Time to Scout for Black Cutworm in Emerged Corn

Black cutworm (BCW) is a migratory pest that arrives in Iowa each year with spring storms. The female BCW moths lay eggs in and near crop fields. The eggs hatch and the larvae can feed on corn leaves or even cut corn seedlings. Larvae must attain a certain size (4^th instar) to be large enough to cut corn plants, and cutting can occur until plants reach the V5 stage (five leaf collars present). 

Black cutworm is unpredictable, making it essential to scout to determine whether BCW larvae are present in a field and if management is required. Fields with cover crops or other vegetation (i.e., weeds) this spring should be prioritized since they serve as attractive egg-laying sites for migrating moths. Because BCW is sporadic, it is not economical to use preventative insecticide applications; however, rescue treatments can be very effective if scouting reveals larvae are present at economic levels. 

Predicting cutting dates

Each spring we monitor moth activity around the state with our Iowa Moth Trapping Network. There has been quite a bit of BCW moth activity this spring according to the trapping network. 

We predict when BCW will reach the 4^th instar (big enough to cut corn plants) based on growing degree day data and the occurrence of significant flights. A significant flight is defined as the capture of eight or more BCW moths over a two-night period. Larvae reach the 4^th instar after 300 GDD have accumulated since a significant flight.

This year we have 18 trapping sites across 13 counties in Iowa. Counties that have reported significant flights of BCW moths in 2025 include Hamilton (21), Lucas (April 28), Polk (April 25), Pottawattamie (April 29), Story (April 17), and Washington (April 21), the date in parentheses is the date of the first significant flight in that county. As of May 5, there have been a total of 8 significant flights reported, with a couple sites having multiple significant fights. 

Figure 2 shows the predicted cutting dates for BCW in each crop reporting district. These cutting dates are estimated by combining current and historical growing degree day data from when significant flights occurred. These are estimates of when cutting will begin, but additional large flights may result in prolonged feeding by black cutworm larvae.

This spring the moth trapping network started later than normal, and there is a chance we may have missed some early significant flights of BCW moths that surrounding states, like Missouri and Illinois reported. Consequently, we could see some BCW activity earlier than our predicated cutting dates, particularly for southwest Iowa and south-central Iowa. 

Corn, including the first planted corn, will likely be susceptible to BCW cutting this year. Most of the first planted corn around the state has emerged and ranges from VE to V2. Looking at current and historic growing degree day data from the U2U website to estimate corn growth stages, even the emerged corn will have not surpassed the V5 stage with our predicated cutting dates, making it vulnerable to BCW cutting.   

As a reminder, capturing BCW moths in a pheromone trap does not necessarily mean economic infestations will occur in a particular location. Scouting fields is the only way to determine if BCW are present and whether management is warranted. 

Combine scouting for BCW with early season stand assessments. Missing or cut plants are signs of black cutworm feeding but dig in the soil near affected plants to confirm larvae are present. Refer to this encyclopedia article to learn more about black cutworm identification, biology, scouting, and management.

Keep up with moth flights!

BCW trapping will continue throughout May, and any additional significant flights that occur will be included in our weekly ICM Blog updates. Additional significant flights in an area may represent prolonged feeding of BCW larvae, so continue to scout until corn reaches V5.

If you live near the Iowa border, information from trapping networks in other states may be useful to you. Some links to out-of-state resources are listed below, but others may be available:

• University of Minnesota
• University of Missouri
• University of Illinois
• University of Nebraska-Lincoln

Report larvae!

If you see any fields with BCW larvae or injury while scouting, please let us know! You can contact us via email (crops@iastate.edu) or X (formerly Twitter) (@erinwhodgson and @ISUCropNews) This information will help us refine cutting predictions and scouting recommendations in the future.