Cutworm Alert


By Emily Unglesbee
DTN Staff Reporter

ROCKVILLE, MD. (DTN) -- A nighttime vandal is sneaking through Midwestern fields.

Black cutworms -- nocturnal corn cutting pests -- are already being spotted in fields this spring. Growers are being urged to scout carefully; particularly those who opted to plant conventional corn this year.

Cutworm moths have been winging their way into the Midwest since late March and worm damage will soon follow, entomologists warn. In Illinois, corn cutting could begin as early as May 13 for growers in central counties, according to Kelly Estes, the agricultural pest survey coordinator for the Illinois Natural History Survey. Farther north, University of Minnesota entomologist Bruce Potter expects damage to surface in the state's southwestern cornfields by the third week of May.

Black cutworm moths may have been flying north earlier than agronomists were setting traps, so damage could come even earlier than expected, Syngenta agronomist Todd Thumma warned in an email to DTN. First instar larvae have already been found on winter annuals in central Illinois, he reported.

Cutworm damage predictions are made possible by the cutworm's predictable seasonal migration, which starts along the Gulf Coast in February and ends in the northern Midwest in April and May. Female moths lay their eggs in crop residue or weedy and grassy fields.

"While Black Cutworm can be found in any field, I would pay special attention to corn fields that have had cover crops, weeds growing, soybeans as a previous crop, no-tilled/strip-tilled and fields with heavy residue," Thumma wrote.

Cutworm damage starts small. Often a "shot-hole" pattern is detected on leaves of seedlings, formed by young larvae munching through in a straight line. As they grow, they become capable of "cutting" young corn plants off above and below ground, resulting in wilted or dead plants.

Because this cutting action is so damaging, most Midwestern scouting guides recommend that growers scout for cutworm immediately after corn emergence and continue through the 4- to 5-leaf stage, when corn is capable of withstanding more damage.

"Fields at greatest risk to cutting and economic damage are in the 1-to-4 leaf stage of plant development," Estes wrote in a University of Illinois pest Bulletin article. "Producers are encouraged to look for early signs of leaf feeding as a potential indicator of cutting, rather than waiting for cutting to take place."

Thumma's Syngenta scouting guide recommends growers check 20 to 25 plants in five locations for every 25-30 acres of corn.

Proper cutworm scouting often involves a backache and a flashlight. Like little vampires, cutworms emerge and do their feeding after sunset throughout the night and bury themselves in the soil and debris during the day.

Estes recommends an overall treatment threshold of 3% damage. If your stand has dropped 15% below its optimum level, the Syngenta guide suggests dropping that threshold to 2%.

Growers who have opted for Bt-free, conventional corn will need to be especially vigilant in scouting. Bt proteins are among the first line of defense against black cutworm.

However, not all Bt proteins in corn hybrids will keep cutworms at bay, Estes warned. You can use this Bt trait table guide from Michigan State to determine if the proteins in your hybrids work against black cutworm:….

Even if your Bt hybrids do target black cutworm, they aren't bulletproof, Potter added. "The Herculex and Viptera above-ground Bt traits provide protection against black cutworm attack," he wrote in a university newsletter. "However, very large populations of large cutworm larvae might still damage small corn."

Finally, insecticide-treated seed can deter feeding cutworms, but only if the plant is actively growing and taking up nitrogen. Cutworms might feed past the seed treatment if early planted corn plants have stalled in cool, wet conditions, according to Purdue entomologist John Obermeyer.

Although rare, cutworms can attack soybeans, too, when populations are high in corn fields. They feed on the seedling plant, below the cotyledon.

You can find the Illinois pest Bulletin here:….

Potter's university newsletter on cutworm can be found here:….

See a standard Black Cutworm management guide from Missouri here:….

Emily Unglesbee can be reached at

Follow her on Twitter @Emily_Unglesbee