Key Points:

• Three main species of armyworms attack field crops in the Midwest:  True armyworm, fall armyworm, and yellowstriped armyworm.
• True armyworm attacks only grass species, but yellowstriped and fall armyworm will also attack soybeans and alfalfa.
• Outbreaks are typically localized to specific fields, where habitat and environment favor development of the pest.
• Armyworm larvae often go undetected until damage is severe, so timely scouting is essential.
• If threshold damage is reached, a wide range of foliar insecticides will provide adequate control.

Introduction 
The three main species of armyworms that attack field crops are true armyworm, fall armyworm, and yellow striped armyworm.  Armyworms are named because of their behavioral tendency to 'march' in large numbers across open areas toward food resources.  True armyworms only attack grass species, but the fall and yellowstriped armyworms will also feed on soybeans, alfalfa, and vegetable crops.  Distinguishing between these species is essential to evaluate the nature of the threat level presented by these pests in any given cropping environment. 

Life Cycle and Identification
True armyworm (Pseudaletia unipuncta Haworth) moths migrate into our operating territory from the south in April and May.  Moths lay eggs in densely vegetated areas, like field borders, cover crops, standing cereal crops, or weedy areas.  Larvae hatch in about a week and are initially pale green in color.  More mature larvae vary in color from dark greenish brown to near black, with long pale whitish, orange, and dark brown stripes running down the full length of the abdomen.  The head capsule is a light brown-orange with a mottled appearance.  Identifying features are black bands on the outer sides of its 4 pairs of pro-legs.  Mature larvae are about 1 ½" in length. 

Fall armyworm (Spodoptera frugiperda) moths migrate into the Midwest from the Gulf Coast in May, with only onegeneration per year.  Larvae vary in color from nearly black to light tan.  Larvae have three yellowish-white lines down the dorsal side, with a wide dark stripe and yellow-red stripe on each side of the body.  Body segments have six distinctive black tubercules, each spiked with hairs.  The head is dark brown with an inverted "Y".  They feed deep inside the whorl of corn, sometimes killing the tassel and later entering the ear.  The larvae may cut soybean seedlings and feed on pods and stems.     

Yellowstriped armyworm (Spodoptera ornithogalli) are quite distinct in appearance with dark grey to black bodies and two prominent yellow stripes along each side of the body.  Two dark triangles may be visible on the top of each body segment and a dark spot above and behind the true legs, although these markings may be more difficult to see on darker colored larvae.  A dark head capsule is marketed with an inverted white "Y" and a net-like pattern.  Mature larvae are about 1 ¾" in length.  Yellowstriped armyworm overwinters as pupa in the soil and produces 3 generations per season.  Moths emerge in April and May and females lay clusters of 200 to 500 eggs on the underside of foliage.  Eggs hatch in 5-7 days and larvae feed for about 3 weeks, before then pupating and completing the cycle.  Yellowstriped armyworm is primarily a foliage feeder and may cause severe injury to young seedlings.    

Scouting and Crop Injury
Fall armyworm should be scouted in mid-late June through the corn silking stage.  True armyworms should be scouted late May to early June, and following burndown applications or maturity of cereal crops.  Yellowstriped armyworms are present from June through August.

Armyworm larvae are generally nocturnal feeders and often go undetected until crop damage is severe and obvious.  Early warning signs of armyworms include square notch feeding damage on the leaf edge and the presence of dark green fecal pellets, or frass, on the ground near the base of the plant or in the whorl.  As the larvae grow, leaves may take on a tattered appearance.  During the day, armyworms can be found taking refuge near the soil surface under vegetative residue or clumps of grass, or possibly hiding in the whorl of a corn plant and even retreating under the soil surface. 

Scouting is best done in the early morning while the larvae are still actively feeding in the plant.  When scouting, attempt to dislodge the larvae by shaking the plants, then look for them on the soil surface, near the crown of the plants, and under debris.  Pay particular attention to fields that were planted no-till, with cereal rye cover or emerged winter annual weeds, and poorly drained areas.  Also scout adjacent habitats like grass borders.  If you find armyworms, the following guidelines are used for treatment decisions:

• Wheat:  six (6) or more non-parasitized worms ¾"-1 ¼" in length per linear foot of row.
• Corn:  25% of corn plants injured and worms less than 1 ¼" in length still present.
• Soybeans: Consider treatment if yellowstriped or fall armyworm are present, larvae are less than ¾ - 1 ¼" long, and 20% of the plants are damaged or missing.

Management
Host plant resistance as Bt traits in corn may provide some measure of control, depending on the species.  The Cry1F protein found in Genuity SmartStax hybrids will provide some control against fall armyworm.  Only Vip3A is labeled for control of true armyworm.   Smaller larvae are easier to control and 80% of feeding injury occurs in the final instar.  Treatment decisions should consider the size of the larvae, the developmental stage of the crop, and the presence of natural enemies to predict whether economic damage will occur.  Border or spot treatments may be effective.  If threshold damage is detected in a timely manner, a number of different products are labeled for control.  Product choices include several of the common synthetic pyrethroids, carbamates, and spinosyns.  Please refer to product labels for suitability to crop and application rates.  Always read and follow pesticide label directions.         

Contact your FS Crop Specialist for your agronomic information.