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Armyworms and Their
Control in Fields

Introduction

Livestock and hay producers in Missouri should be on the lookout each spring for a severe outbreak of armyworms in pastures and hay fields.  These infestations can be attributed to winter die-off of natural predators and parasites coupled with a cool wet spring that encourages lush growth.

Importance and Type of Injury

The armyworm fluctuates greatly in abundance, undergoing cycles which reach destructive peaks at greatly varying periods of years.  During epidemics, it often destroys much of the vegetation over many hundreds of square miles.  Corn under 8 in. (203 mm) in height that is attacked by armyworms will usually have the leaves eaten off entirely.  With larger corn the midrib of the leaves will sometimes be left, but the center of the young stalk is so eaten out that it dies.  The dark-green worms, up to 2 in. (50.8 mm) in length, with white stripes on the sides and down the middle of the back, will be found hiding under clods and stones or in the center leaves of the plant during the day.  The damage usually starts at the sides of the field, where the worms have moved in from some other crop.

Plants Attacked

All grass crops, especially corn, timothy, millet, bluegrass, small grains, and some legumes, and under stress of hunger, many other plants.

Distribution

United States and Canada, east of the Rockies, and many other parts of the world.

Life History, Appearance, and Habits

The winter is passed mainly in the partly grown larval stage, but the fact that the moths are abroad very early in the spring in the northern states would indicate that some of the insects winter as adults, or as pupae, or that there is a spring flight northward from the southern part of the range of the insect.

The moth is a light brown or tan and is identified by a salmon-colored 'apostrophe' or 'exclamation mark' on its back in the middle of the forewing  (see Photo #1 and #2). Their rate of development is temperature related, so it may be hastened by warm weather or slowed down by cool weather.  Initially, because the spring temperatures are typically cool, it takes 2-3 weeks from the time the moth lays the egg until they have hatched and grown through the early instars.  Instar is a rating, or size that describes the degree of development or maturity of the larvae. It is at the 3rd through 5th instars that the caterpillar or worm feeds, and the stages which are most destructive. It will take approximately 5-7 days for them to grow from the 1/2 inch to the 1 1/2 inch length.

Photo #1 Pupae and Adult Moth Photo #2 Adult Moth
Photo from Lee Jenkins Slide Collection, Univ. of Missouri Photo from Wayne Bailey, Univ. of Missouri

The female moths lay their greenish white eggs in long rows or clusters on the lower leaves of grasses to the number of 500 or more.  The leaf is generally folded lengthwise, and fastened about the eggs with a sticky secretion.  The young worms are pale green in color and have the looping habit of crawling until about half-grown.  They begin feeding early in the spring, become full-grown by the early to mid-May in the latitude of southern Missouri. They may often be found by thousands in fields of grass or small grains, and because of their habit of feeding at night, their presence is generally not suspected until the crop is nearly destroyed.  When the food supply becomes exhausted in the fields where they have hatched, these caterpillars move out in hordes or armies and attach crops in nearby fields.  These crawling masses of worms have given them their common name (see Photo #3).

Photo #3 Armyworm Larvae on Wheat


Photo from Lee Jenkins Slide Collection, Univ. of Missouri

On becoming full-grown, the worms are nearly 1 1/2 in. (38 mm) long. Larvae have a greenish brown body, nearly hairless, smooth, with a thin stripe down the center and two orange stripes along each side (see Photo #4).  The head is brown in color and often exhibits a honeycomb pattern of markings, or inverted V marking.  Each proleg has a dark band on its outer side and a dark tip on the inner side. 

Photo #4 Armyworm Larvae full-grown


Photo from Lee Jenkins Slide Collection, Univ. of Missouri

When the larvae reach about 1 1/2 inches in length, they pupate, or go into the resting stage by burying into the ground and making a cocoon.  The pupae are dark brown, about 3/4 in. (19 mm) long, tapering sharply at the tail, and blunt at the head end (see Photo #1).  They remain in this stage for 10-14 days, or longer if the weather is cool, and then transform to uniform, pale-brown or brownish gray moths with a wing expanse of about 1 1/2 in. (38 mm). They complete their life cycle when they emerge as the adult moths. The moths are strong fliers, but remain hidden during the day, becoming active at night.  They are attracted to lights, and strongly so to sweets or decaying fruit.

