There are two to three generations of armyworm each year in Missouri, but the larvae of the first generation in May and June usually cause the most feeding damage.
There are two to three generations of armyworm each year in Missouri, but the larvae of the first generation in May and June usually cause the most feeding damage.

Armyworm feeding has been reported in wheat in Central Missouri as well as some severe feeding in some grass pastures in Western and NW Missouri. Here are some facts for scouting and threshold information on this occasional pest to Missouri crops.

The true armyworm, Pseuduletia unipuncta (Haworth), overwinters as a partly grown larva in Missouri. Resident armyworms are further supplemented by migrants that arrive during the first week of April. There are two to three generations each year in Missouri, but the larvae of the first generation in May and June usually cause the most feeding damage. This insect requires 41 to 66 days to complete a generation (egg to adult). Female moths prefer to lay their eggs in dense, grassy vegetation. True armyworm larvae have two characteristics that distinguish them from other armyworm larvae by:

  • White-bordered, orange line down each side of the body.
  • A large, single dark spot at the base of each fleshy, abdominal proleg.

Armyworm Facts at a Glance

  1. Several armyworm species may attack field and forage crops in Missouri.
  2. True and yellowstriped armyworms are more early-season pests, whereas the beet and fall armyworms are generally late-season ones.
  3. The true armyworm primarily attacks grass crops (e.g., corn, fescue, sorghum, wheat) and weedy grass species, whereas the other three armyworm species also may attack alfalfa, cotton, and vegetables.

Armyworms can be serious pests of wheat when populations reach large numbers. Armyworms get their name from their migrating habit, as they sometimes start at one portion of the field and devour everything in their path. Damaging infestations of true armyworm normally occur in the spring. Besides feeding on foliage, larvae will sometimes cut the heads of maturing wheat plants.

Wheat

armyworm in wheat field

True Armyworm larvea found in wheat field.

Rank or dense fields of grasses or wheat are the most common infestation sites for true armyworm larvae. Scouting for true armyworms is best done late in the evening or during early morning hours because the larvae are primarily nocturnal feeders. They usually remain hidden on bright, sunny days. Producers should beginning scouting their wheat fields for this pest from mid April until harvest. Take note of larval sizes, percent parasitism and the insect's pepperlike droppings on the ground.

Economic threshold

Corn

For the true armyworm, treatment is recommended when 25 percent or more of the seedling corn plants are damaged and larvae are still present. Control of fall armyworm larvae is recommended when 75 percent or more plants are infested and larvae are less than 1.25 inches in length. Insecticide applications to control larger larvae or those within the ears are not effective. In general, it is not economical to treat for fall armyworms unless infestations are severe and plants are stressed by environmental factors. Precaution: Before you select and apply an insecticide, review the manufacturer's label for information on the proper and safe use of the material.

Grass pastures

Insecticide treatments are justified when four or more nonparasitized, half-grown or larger larvae are present per square foot.

Wheat

Insecticide treatments are warranted when an average of four or more nonparasitized, half-grown or larger true armyworm larvae per square foot are present during late spring and before two to three percent of the heads are cut from the plants. Parasitic flies commonly attack true armyworm larvae, and the flies' small, oval, yellowish eggs are laid on the larva's body or behind its head. The probability of yield loss increases when larvae destroy the flag leaf and before the plants complete the soft dough stage. Insecticides should be applied late in the afternoon to maximize larval exposure.

Numerous insecticides are labeled for armyworm control. Always read and follow label directions.

For further information on Armyworm:
Management of the Armyworm Complex in Missouri Field Crops
MU IPM True Armyworm ID Guide