Scout for cutworms in seedling corn

The black cutworm is an occasional pest of corn, yet it deserves our attention because of its potential for causing economic damage. First cutting was predicted to occur in southern Iowa during the May 15-16 weekend. Scouting of fields where corn plants have emerged will help determine the presence or absence of black cutworm injury.

Do predicted cutting dates indicate a cutworm problem?

Definitely not. Our predictions cannot project the amount of cutting that will occur, nor can they predict where cutting will occur. Therefore, don't be mislead by advertising claims that a cutworm "outbreak" will occur in your county simply because cutting is predicted to occur on a certain date. Scouting of seedling corn is the only reliable method for determining if a problem exists. Then, insecticides can be applied if needed.

How should a field be scouted?

Begin walking fields now and looking for injury. Problems are more likely to occur in fields with spring weed growth or in soybean stubble. One of the first signs of a potential problem is feeding injury on corn leaves. Both black cutworms and dingy cutworms feed on young corn leaves, but the dingy cutworm rarely cuts corn because they are mostly leaf feeders. If leaf injury on young corn is found, try to find the cutworms to determine if they are blacks or dingys. The field should not have a cutting problem if only dingy cutworms are found. If it is determined that the field has black cutworms, then mark off 100 plants in a row with stakes or flags and scout these same plants for cutting over the next several days. Do this at several locations across the field and then monitor the progression of injury (or lack of it) over time. By monitoring the same set of plants over time, a more reliable picture of cutworm activity can be developed.

Leaf feeding from dingy or black cutworm.

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 diameter 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 and have hornlike projections on the tail. Canada geese feeding on seedling corn will clip the top leaves and this damage 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 to 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 (five 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 insecticides are recommended, with manufacturer label rates: Ambush 2EC (6.4 to 12.8 oz/acre), Asana XL (5.8 to 9.6 oz/acre), Lorsban 4E (1 to 2 pints/acre)*, Pounce 3.2E (4 to 8 oz/acre), and Warrior 1EC (1.92 to 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 soil incorporated because the performance may be reduced.

*This was originally incorrectly published as 2 to 4 oz/acre; it was corrected 6/4/1999.

This article originally appeared on pages 70-71 of the IC-482(11) -- May 24, 1999 issue.

Updated 05/23/1999 - 1:00pm