Armyworms invading St. Lawrence County corn and hayfields
Wednesday, August 7, 2013 - 6:41 am


High numbers of true armyworm (aka common armyworm) were identified Monday, July 22 in a St. Lawrence County mixed alfalfa/reed canarygrass second-cutting hay field.

The grower described the field as “beautiful” on Friday the 19th, but three days later it was being cut for dry hay, and there was very little reed canarygrass left - only stems.

Armyworm larvae were plentiful on standing plants and on the ground. Most larvae were large, one to 1.25" in length, and very active. A few worms were brown and shriveled and a few were observed to have parasitic fly eggs attached.

The farmer was advised to scout adjacent hay and cornfields over the next several days for armyworms that may migrate from this infested field. In the event that larvae do make it to adjacent fields, it is recommended that the farmer spray field edges and boundaries to prevent additional damage.

Unfortunately, by the time this armyworm outbreak was discovered, the larvae were at their largest and were near the end of their feeding stage when they generally do the most damage. If the worms had been discovered one totwo2 weeks earlier, when they were less than half an inch long and causing significantly less damage, insecticide sprays may have been recommended.

Once the larvae are larger than one inch, they are almost ready to stop eating and burrow into the ground to pupate. At this point, spraying is a poor investment. This particular infestation, observed on July 22, is thought to be a second generation of larvae. Three generations are possible, each requiring about five weeks. Armyworm moths migrate from the southern U.S. each spring. The migrations are sporadic and vary from year to year.

Armyworms prefer plants in the grass family and so, as the moths arrive, they lay eggs on forage grasses, corn, small grains, lawns and grass weeds at field borders. The larvae hatch after a one-week incubation and grow over the next three weeks, feeding primarily at night. Under hunger stress, the armyworms will also feed on forage legumes and other plants.

Armyworms have some natural enemies, including several fungi and parasites. Each natural enemy helps to suppress armyworm populations.

The best approach to protecting crops from armyworm damage is to scout fields and detect infestations early, when larvae are small. Large larvae do most of the damage and are capable of destroying whole stands of corn, grasses and small grains.

Armyworm larvae feed at night, and so during the day, it’s best to look for signs of feeding, such as chewed ragged leaves, cut stems, lodged plants, and pellet-like frass on the ground. Larvae avoid sunlight during the day and will often hide under the plant canopy and within surface residue. In corn, look under the canopy and within the whorl for larvae and frass.

Armyworms can move from field to field in large numbers very quickly as food sources becomes scarce. As small grains dry down or are harvested, or as grass hay is cut, larvae can move quickly to another field or to alternate crops, such as corn. For management decisions, refer to the monitoring and management guidelines described in the Cornell IPM factsheet listed at the end of this article. Consider several factors in your decision, including the size of larvae, their relative abundance and impending harvest operations. Larger larvae (1 to 1.5”) are responsible for 80% of the feeding damage, which typically occurs during the last 7 days of larval feeding before pupation.

If sufficient numbers of smaller armyworm larvae and damage is present, an insecticide could be justified. Large armyworm larvae (> 1” in length) are much more tolerant of insecticides, reducing their effectiveness and economic viability. Always read, understand and follow insecticide label recommendations and comply with pre-harvest intervals

For more information about armyworms or other field crop concerns, contact your local Cornell Cooperative Extension office or call or email Kitty O’Neil at the St. Lawrence county office (315 379 9192, ext. 253; [email protected]).

Additional resources:

1. True Armyworm (aka Common armyworm) Alert – Update, Cornell IPM Program.

2. Armyworm (Pseudaletia unipuncta Haworth) Insect Fact Sheet, Univ. Illinois IPM Program.

3. Armyworm Damage to Field Corn, Small Grains, Grass Hay and Pasture – 2001, Univ. Vermont Extension.

This story is also available at

Kitty O’Neil, Ph.D., is a Regional Field Crops and Soils Specialist at Cornell Cooperative Extension, Northern New York.