Armyworm

Overview

Armyworms, a common agricultural pest, can cause significant damage to plants if left unchecked. Feeding on plant leaves and quickly stripping them away, armyworms leave behind a skeletonized leaf appearance and reduce the plant’s ability to carry out photosynthesis.

While armyworms primarily target grasses and cereal crops, they are also known to attack corn, soybeans and vegetables. In the worst-case scenario, they can even devour the developing ears of corn, leading to a complete crop failure.


How Do Armyworms Impact Corn Crops?

Adult moths are attracted to late-maturing corn, where they will lay their eggs. Larvae arrive late in the season and feed on corn leaves, causing damage in patches throughout the field that is similar in appearance to that of hail injury.

When armyworms feed on corn plant leaves, they remove significant amounts of leaf tissue, directly impacting the plant's ability to produce energy through photosynthesis. As a result, corn plants impacted by armyworms may experience:

  • Stunted growth

  • Reduced ear size

  • Decrease in overall yield

  • Newly created entry points for pathogens, leading to the introduction of secondary infections, such as fungal diseases

(Image Credit: Billy R. Wiseman, USDA Agricultural Research Service, Bugwood.org)

How to Identify Armyworms

Armyworms get their name from their behavior of moving in large groups, resembling an army marching across a field.

The pests are typically light green to dark brown in color, with distinct longitudinal stripes running along their bodies. One unique feature to look out for is the inverted "Y" mark on their heads.

When scouting corn for armyworms, look for:

  • Ragged edges and skeletonized leaves, particularly in the lower canopy

  • Dark green or black droppings on the leaves or on the ground

  • The actual larvae themselves, which can often be found curled up in the whorls of the corn plant or hiding in the soil during the day

(Image Credit: Frank Peairs, Colorado State University, Bugwood.org)

Regions Affected by Armyworms

Armyworms can be found in various regions across the United States, but they tend to be more prevalent in the Midwest, Southeast, and parts of the Northeast. These regions provide the ideal conditions for armyworms to thrive, including warm temperatures and a wide range of suitable host plants.

When Do Armyworms Impact Crops?

Armyworm infestations can occur at various stages of the corn crop's growth. While they are more frequently observed during the late season, particularly in late summer and early fall, early-season infestations can also occur, especially when the weather is warm and moist.

How to Manage Armyworm Infestations

To proactively manage armyworms, implement good agricultural practices such as:

  • Proper crop rotation

  • Timely planting

  • Maintaining optimal soil moisture levels

  • Destroying crop residues

Crop rotation helps disrupt the life cycle of armyworms by planting different crops in successive seasons, reducing the availability of their preferred host plants. Timely planting ensures that crops are at a stage of growth that is less susceptible to armyworm infestations. Maintaining optimal soil moisture levels reduces the likelihood of armyworms by avoiding the creation of a hospitable environment. Finally, destroying crop residues after harvest eliminates potential overwintering sites for armyworms, reducing their population in the following season.

However, these approaches may prove insufficient in some scenarios with a substantial infestation. When deciding whether to employ chemical products to control armyworms, farmers should first assess the severity of the infestation and determine the economic threshold for treatment. Applying an insecticide is usually not economical for control unless larvae are less than 1.25 inch long, plants are under stress, and 75% of plants have whorl feeding damage.

After deciding to move forward with a chemical treatment, there are a few insecticide options available, including the following:

Willowood Lambda-Cy 1EC*

This popular insecticide contains lambda-cyhalothrin, which is labeled for use on corn specifically for armyworm control. Apply 2.56-3.84 fl oz/A as required by scouting, or locally prescribed corn growth stages, usually at intervals of seven or more days. Use higher rates for large larvae. 

GCS Bifenthrin 2EC*

  • At planting application: Apply 0.15-0.30 fl oz. per 1,000 linear ft. of row. For use on corn at-planting, apply a 5-inch to 7-inch T-band over the open seed furrow. Center the spray nozzle over the row behind the planter shoe in front of the press wheel. In-furrow pop-up fertilizers may be used alone or in tank mixtures with GCS Bifenthrin 2EC. 

  • Pre-plant incorporated: Apply 3-4 fl oz/A as a pre-plant incorporated treatment either alone or in a tank-mix combination with registered pre-plant incorporated herbicides.

  • Preemergence: Apply 2.56 fl oz/A tank mixed with preemergence herbicides.

  • Foliar: Apply 2.1-6/4 fl oz/A by ground or air. 

Insecticides containing the active ingredient chlorantraniliprole have also shown great efficacy in combating armyworms specifically in corn crops. Many post-plant insecticide products, including those in the pyrethroid, organophosphate, carbamate, and diamide insecticide groups, effectively control armyworms. 

Early morning or evening applications may be most effective, since armyworms spend most of the daylight hours low in the canopy, lodged under plants. As well, high rates of neonicotinoid seed treatments (e.g., Poncho, Cruiser, Gaucho) may provide some early seedling protection against armyworm.

Disclaimer

*GCS Bifenthrin 2EC and Willowood Lambda-Cy 1EC are restricted use pesticides.

*RESTRICTED USE PESTICIDE Due to acute toxicity. For retail sale to and use by certified applicators only – NOT to be used by uncertified persons working under the supervision of a certified applicator.

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Every product that labels ARMYWORM as a weed, pest, or disease the product can be applied against is shown below. Click any product for label and more information.
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