Watch for Cutworm as Corn Emerges

Scouting for black cutworm in Iowa should begin soon.

Black cutworm is an occasional and sporadic pest of corn from the time corn emerges until it reaches the V4 stage of growth. Black cutworm moths fly north from overwintering areas on the Gulf Coast, carried along with the low-level winds associated with spring storms. The moths fly into Iowa and lay eggs, which hatch into cutworm larvae that can damage corn.

ISU has a pheromone trapping network that is used to determine when and where moths have arrived in the state and where egg laying is possible. Black cutworms need about 300 base-50 growing degrees to grow from egg laying to cutting size, so Iowa State University Extension entomologists can project when larvae are potentially cutting in an area and scouting is appropriate.

Cutting may occur in southern Iowa first

"From spring moth pheromone trap capture data this spring, we project that black cutworms could start cutting around May 10 in the southern three tiers of counties in Iowa," says ISU's Rich Pope. "That's based on a sporadic early moth flight that occurred the first week of April. Additionally, cutting is possible from larvae hatched from eggs from a second significant moth flight observed around April 23 to 25. Scouting should start May 17 in southern third of Iowa, May 18 or 19 in central Iowa and May 20 to 21 in the northern third of Iowa."

Another kind of cutworm, called the dingy cutworm, isn't considered to be an economic problem. They chew on leaves of corn seedlings but rarely clip off the plants, says Pope. How do you tell the difference between the two kinds of cutworms? He says black cutworms have two prominent tubercles (dark bumps) of different size on each body segment, whereas dingy cutworms have two tubercles that are the same size.

Apply rescue insecticide if needed

Scouting for black cutworm in Iowa should begin this spring May 10 to May 21, depending on where you are located in Iowa, says Pope.

"Because black cutworm is a sporadic pest, the use of preventive insecticide treatments is not recommended by Iowa State University. Instead, we suggest that you scout your cornfields and if black cutworm does show up and reaches the economic threshold for damage, then you should apply a rescue insecticide treatment," he explains.

Economic treatment thresholds are when 2% to 3% of the corn plants are cut off or wilted and when the larvae are less than 1-inch long. Or when 5% of the plants are cut off or wilted when the larvae are more than 1-inch long.

More information is available, listing the treatment options and recommendations, in the May 7 ISU ICM newsletter at www.ent.iastate.edu/ipm/icm.

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