Iowa State University Extension issued a black cutworm scouting advisory for the state's young 2010 corn crop on May 11. Small black cutworm larvae have been reported feeding on the leaves of corn plants in a handful of fields in eastern Iowa. Based on the catches of adult black cutworm moths in ISU's monitoring traps across Iowa earlier this spring—the highest risk areas of the state so far are central, east central and southeast Iowa.
Keep an eye on cornfields for signs of leaf feeding and cut plants; an insecticide spray may be needed if damage reaches the economic threshold. That advice comes from Erin Hodgson, ISU Extension entomologist. Hodgson, along with ISU Extension colleagues Adam Sisson and Laura Jesse, offer the following information and recommendations regarding the black cutworm alert.
Damage in fields 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 fields in the high risk areas, especially in central, east central and southeast Iowa.
Predicted black cutworm cutting dates for various Iowa areas
While black cutworms are already feeding on leaves of corn plants in a small number of fields in eastern Iowa—the predicted cutworm cutting dates for all of the state, based on moth trap catches and degree day data, are now available.
In Benton County in east central Iowa, there was a peak flight of black cutworm moths recorded much earlier than the surrounding regions on April 6, so it is advisable to be checking fields in this area at the present time.
The map shows that 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 this insect is a sporadic pest and fields should be scouted to determine the presence of the insect prior to spraying.
How to scout corn for black cutworm larvae
Farmers and crop scouts 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 other fields. According to Jon Tollefson, Iowa State University Department of Entomology, cornfields that are low lying and poorly drained, next to areas of natural vegetation, and are weedy, and have other characteristics attractive to the egg-laying black cutworm moths, should be scouted first for signs of cutworm larvae (worm stage) activity.
Fields should be scouted for larvae (Figure 2) weekly until the V5 stage of corn growth. You scout by examining 50 corn plants in five areas in each field. Look for plants that show wilting, leaf discoloration and damage, and those plants that are missing in the row. That's a sign the cutworms are cutting off plants. Note areas with suspected damage and return later to assess further damage. The larvae can be found by carefully digging the soil around a damaged plant.
Use economic threshold to make treatment decisions
If larvae are found in the field and they are smaller than three-fourths inch long, and 2% to 3% of the corn plants are wilted or cut, that field may warrant an insecticide application. If larvae are longer than three-fourths inch, the threshold increases to 5% wilted or cut plants.
Remember to take into consideration the corn 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 the spreadsheet can be used to aid management decisions regarding black cutworm.