Latham ArmywormThe true armyworm – not to be confused with the fall armyworm – can pose a threat to corn stands and overall yield as an early season pest. Armyworm larvae are characterized by alternating light and dark bands.  

The larvae feed primarily on seedling leaves and are associated with causing defoliation. When feeding, armyworms start at the base of the corn plant and work their way up, primarily feeding on softer leaves and during the nighttime. During the day, armyworms will migrate to the whorl of the corn plant as well as inside of soil cracks.  

Cool and wet weather in the springtime supports the development of armyworms and fields with a heavy presence of weeds and grasses are conducive for allowing them to thrive. Armyworm moths migrate up from the south and look for the grassy fields or areas with lush vegetation. Therefore, winter rye cover crops have been associated with a higher prevalence of armyworm infestation. That is why it is important to terminate cover crops at least two weeks prior to corn planting.  

Treatment and Mitigation  

Death in corn plants from armyworm is not common, but if the armyworm feeds on a budding corn plant, this can lead to poor stands and significant loss in yield. When armyworm infestation causes defoliation in more than 25% of corn plants, it is advised to begin using an insecticide. Additionally, when armyworms are still developing and larvae is only ¼ to ¾ inches in length, treatment is considered economical and is advised.  

Controlling excess grass growth will prevent moths from laying eggs and will offer protection from future infestation. If herbicides are applied, the loss of a weed habitat can drive armyworms to feed predominately on corn plants.  

Treating an armyworm infestation with small corn plants requires quick decision making because large populations of armyworms can spread quickly across a field. Keep an eye on those fields that have a cover crop or are close to armyworm habitat. Be prepared to act if conditions warrant treatment!