By Adam Sisson, Corn and Soybean Initiative; Laura Jesse, Plant and Insect Diagnostic Clinic; and Erin Hodgson, Department of Entomology
Small black cutworm larvae have been reported feeding on leaf tissue in a handful of isolated fields in eastern Iowa. Damage at the field level has been well below economic thresholds and is consistent with the low levels of moths captured before peak flights occurred. However, with trap captures occurring over an extended period of time before peak flights and recent fluctuating temperatures, it is advisable to be scouting high risk areas, especially in the central, east central and southeast districts of the state. In Benton County, located in the east central district of the state, there was a peak flight recorded much earlier than the surrounding regions on April 6, so it is advisable to be checking fields around this area at the present time.
The map (Figure 1) shows predicted cutting dates, based on trap catches and degree day data, should begin about May 19 in the southwest and south central districts and on May 25 in the southeast. Thresholds should occur about May 23 in the west central district and May 27 in the central and east central portions of the state. Cutting dates are about May 24 in the northwest and north central districts and May 25 in the northeast. Keep in mind that preventative black cutworm insecticide treatments applied as a tank-mix with herbicides are of questionable value as the insect is a sporadic pest and fields should be scouted to determine the presence of the insect prior to spraying.
Figure 1. Projected black cutworm 2010 cutting dates for Iowa's crop reporting districts. *Benton County experienced an early peak flight on April 6, so it is advised that growers in this area be scouting at the present time.
Growers should be looking for small larvae and keeping track of fields that may be good candidates for cutting. Certain fields may be at a higher risk for black cutworm damage than others. According to Jon Tollefson, Iowa State University Department of Entomology, fields that are low lying and poorly drained, next to areas of natural vegetation, weedy, and other characteristics should be scouted first.
Fields should be scouted for larvae (Figure 2) weekly until V5 by examining 50 corn plants in five areas in each field. Look for plants with wilting, leaf discoloration and damage, and those that are missing. Note areas with suspected damage and return later to assess further damage. Larvae can be found by carefully excavating the soil around a damaged plant.
If larvae found in the field are smaller than three-fourths inch, 2 to 3 percent wilted or cut plants may warrant an insecticide application. If larvae are longer than three-fourths inch, the threshold increases to 5 percent wilted or cut plants.
Remember to take into consideration the plant population in a particular field and adjust threshold numbers accordingly. However, with corn price and input fluctuations, a dynamic threshold may be more useful. An Excel spreadsheet with the calculations built in can be downloaded here and can be used to aid management decisions regarding black cutworm.
Figure 2. Fourth-instars (shown here) are capable of cutting corn.
Adam Sisson is a program assistant with responsibilities with the Corn and Soybean Initiative. Sisson can be contacted by email at email@example.com or by calling (515) 294-5899. Laura Jesse is an entomologist with the Iowa State University Extension Plant and Insect Diagnostic Clinic. She can be reached by email at firstname.lastname@example.org or by phone (515) 294-5374. Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at email@example.com or phone (515) 294-2847.