Armyworms are often go unnoticed in fields until injury is severe. Larvae feed primarily at night and may cause damage before being detected. In corn: Examine 20 plants in five locations in a field for a total of 100 plants.
Determine the percentage of damaged plants in the field. Also note the number of larvae found and estimate their size. On warm, sunny days, larvae may be found under plant residue or in the whorl of the corn plants.
In cereal grains: Examine three linear feet of row in five or more areas of a field. Shake the plants vigorously and then look for armyworms on the ground and under debris.
Estimate the size of the larvae and the percentage of plants damaged. Also note the stage of development of the plants.
Management:
There are several species of armyworms that are present every year in the Midwest, however, economic infestation usually do not occur on an annual basis.
The development of economically damaging populations depends on several factors, including, spring moth flights, cropping practices, weather conditions, natural enemies, and others.
Most years, armyworm populations are kept below damaging levels by several predators, parasites, and pathogens. Naturally occurring predators such as ground beetles and rove beetles may be important in keeping armyworm populations low each year. Research conducted by Michigan State University has found over 12 species of parasitic wasps and several flies that commonly attack armyworm larvae.
Tachinid flies, such as Nemosturimia rufopicta, lay eggs on the body of armyworm larvae. Eggs are usually deposited on the thorax near the head so the armyworm can’t bite them off. Parasitic wasps also parasitize armyworms.
Cotesia marginiventris, a braconid wasp is one of the most common. The parasitoid, Glyphanteles militaris inserts as many as 60 eggs at a time and is known as a gregarious parasitoid since many larvae develop in one host.
On the other hand, Meteorus communus lays only one egg per host (solitary parasitoid) and attacks second to fourth instar armyworms.
There are several types of viruses, a microsporidium, and fungi that infect armyworm larvae. A cool and wet spring favors armyworm development while hindering parasite and predator populations.
The risk of potential armyworm infestations can be minimized by control or elimination of grassy weeds in from fields or field borders. Not only will moth egg laying activity be reduced, but so will the migration of larvae into the field.
Watch for movement of armyworms from grass or weedy areas to field borders. If armyworms move into fields and infest crop areas, and insecticide treatment may be necessary.
In seedling corn, control may be justified when 25% of the plants are being damaged. After pollen shed, insecticide control may be justified when feeding occurs above the ear level. In small grains, treat for armyworms when there are 6 or more non-parasitized larvae (3/4 to 1 ¼ inches long) per linear foot or row before extensive head cutting occurs.
Mature larvae are approximately 1 ¼ to 1 ½ inches long. Armyworm larva complete six instars. The sixth instar requires about 7 days to complete development, depending on temperature. The sixth instar consumes about 80% of all the foliage eaten during larval development.
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