Get ready to scout for cutworms

The black cutworm is an occasional pest of corn, yet it deserves our attention because of its potential for causing economic damage. Two significant flights of black cutworm adults (moths) were observed in parts of Iowa on April 11-13 and on April 18-20. The date that moths arrived indicates that eggs may have been laid in Iowa fields. Trap catch data were reported from cooperators in Iowa State University Extension's pheromone trap network. We have taken the distribution of moth captures across Iowa and coupled it with actual and projected degree-day accumulation, and present a potential first cutting dates forecast across Iowa. Black cutworms require 300 base-50°F degree days for larvae to be large enough to cut corn plants, so by calculating cutworm hatch and development over time, we can anticipate when to look for damage.

Leaf-feeding by cutworms usually appears before cutting occurs.
Black cutworm (top) and dingy cutworm (bottom).

When should scouting begin?

The map lists dates of first cutting based on pheromone trap information and degree-day predictions. Dates range from May 11 in south central and southeastern Iowa to May 20 in northeastern Iowa. This map is probably the most complex map we have produced in the last 10 years, but it reflects our best estimates of first cutting based on the two moth flights.

Projected dates of first black cutworm cutting, 2000

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. 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 trap catches nor anyone's interpretation (including our own) of the trap catches can accurately predict the amount or magnitude 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.

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 cut corn. If leaf feeding is detected, 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 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).

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 hornlike projections on the tail. Canada geese feeding on seedling corn 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 they 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 or 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 labeled for rescue treatments?

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

This article originally appeared on pages 61-63 of the IC-484 (8) -- May 8, 2000 issue.

Updated 05/07/2000 - 1:00pm