Black cutworm is a sporadic pest that can clip off young corn plants and reduce the stand. You should start watching for signs of black cutworm larvae in fields as soon as the corn emerges, advises Iowa State University Extension entomologist Erin Hodgson. If an infestation reaches the economic threshold and isn't promptly treated with an insecticide applied as a rescue treatment, this hungry insect pest can decimate stands of early vegetative-stage corn.
Scouting for black cutworm larvae helps you determine if applying an insecticide as a rescue treatment will be a cost-effective decision. Hodgson, along with ISU Extension colleagues Adam Sisson and Laura Jesse, offer the following cutworm scouting guidelines. They also offer advice for deciding whether or not to treat this pest with a rescue insecticide application in cornfields.
Begin scouting for cutworm on these predicted dates
Each spring, ISU Extension issues predicted dates indicating when the black cutworm larva will begin to be active in various areas of the state. These dates indicate when to begin scouting corn in these areas. The scouting date predictions are based on peak flights of black cutworm moths arriving in Iowa and on the accumulating degree days after the peak flight. The adult black cutworm moths migrate north into Iowa from southern states each spring; upon arrival in Iowa these moths lay eggs in fields, and the eggs hatch into larvae. The larvae or worm stage can damage the small corn plants.
It can be difficult to determine exactly when the moths arrive each spring. After the female moths arrive in Iowa and lay eggs in fields, the number of growing degree days is estimated to determine when the cutworm larvae are capable of cutting corn.
ISU has network of moth monitoring traps across the state
To find out when black cutworm moths arrive in Iowa, a volunteer network is established early each spring to monitor a series of moth traps scattered across the state. Hodgson and her ISU colleagues asked the volunteers this year to start checking the traps at the end of March, and so far the volunteers have contributed more than 1,000 data reports in 2014. The first black cutworm moths flying into Iowa this spraying were recorded in Muscatine County in southeast Iowa on April 3.
The map accompanying this article shows the predicted cutting dates for the nine Iowa climate divisions or districts in the state. These predictions are based on actual degree day data and also on historical degree day data. In the east central, southeast and south central districts, a peak flight occurred in mid-April, and estimated cutting dates are shown in orange on the map below. The dates in black are predicted cutting dates for peak flights of cutworm moths that occurred in late April. Large numbers of moths were collected in the monitoring traps over several days in early May, so you should continue to scout fields past the predicted cutting dates.
BEGIN SCOUTING: Estimated black cutworm larvae cutting dates for each district are based on peak flights of moths occurring in April 2014. Dates in orange represent estimated cutting dates from an early peak flight of moths. Scouting should begin and continue from these dates as black cutworm moths continue to arrive in Iowa.
Follow these guidelines when scouting for black cutworm
Fields of corn that may be at higher risk for black cutworm damage include those that are poorly drained and low lying, those next to natural vegetation such as wooded areas, and those that are weedy or fields where reduced tillage and no-till are used. Also, note that late-planted corn can be smaller in size and thus more vulnerable to cutworm larval feeding. Some Bt corn hybrids provide suppression of black cutworm, but young corn plants, even if they have the Bt trait, can still be clipped by larvae. So you need to scout corn fields for this pest, even if you've planted Bt corn.
Scouts are encouraged to start looking at cornfields several days before the estimated cutting dates. That's because local larvae development may be different, depending on weather specific to that area. Fields should be scouted for larvae weekly until corn reaches V5 growth stage. Examine 50 corn plants in five areas in each field. Look for plants that have wilting, leaf discoloration and damage, or those that are missing or cut (see photo on the next page). Make a note of the areas of your fields that have suspected black cutworm damage (you can mark these areas with a flag). Then you can return later to assess further damage. Larvae can be found by carefully digging in the soil around a damaged plant.
Identification tips are helpful for scouting black cutworm
Black cutworm larvae are light grey to black, notes Hodgson. Their skin appears grainy and there are four pairs of fleshy prolegs on the hind end (see photo below). On each body segment, the pair of tubercles closest to the head is about one-third to one-half the size of the pair nearest to the abdomen (see bottom photo). Black cutworm larvae can be confused with armyworms and other cutworms. Some characteristics can be used to set these species apart, which are outlined further in this article on cutworm identification.
BLACK CUTWORM: How do you tell the difference between black cutworm and other kinds of cutworms? Black cutworm larvae have light grey to black, grainy skin.
AVOID CUTWORM CONFUSION: Black cutworms are best distinguished by the dark tubercles on the middle of the back. On each body segment, the pair of tubercles closest to the head is about one-third to one-half the size of the pair nearest to the abdomen.
DAMAGING YOUNG CORN: Damage from black cutworm larvae usually starts above the soil surface. Leaf feeding (left) can occur. As larvae mature, they can cut off the plants (right). Photos copyright Marlin Rice.
Use economic thresholds to make smart treatment decisions
Common thresholds for seedling, V2, V3 and V4 stage corn plants are 2, 3, 5 and 7 plants cut out of 100 plants, respectively. But with corn price and input fluctuations, a dynamic threshold may be useful. An Excel spreadsheet with the calculations built in can be downloaded here and can be used to help you make management decisions regarding black cutworm.
Preventive black cutworm insecticide treatments applied as a tank-mix with herbicides are of questionable worth. Black cutworm is a sporadic pest and therefore every field should be scouted to determine the presence of the insect prior to spraying insecticides.
Trap catches of black cutworm moths in Iowa
In 2014, cooperators in ISU's moth flight monitoring effort have reported data from 49 Iowa counties, with several counties having multiple traps. The number of moths trapped in Iowa can be viewed by going to www.ncipmpipe.org, selecting "View all maps" and clicking on "Iowa Black Cutworm Monitoring 2014." A word of caution: "Please consider that adult moths captured do not necessarily mean there will be economically significant black cutworm infestations in a particular location," says Hodgson. "Field scouting is essential to determine if an economically damaging infestation exists."
If you see any damage from black cutworm larvae while scouting, please let ISU Extension entomologists know by sending them a message. Send it to this email address: [email protected]. "The information you send to us can help us refine our black cutworm prediction efforts in coming years," Hodgson adds.