Scout for cutworms in southern Iowa

The black cutworm is an occasional pest of corn, yet it deserves our attention because of its potential for causing economic damage. As mentioned in last week's newsletter, first cutting was predicted to occur in parts of Iowa over Memorial Day weekend. However, some cutting was found May 14-15 in eastern Iowa. These cutworms would have come from moths that migrated into the state prior to March 21, before we had any pheromone traps in the field. Therefore, we couldn't predict this early cutting.

What does this early cutting suggest? It strongly emphasizes the point we've made in the past; the pheromone traps we station in 69 counties across the state during April don't catch early flights or all the moths in an area. Sometimes they fly into a county very early. Sometimes they fly in later and the traps don't catch them. When cutting is found prior to our predictions, then we should modify our estimate of first cutting.

Leaf feeding from dingy or black cutworm.

When should scouting begin? In last week's newsletter, we suggested that scouting should begin May 23 in southeastern and southwestern Iowa. This was before extension field crops specialist Jim Fawcett found black cutworms cutting seedling corn planted into a Cedar County sod field on May 15. It also was before we received in the mail a 1/2-inch black cutworm from Keokuk County that was cutting corn on May 14. Neither of these fields exceeded one percent cutting. Because of these cutworm observations, fields in Cedar, Keokuk, and adjacent counties should be scouted now. All other counties south of Interstate 80 should be scouted May 23 or May 27, as predicted in last week's newsletter.

Do predicted cutting dates indicate a cutworm problem? No. Pheromone traps only catch moths and they cannot predict the amount of cutting that will occur, nor can they predict where cutting will occur. Don't be mislead by advertising claims that a cutworm "outbreak" will occur in your county just because moths were trapped there several weeks ago and we have predicted cutting at a certain time. Scouting of seedling corn near the first cutting date is the only reliable method to determine if a problem exists. Then, insecticides can be applied if needed.

How should a field be scouted? Begin walking the field a couple of days before cutting is predicted. Problems are more likely to occur in fields with spring weed growth or in soybean stubble. Look for cutworm injury on corn leaves. Dingy cutworms also feed on young corn leaves, but rarely will they cut corn. If leaf feeding is found, try to find the cutworms to determine whether they are blacks or dingys. If you find only dingys, then you shouldn't have a cutting problem. If you find only 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 time. Do this at several locations across the field. Then you can monitor the progression of damage (or lack of it) and better determine what is happening in the field.

How do you distinguish blacks from dingys? On the top of each body segment, black cutworms have four black tubercles (warts or spots) with the pair closest to the head about half the diameter of the pair closest to the tail. Dingy cutworms have these four tubercles equal in diameter.

Black cutworm (top) and dingy cutworm (bottom).
Black cutworm has four tubercles of unequal diamter on the top of each body segment.

What could be confused with cutworms or their injury? Crane fly larvae are common in no-till or conservation tillage fields. They feed on dead organic matter and do not cut plants. They lack legs, but have horn-like projections on the tail. Canada geese feeding on seedling corn will clip the top leaves and this may be confused with cutworm injury. Geese, however, often clip several plants in a row, eating only the top leaves and do not cut the plant at ground level.

Crane fly larva, which has no legs, can be confused with a cutworm.
Goose injury to corn.

What economic thresholds should be used? When cutworms average less than 3/4-inch in length, an insecticide should be considered if 2-3 percent of the plants are wilted or cut. 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.

When can field scouting stop? Stop scouting when the field is sprayed or plants have five fully developed leaves. Cutworms have difficulty cutting plants in the V5 stage (5 true leaves) because of the larger diameter of the stalk.

Should fields be scouted if a corn rootworm insecticide was applied at planting? Yes. At-planting insecticides may not provide adequate control for large black cutworm infestations and a rescue treatment still may be needed.

What insecticides are recommended for rescue treatments? The following are recommended, with manufacturer label rates: Ambush 2EC (6.4-12.8 oz./acre), Asana XL (5.8-9.6 oz./acre), Lorsban 4E (2-4 oz./acre), Pounce 3.2E (4-8 oz./acre), and Warrior 1EC (1.92-3.2 oz./acre). If the soil surface is dry, rotary hoeing just after application will increase the effectiveness of Lorsban. However, Ambush, Pounce, or Warrior should not be incorporated.

This article originally appeared on pages 70-71 of the IC-478(10) -- May 26, 1997 issue.

Updated 05/25/1997 - 1:00pm