Black cutworm moths fly into Iowa each spring and lay eggs in fields. The eggs can hatch into tiny cutworms if conditions are favorable, and the hungry little worms can eat and destroy emerging corn seedlings. So you need to keep an eye on cornfields when the corn begins to emerge and you should continue to watch cornfields for signs of feeding and cutting off of plants. If the damage reaches economic threshold levels, you may need to treat the corn with an insecticide rescue application.

Iowa State University Extension will issue warnings when and where the worms are hatching and threatening young corn plants in the state. The following information is provided by Adam Sisson of the ISU Department of Plant Pathology, Laura Jesse of ISU's Plant and Insect Diagnostic Clinic, and Erin Hodgson, ISU Extension entomologist.

Monitoring for black cutworm, an occasional corn pest in Iowa, has begun. Cooperators in the black cutworm monitoring network are observing the arrival of adult moths (Figure 1) throughout the state. Since April 1, reports of moths captured in pheromone traps are coming from a number of counties, but most captures have been below peak flight levels.

The black cutworm monitoring network helps growers determine when they should start scouting. Scouting fields is important in determining if cutting larvae are a problem in a specific field. Adult moth trap captures alone are not enough to justify an insecticide treatment. Scouting should be done before treating for black cutworm is considered.

Figure 1. The black cutworm adult life stage

Why you need to understand the biology of the black cutworm

The black cutworm, a pest of corn in Iowa, causes damage early in the season. The insect does not overwinter here. Instead, adult moths migrate on the wind from southern states near the beginning of spring, mate and lay eggs.

Approximately 1,300 eggs are laid by a single mated adult female. Eggs are laid in crop stubble, low spots in the field and in weedy areas. Younger larvae (Figure 2) injure corn plants by feeding on leaf tissue and older larvae can cut seedlings (Figure 3). Once corn reaches the V5 stage, it becomes harder for the pest to cut plants. Three generations of black cutworm occur per year.

Black cutworms can be confused with another insect that may be found in fields during spring, the dingy cutworm.

Figure 2. Black cutworm larva

Figure 3. Damage to corn plant from the black cutworm

The Iowa monitoring network can help farmers head off problems

After checking pheromone traps for the presence of the male moths, monitoring network cooperators enter the number of captured moths on the network website. The arrival of moths indicates egg laying will soon take place. Once a large number of moths are recorded in a particular region in Iowa, we use degree days to estimate insect development. Degree days are based on temperature which is a better way to estimate insect development than calendar days. If it is warm insects will grow faster than when it is cold.

We use past temperature data combined with this year's temperature information to determine when hatched larvae will begin cutting corn (cutting date). Cutting dates are projected to occur at about 300 accumulated base-51 F degree days from a peak flight. 

As the season progresses, we'll keep you posted on the status of black cutworm in Iowa. Keep your eyes open for the black cutworm predicted cutting dates in an article on the ISU Integrated Crop Management News website. If you wish to join the monitoring effort in coming years, send an e-mail to [email protected] with your name and address and we will add you to the list for 2011.