There are from two to three generations each year.  Typically, the second generation of moths do not pose the serious problem of the first generation, because (1) the moths that emerge typically move north, and (2) crops and weather conditions change by the time the second generation comes around. The larvae of the first generation do most of the damage, in April-May in the latitude of southern Missouri.  The larvae of the last generation are abundant in late August and September.

Control Options in Fescue Fields

If the fescue is to be used for hay, it can be mowed. As the hay cures, it becomes less desirable as a food source for the armyworms. Armyworms tend to avoid fields that contain legumes (alfalfa or clovers).

Eggs of this pest are often laid in the thickest vegetation so look at these areas first when scouting.  Larvae will begin feeding during the night time on lower plant tissue and spend the daylight hours in plant debris on the ground surface.  As larvae grow in size, they will spend more time feeding during daylight hours and feeding on the upper tissues of the plant.  In some years larvae will cut seed heads from fescue and wheat plants.  The best time to scout for this pest is at dusk, dawn, or during the night.

In fescue seed fields and pastures treatment for this pest is justified when an average of 4 or more non-parasitized, half-grown or larger worms per square foot are present during late spring and before more than 2 to 3% of seed heads are cut from stems by this pest. Scout at dusk, dawn, or at night for best results. Small larvae feed on foliage at night and remain in plant debris near ground surface during day.

One hour of dry weather should be sufficient to insure the efficacy of most insecticides, once they are applied.  A little rain may actually enhance the activity of the insecticide by getting it down on the ground.  There typically should not be a problem of the sprays reaching the worms even if there is a heavy cover.

If an insecticide application is necessary on fescue or grass pastures, use one of the following insecticides listed here as a foliar broadcast spray. Follow all label directions, precautions, and restrictions.

Prolonged wet weather can be hard on the armyworm larvae by creating conditions whereby a fungus knocks down the population.  They do not like sun and heat, so they respond to those conditions by crawling under trash on the ground during the day and coming out at night to eat. (If you are scouting for armyworms in a field in the heat of the day, for example, look at the base of the plant and meticuously uncover the trash in the row.)  Cool weather is what they like.

There is also a wasp that can parasitize the armyworm.  The wasp lays an egg right behind the head on the worm that  can eventually kill it.  This is a natural predator, but one also must not assume this will occur.  It is important to check for parasitism if trying to determine threshold levels for spraying. This will be apparent by mummified bodies of armyworms clinging to the plant stalks or laying in the plant debris on the ground.

Control Options in Wheat Fields

In wheat, treatment for true armyworm is justified when an average of 4 or more non-parasitized, half-grown or larger worms per square foot are present during late spring and before more than 2 to 3% of seed heads are cut from stems by this pest. Scout at dusk, dawn, or at night as small larvae feed on foliage at night and remain in plant debris near ground during day.

If an insecticide application is necessary on wheat, use one of the following insecticides listed here as a foliar broadcast spray. Follow all label directions, precautions, and restrictions.

Control Options in Field Corn

Control of armyworm in field corn is justified if 25% or more of corn seedlings are significantly damaged through foliage feeding by larvae. Control is justified after pollen shed if leaves above ear zone are being consumed by larvae.

If an insecticide application is necessary on field corn, use one of the following insecticides listed here as a foliar broadcast spray. Follow all label directions, precautions, and restrictions.

For more information

Producers can monitor insect activity in Missouri through the Missouri Pest Monitoring Network at http://ipm.missouri.edu/pestmonitoring/index.htm

_________________________________________________________________________
Information compiled 5/23/2000, last updated 5/9/2012 by:
Bob Schultheis, Natural Resource Engineering Specialist, Webster County Extension Center, 800 S. Marshall St., Marshfield, MO 65076


University of Missouri Extension Web site Manager:  Bob Schultheis
Webster County Extension Center
Email comments to:  schultheisr@missouri.edu
Last revised: 05/09/2012

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