Armyworm (Pseudaletia unipuncta) is often called true armyworm and should not be confused with the “fall armyworm”, another corn insect pest that occurs later in the season, usually only on very late planted corn. Armyworm is an early season pest that attacks grass crops like small grains and corn. It is not a common economic pest, but occasionally can occur in high numbers and cause severe damage to seedlings and whorl stage corn. Armyworm, in field corn, is always associated with the presence of grassy weeds within fields prior to planting or during the season. Moths will not lay eggs on corn plants but prefer other grass plants. Significant numbers of armyworms may be present on existing grass vegetation in no-tillage or reduced tillage corn. When this grass is killed with a contact herbicide, or consumed by the armyworms, caterpillars are forced to the corn plants. In this instance serious armyworm damage is restricted to fields with large quantities of wild grasses or small grain cover crop. Also, in whorl-stage corn poor weed control may result in heavy stands of grass weeds (e.g. crabgrass) that are colonized by armyworm moths. Very high numbers of armyworm caterpillars may consume all the grass and then defoliate the corn plants.
Armyworm management focuses on controlling grassy weeds and insecticidal measures when needed. In seedling corn caterpillars may be avoided by killing the grassy vegetation before planting, by two weeks or more. Also, favorable grass control in seedling corn will prevent moths from colonizing fields. Cultural practices that help insure rapid seedling growth can be important in minimizing damage. To detect caterpillar populations in no-tillage corn seedlings should be scouted and treated with an appropriate insecticide when thresholds are exceeded. Foliar applied insecticide works well but at-planting, soil insecticides are not effective against armyworm. Finally, some Bt proteins are effective to control this insect.
No-tillage corn growers sometimes automatically apply an insecticide in their contact herbicide solution for armyworm and cutworm control. This practice is not recommended in most instances since the occurrence of these caterpillars is very erratic. Scouting procedures and as-needed insecticide treatment are effective but automatic treatment is seldom economically or environmentally justified. See scouting guidelines for seedling pests.