Time to scout for black cutworms

The black cutworm is an occasional corn pest, yet it deserves our attention because of its potential for damage. The Integrated Pest Management Program at ISU coordinates a moth-trapping effort in about 75 Iowa counties. These traps contain a pheromone (a chemical attractant) that catches male moths.

1996 black cutworm scouting advisory

Leaf-feeding by cutworms usually appears before cutting occurs.

Corn cut by black cutworm above the soil surface.

Corn drilled by black cutworm below the soil surface.

Weedy fields often attract the most black cutworm moths.

How is trap information used?

Significant numbers of moths (eight or more moths on two consecutive nights) were caught across the southern two-thirds of the state in April. Growing degree days are then used to estimate when first cutting is expected to occur from larvae hatched from eggs laid by moths in the area.

Do the trap catches indicate a cutworm outbreak this year?

Definitely not. Moth catches were on the sparse side this spring, but we did catch a few in very scattered locations. It is very important to remember that the trap catches only indicate that moths have arrived in an area. Traps do not predict the amount of cutting. Traps do not predict where the cutting will occur. And traps don't predict how many cutworms will occur in a field or county. Don't be mislead by advertising claims that a cutworm outbreak will occur in your county just because moths were trapped. 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.

When should scouting begin?

Scouting could begin May 21 in central and south-western Iowa. Scouting in other regions of the state should begin based upon the dates on the map (see opposite page).

No cutting problems are expected in northwestern, extreme north central, and northeastern Iowa because no significant moth flights were trapped in these areas.

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 the corn leaves. As mentioned in the May 5 ICM newsletter (pages 54-55), dingy cutworms also feed on leaves but rarely cut plants. Therefore, try to find cutworms to determine if they are blacks or dingys. When leaf feeding is found, mark off 100 plants in a row with stakes or flags and scout these same plants for cutting over time. Do this at several spots in the field. This way you can monitor the progression of damage and better observe what is happening in the field.

What economic threshold should be used for deciding on treatment?

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?

You can stop looking for black cutworms when the field is sprayed or plants have five fully developed leaves. Once plants reach the fifth-leaf stage, it becomes very difficult for cutworms to cut the plant, although they may drill into the side.

Should fields be scouted if a corn rootworm insecticide was applied at planting?

Yes. In corn following corn, many farmers want to get control of both corn rootworms and black cutworms with the same insecticide application. However, the at-planting insecticide may not give 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 given in parentheses: Ambush 2EC (6.4 to 12.8 oz./acre), Asana XL (5.8 to 9.6 oz./acre), Lorsban 4E (2 to 4 pints/acre), Pounce 3.2E (4 to 8 oz./acre), and Warrior (1.92 to 3.20 oz./acre). If the soil surface is dry, rotary hoeing just after application will increase the effectiveness of Lorsban. However, Ambush, Asana, Pounce, and Warrior should not be incorporated.

This article originally appeared on pages 78-79 of the IC-476(11) -- May 27, 1996 issue.

Updated 05/26/1996 - 1:00pm