Keep An Open Eye For Armyworm Infestations

MT. VERNON, ILL.

    Looking around in southern Illinois, the wheat crop looks pretty good. There have been few reports here of aphid pests, and the weather seems to be cooperating with timely rain showers and moderate temperatures. Everything seems good … so far. But, it is again time to be on the lookout for armyworm infestations. Typically, armyworm development is favored by a spring that is cool and wet, while armyworm natural enemies are hindered by the same weather conditions.

   The armyworm, which is native to North America, feeds on a variety of grasses and similar crops. Armyworm moths move from the southern states in April and May to northern areas, and they lay eggs in rows or clusters on the lower leaves of grasses. One to two weeks later, pale green larvae hatch and begin feeding. These larvae are frequently found in the top couple of inches of soil around their food plants during the day. Active at night, they go through six instars before pupating in the soil. One to two weeks later, adults emerge and repeat the life cycle. There can be two or more generations per year.

   Armyworm moths are most active during the evening. Because they prefer to lay their eggs in dense vegetation, infestations generally begin in heavy grass areas such as fencerows and also in reduced tillage fields. When the host plant is consumed, they begin to search for more food and can spread like an invading army across fields of grasses, small grains and corn. Feeding in corn is usually confined to the leaf margins; however, seedling corn is more susceptible to significant damage.

   Last year, large numbers of armyworm adults were caught in pheromone traps in southern Illinois and western Kentucky. But, the damaging infestations were experienced further north into Ohio and southern Canada. This year, it is anybody’s guess where the damaging horde might turn up. Therefore, it is imperative that you keep a watchful eye. Armyworms often go unnoticed in fields until injury is severe. Because their larvae feed primarily at night, they can cause significant damage before being detected.

   When scouting for armyworms in corn, pick five random locations in the field and closely examine 20 plants at each location. Record the numbers of larvae found and make estimates of their size and the percentage of damaged plants. Remember that they are frequently found under debris and just under the top of the soil. In cereal grains, carefully examine about 3 row feet in those five locations.

   Reduce your armyworm risk by eliminating grassy weeds from fields and their borders. This will reduce egg laying and migration into your field by hungry larvae. Keep an eye on field borders in order to spot worms beginning to migrate into your field. If armyworms do make it into fields, control may be justified when more that 1/4 of seedling corn plants are damaged.

   Older corn (post pollen shed) may need insecticidal control when armyworms are feeding above ear level. Growers of small grains may need to take action when there are more than six smaller armyworms per foot of crop row; however, once the larvae are longer than 1¼ inch and head cutting is observed (the most damaging stage), treatment may be justified with fewer observed worms. Δ

Dr. Douglas B. Jones is Extension Specialist, Integrated Pest Management, with the University of Illinois at the Mt. Vernon Extension Center.