fields to lay their eggs. Fields full of chickweed, shepherd’s purse, peppergrass, and yellow rocket are highly attractive to BCW moths.
  • True Armyworm Identification2: 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: Tend to prefer more grassy type environments. Also, corn no-tilled into rye grass cover crop, wheat fields, pastures, and grass waterways are more attractive to them.

1 Black cutworm Identification and picture source: University of Missouri, https://ipm.missouri.edu/pestmonitoring/bcw/identification.cfm
2 True Armyworm Identification and picture source: University of Missouri, https://extension2.missouri.edu/g7115

Scouting Best Practices:

  • Start with your high risk fields such as emerged corn fields that had heavy green growth over the last couple months (Cover crops or weeds).
  • You will want to determine three key items in the field being scouted.
    1. Percentage of cut or damaged plants.
    2. Growth stage of the corn.
    3. Average stage of development of the larvae found in the field.
  • Select 5 different areas of the field to get a good representation of the field. Note: You may want to flag these areas so you can return to these area to monitor developing populations.
  • At each location, randomly select a plant to inspect for leaf feeding or plant cutting. 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.
  • Additionally, scout for live larvae. Collect around 10 in total to determine average stage of development.
  • Scout early morning or near dusk, as both pests are more active during night-time hours.

Management-(Corn with Insect Traits)

Black Cutworm & True Armyworm
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. Viptera™ is labeled for control of both Black Cutworm and True Armyworm. Please refer to the following link for how Viptera™ compares to competitive traits for control of Black Cutworm, True Armyworm and other above ground insects.

Check out these comparison charts for yoru reference: https://www.syngenta.ca/agrisure-viptera, https://utcrops.com/wp-content/uploads/2021/01/BtCornTraits.pdf or https://lubbock.tamu.edu/files/2018/01/BtTraitTableJan2018.pdf.

Management-(Corn without Insect Traits) Black Cutworm
Historically, 3% cut plants has been the trigger point, but growers should consider cost of treatment, price of corn grain, their expected yield, and how many plants they will have to accomplish that yield target. Iowa State University has an Excel based threshold calculator that helps consider all these factors.

This Excel calculator is a great rBlack Cutworm Development Guideesource: http://crops.extension.iastate.edu/cropnews/2009/05/dynamic-black-cutworm-action-threshold

We would also encourage growers to factor in size of the larvae and how much longer they will be feeding. The smaller the average larvae size the lower the threshold. Larvae greater than ¾” in size increases threshold levels. The Iowa State University calculator doesn’t factor larvae size into their threshold equation. 

Source: Purdue University, https://extension.entm.purdue.edu/fieldcropsipm/insects/black-cutworms.php


Black Cutworm University Treatment Thresholds

U of IL: http://extension.cropsciences.illinois.edu/fieldcrops/insects/black_cutworm/
Purdue: https://extension.entm.purdue.edu/fieldcropsipm/insects/black-cutworms.php
ISU: https://crops.extension.iastate.edu/cropnews/2009/04/black-cutworm-treatment-options
U of MN: https://www.extension.umn.edu/agriculture/corn/pest-management/black-cutworm/
U of KY: https://entomology.ca.uky.edu/ent59
UNL: http://extensionpublications.unl.edu/assets/pdf/g1153.pdf
PSU: http://ento.psu.edu/extension/factsheets/black-cutworm
MSU: http://msue.anr.msu.edu/news/scout_now_for_black_cutworms_and_armyworm_moths
U of TN: https://extension.tennessee.edu/publications/Documents/SP290-J.pdf
U of MO: https://ipm.missouri.edu/pestmonitoring/bcw/identification.cfm
OSU: https://ohioline.osu.edu/factsheet/ENT-35
U of WI: http://labs.russell.wisc.edu/vegento/pests/black-cutworm/
SDSU: https://openprairie.sdstate.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=1497&context=extension_circ
NDSU: https://www.ag.ndsu.edu/extensionentomology/field-crops-insect-pests/Documents/corn/corn-insects-of-north-dakota-affecting-the-crop-after-emergence
NCSTATE: https://entomology.ces.ncsu.edu/field-corn-insects/scouting-and-thresholds/scouting-for-seedling-insects/
KSU: https://entomology.k-state.edu/extension/insect-information/crop-pests/corn/cutworm.html


True Armyworm

Historically it takes quite a bit more feeding to cause economic damage because armyworm don’t often cut plants, 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 average size of 1” or less.


True Armyworm University Treatment Thresholds

NDSU: https://www.ag.ndsu.edu/extensionentomology/field-crops-insect-pests/Documents/corn/corn-insects-of-north-dakota-affecting-the-crop-after-emergence
MSU: http://msue.anr.msu.edu/news/scout_now_for_black_cutworms_and_armyworm_moths https://ipm.missouri.edu/pestmonitoring/taw/taw.pdf
U of MN: https://swroc.cfans.umn.edu/ipm-stuff-2017-true-armyworm
U of KY: https://entomology.ca.uky.edu/ef109
Purdue: https://extension.entm.purdue.edu/fieldcropsipm/insects/corn-armyworm.php
PSU: http://ento.psu.edu/extension/factsheets/armyworm
KSU: https://entomology.k-state.edu/extension/insect-information/crop-pests/corn/armyworm.html
OSU: https://agcrops.osu.edu/newsletter/corn-newsletter/2017-13/armyworm-corn-and-small-grains
U of WI: http://ipcm.wisc.edu/blog/2013/07/armyworms-in-corn-and-wheat/
ISU: https://www.extension.iastate.edu/winneshiek/sites/www.extension.iastate.edu/files/winneshiek/CropNotes/First%20Generation%20True%20Armyworm.pdf
UNL: https://cropwatch.unl.edu/armyworm-moths-abundant-watch-armyworm-larvae
U of DE: http://extension.udel.edu/weeklycropupdate/?tag=true-armyworm
NC State: https://entomology.ces.ncsu.edu/field-corn-insects/scouting-and-thresholds/scouting-for-seedling-insects/
CSU: http://extension.colostate.edu/topic-areas/insects/armyworms-5-503/

Speak to your local NK® Agronomist for more information and to get advice for your field. Or, join the conversation online – connect with us at @NKSeedsCanada on Facebook and Twitter.

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