What are true armyworms?
True armyworms (Mythimna unipuncta) are a species of moth that lays their eggs in lush, green grasses. They mainly affect corn and wheat production. Pastures and grassy hay fields can also be at risk of damage.
What do true armyworms look like?
Larvae: The larvae range from brownish-green to black with orange, brown, and white stripes running down their backs. The head is light brown with dark markings. Full-grown larvae are about 2 inches long before pupating.
Adults: The adult moths are beige-colored with a wingspan of about 1.5 inches. The moths are identifiable by a distinctive small, white dot in the center of each forewing.
Photo of a 5th instar true armyworm larvae. Photo credit: Krista Hamilton, DATCP.
What is the life cycle and biology of true armyworms?
True armyworm moths are nocturnal and lay their eggs in the evening on the leaf blades of their host plants. Starting in April or May, they produce 2-3 generations per season in Wisconsin with the second generation in early July being the most damaging.
The eggs hatch after 7 to 10 days, and the larvae then feed on the corn and wheat foliage, as well as grass in hay fields or pastures for about 25 days. Armyworm larvae develop through six instars before pupating and emerging as moths. The pupal stage requires approximately 21 days to complete.
They do not overwinter in Wisconsin, but instead migrate to from the milder climate of the southern United States and northern Mexico, returning each spring with weather systems.
Source: Purdue University Extension; Illustration: Farm Journal
What are the symptoms of true armyworm damage?
True armyworms eat the leaf tissue of plants, in severe cases entirely stripping the leaves.
Once they consume all of the grassy plants in the early summer, they move onto corn and small grains.
Infestations are more common in fields planted after cover crops such as those with little to no tillage, and those with a high density of grassy weeds.
Photo of true armyworm leaf feeding on corn (light). Photo credit: Krista Hamilton
How do you scout for true armyworms?
Corn should be scouted in early June for any sign of true armyworm damage, taking care to look in the whorls and under the plants on sunny days. Check 5 sets of 20 plants around the field at random for the presence of larvae or damage, noting the size of the worms.
In small grains, 3 feet of a row should be examined in 5 different areas. Shake the plants vigorously to dislodge the worms and count the number found while noting their size.
Photo of a true armyworm feeding in a corn whorl. Photo credit: Claire LaCanne
What is the threshold to reach for true armyworm treatment?
The economic threshold for corn is 25% of plants with 2 or more larvae or 75% of plants with 1 larva. In small grains, the threshold is 3 larvae per square foot.
Integrated pest management (IPM) strategies for true armyworms
Cultural Control: Fall termination of forage crops in rotation to corn is a good way to lessen the number of egg-laying sites in the spring. Controlling the amount of grassy weeds in the field can have the same effect if done before the eggs are laid in early April.
Biological Control: Certain species, such as ground beetles and parasitic hymenoptera, prey on the true armyworms and limit their populations. Another species, the red-tailed tachinid fly (Winthemia quadripustulata), lays eggs on the backs of the larvae which hatch and bore into the worm to eat.
Chemical Control:
Refer to A3646, Pest Management in Wisconsin Crops, by following these links for true armyworm insecticide recommendations in corn, pasture, alfalfa (pg. 177), and small grains (Pg. 197).
For any plant, chemical treatment is only suggested if the larvae are ¾ of an inch or smaller because larger true armyworms do not respond to insecticide. Be mindful that larvae may migrate from dead grass to crops if an herbicide is used on weeds.
Refer to A3646, Pest Management in Wisconsin Crops, by following these links for true armyworm insecticide recommendations in corn, pasture, alfalfa (pg. 177), and small grains (Pg. 197).
For any plant, chemical treatment is only suggested if the larvae are ¾ of an inch or smaller because larger true armyworms do not respond to insecticide. Be mindful that larvae may migrate from dead grass to crops if an herbicide is used on weeds.
Monitor weather conditions and alfalfa growth stage to determine optimal timing for harvesting or insecticide applications, as needed.
Apply insecticides according to label instructions and consider factors such as application method, rate, and pre-harvest intervals.
Consider alternative forage crops or management practices in severely infested fields.
Environmental and ecological considerations for true armyworm treatment
Minimize pesticide use whenever possible to reduce potential impacts on non-target organisms and environmental health.
Implement IPM practices that promote biological diversity and ecosystem resilience.
References
Additional resources
For assistance with alfalfa weevil management and other agricultural pest issues, contact your local agricultural extension office or entomology expert.
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