Black cutworms, dingy cutworms, and predicted cutting dates

Significant numbers of black cutworm adults (moths) were captured in pheromone traps in Iowa on April 18-22, April 27-30, and again during May 7-12. Even though this insect is an occasional pest of seedling corn, it deserves our attention because of its potential for causing economic damage. Based upon these data, we can anticipate when the first cutting should occur.

The dorsal (top side) tubercles on a black cutworm are unequal in size and the skin texture appears grainy..

The dorsal (top side) tubercles on a dingy cutworm are equal in size and the skin texture appears smooth..

On the black cutworm (top), the inside pair of tubercles is about 1/3 to 1/2 the size of the outside pair. On the dingy cutworm (bottom) these tubercles are about the same diameter..

Predictions for the first cutting dates are as follows: May 24 (southern three tiers of western and central counties), May 26 (central three tiers of western and central counties), and May 28 (northern three tiers of western and central counties). No significant moth captures were collected east of a line running through Mason City, Marshalltown, Iowa City, and Muscatine, but this should not imply that cutting can't also occur in northeastern and east central Iowa--the moths simply may have missed the traps.

By scouting fields several days before the first cutting, you may be able to find "hot spots" based upon leaf feeding and get a head start on management decisions. These dates represent the earliest possible cutting dates, based on normal temperatures. However, it is possible that the cutting period may stretch over two to three weeks because moths lay eggs over an extended period, and the emergence of later planted corn would still be susceptible to cutting.

Pheromone traps only catch moths so these captures cannot predict the amount of cutting that will occur, nor where cutting will occur. Each year, one of our concerns is that radio advertisements may predict a cutworm "outbreak" in your county just because moths were trapped there several weeks ago. Neither the traps nor anyone's interpretation (including our own) of the trap catches can predict the amount of cutworm injury. Scouting of seedling corn near the first cutting date is the only reliable method to determine whether a problem exists. Then, insecticides can be applied if needed.

Scout the field a couple of days before cutting is predicted. Look for cutworm injury on corn leaves. Dingy cutworms also feed on young corn leaves but rarely cut corn. If leaf feeding is detected, try to find the cutworms to determine whether they are black or dingy (see photos). Very large cutworms found during the earliest black cutworm cutting dates are often dingy cutworms because dingys overwinter in Iowa as partially grown larvae. We have already seen dingy cutworms this spring 1-inch long. If the field has only dingys, then you shouldn't have a cutting problem.

If you find leaf feeding and only black cutworms, then mark off 100 plants in a row with stakes or flags, and scout these same plants for cutting over a period of several days at several locations across the field. Then you can monitor the cutworm activity and determine progression of damage (or lack of it).

The economic threshold is when cutworms average less than 3/4 inch in length, an insecticide should be considered if 2 or 3 percent of the plants are wilted or cut, or if cutworms are longer than 1 inch, treatment should be applied if 5 percent of the plants are cut. If the field has a poor plant population, 20,000 or less, these thresholds should be lowered.

Stop scouting when the field is sprayed or when plants have five fully developed leaves (stage V5). Cutworms have difficulty in cutting plants in the V5 stage because of the larger stalk diameter, but occasionally they chew into the side of the stalk and kill a larger plant.

Several insecticides are labeled for black cutworms in corn. Several years ago, research showed that after application, rotary hoeing in dry soils increases the effectiveness of Lorsban, but that the pyrethroids (such as Ambush, Pounce, or Warrior) should not be incorporated.

Insecticides labeled for black cutworms in corn

Insecticide Rate
Ambush 6.4-12.8 oz/acre
Asana XL 5.8-9.6 oz/acre
Baythroid 2 0.8-1.6 oz/acre
Capture 2EC 2.1-6.4 oz/acre
Discipline 2EC 2.1-6.4 oz/acre
Lorsban 4E 1-2 pt/acre
Mustang Max 1.28-2.8 oz/acre
Nufos 4E 1-2 pt/acre
Penncap-M 4 pt/acre
Pounce 3.2EC 4-8 oz/acre
Sevin XLR Plus 2 qt/acre
Warrior 1.92-3.2 oz/acre

This article originally appeared on pages 81-82 of the IC-494(10) -- May 16, 2005 issue.

Updated 05/15/2005 - 1:00pm