Black Cutworm Alert – Scout Your Fields

Latham is a partner in the Iowa State University Extension Corn and Soybean Initiative. That means we get timely alerts when any farming condition emerges that could impact a number of growers.

We recently received an alert about possible emergence of black cutworm in Iowa. The alert is reproduced below. Also, we have a simple spreadsheet calculator available for estimating stand-loss threshold of black cutworm as insecticide is applied.

Just give me a call at 1-800-798-3258 or email me at and I’ll be glad to send you the calculator document and explain it to you. For example, one simple calculation on the spreadsheet shows a corn stand planted at 32K plants per acre with an expected yield of 200 bu/acre at a market price of $6 per bushel and receiving insecticide application at a cost of $15/acre could realize an estimated loss of 400 plants per acre, or 1.25%.

Black cutworm to start cutting May 24: scouting is advised

From Iowa State University Extension Corn and Soybean Initiative

Black cutworms could start cutting corn May 24-26 in the southern two-thirds of Iowa, and the first week of June in the northern three tiers of counties. Growers should scout for cutworms as soon as corn emerges, paying special attention to late-planted or weedy fields.

Most at-risk fields:

• Late-planted
• Reduced tillage
• High weed populations – especially of winter annuals
• Near natural vegetation areas
• Poorly drained or low-lying, such as in the floodplain of a stream or river.

Damage signs
• Small, irregular holes in leaves (from young, above-ground larvae feeding)
• Cut stems or clipped leaves (from older larvae, in 4-6 instar – usually feed underground at night)

When scouting

• Examine 250 plants (50 plants in five locations) weekly until corn reaches V5.
• Check for wilted, discolored or damaged leaves and missing plants.
• If damaged leaves are found: dig around the base of the plant and look for larvae. If soils are dry or crusted, larvae  may burrow to moist soil and move to new plants.
• Flag suspected “hot spots” and monitor larval feeding (or lack of it) for a few days.

Economic Threshold & Treatment Options
•Economic thresholds for black cutworm were adjusted last year due to higher crop value and corn stands.
•A tool to calculate treatment thresholds for stand loss will be published in ICM News next week.

To tell black from dingy cutworm larvae

Both feed on corn, but dingy cutworms rarely cut leaves. Black cutworms have two prominent tubercles (dark bumps) of differing sizes on each body segment; dingy cutworms have two of the same size.

Long story short: Black cutworms may start cutting corn May 24 in the southern two-thirds of Iowa. Scout for corn as soon as it emerges, especially in late-planted, weedy or reduced tillage fields. Use the attached threshold calculator to determine the best treatment for your fields.

Source: Erin Hodgson and Jon Tollefson, ISU Department of Entomology