Black cutworms return, 2006 cutting dates predicted

Significant numbers of black cutworm adults (moths) were captured in pheromone traps during April across Iowa. 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 trap captures, we anticipate that first cutting should occur May 8 (southwestern corner of Iowa), May 11 (the remainder of Iowa except the northeastern corner), and May 18 (northeastern corner of the state).

Black cutworm 2006 cutting dates
Predicted black cutworm cutting dates for 2006.
Fourth-instar black cutworm larvae
These early 4th-instar cutworms are not quite the length of a dime, but they can cut seedling corn plants. (Marlin E. Rice)

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 April and May 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.

As a reminder, pheromone traps do not predict the amount of cutting in a field nor the counties 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 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. Also, fields with winter annual weeds are more likely to have cutworms than clean fields, and soybean stubble is more attractive to the moths than corn stubble.

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 whether they are cutting plants and the percent cut plants.

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 105-106 of the IC-496 (9) -- May 1, 2006 issue.

Updated 11/09/2006 - 9:56am