Armyworm in Corn and Small Grains


In April we reported that University of Kentucky true armyworm moth counts were higher than average.  These moths migrate northward, so if our southern neighbor reported high catches, many moths also likely made it into Ohio. After migrating and establishing, armyworms begin to lay eggs in grasses, including wheat fields and cover crop fields (that may have corn planted soon). Larvae feed for about 3 weeks before pupating. This article discusses armyworm management in corn and small grains.

Corn.  Field corn planted no-till into grassy habitats should be monitored closely at this point in time for armyworm activity. Fields that may be at risk for significant armyworm infestations include corn planted no-till in rye cover crops and corn planted no-till into old hay fields. A severe infestation of armyworm can reduce stand when an infestation occurs in the pre-whorl stage and cause significant defoliation when corn is hit in the whorl stage. Total destruction of a field of no-till corn can occur if a severe infestation is allowed to develop without application of a rescue treatment.

When armyworm feeding damage is found on 15 to 20% of a stand this is an indicator of a potential problem, and the field should be rechecked within a few days to determine if defoliation is increasing and if a rescue treatment is warranted. In general, a severe infestation will impact almost 100% of a stand and defoliation of the plants will exceed 50%.  If defoliation remains less than 50% and the new growth exhibits minimal feeding injury, the stand will likely recover with minimal impact on yield. Rescue treatment in corn may be needed if stand infestation is greater than 50% and larvae are not mature.

Since armyworms are foliar feeders, they are relatively easy to control with most foliar treatments. During the day, armyworm larvae will most likely be found seeking shelter in the whorl or possibly in the ground cover. It may be easier to scout at dawn or dusk, or on cloudy days when larvae are more visible. In general, armyworm larvae will feed first on the lower leaves and then progress to the new growth - especially when corn approaches the early whorl stage.

Small Grains.  Wheat fields and other small grains should also be checked at this time for armyworm. Carefully examine a 4 square foot area in five locations spread around the field.  Count the number of caterpillars that are between ½ and 1 inches long.  If the average number of these caterpillars is 16 or more per 4-foot area, or if head-cutting is occurring, a rescue treatment is recommended.   Other thresholds based on caterpillars per row foot range from 3 to 5 per foot, with lower numbers being used when wheat is selling at higher prices. Treatment of armyworm larvae reaching maturity (1¼ inch or more) is not judged economical because most of their feeding is already done and they will soon pupate.  More information on armyworm management in wheat can be found at

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.