Black cutworms and preventive treatments

Most corn is planted in southern Iowa but much remains to be planted in the northern half of the state. For fields that have yet to be planted, the question of whether to use a preventive insecticide for black cutworms is valid. Below are several points to consider before purchasing and applying an insecticide solely for the prevention of black cutworms.

Concern 1: Necessity of preventive insecticide treatments. The basic principles of integrated pest management ask the following questions: Can the insect be scouted? Can the economic damage be predicted based upon field scouting? Can a rescue insecticide be applied if needed? and Can the rescue insecticide provide equal or better control than the preventive insecticide? The answer to all four questions is "yes." The use of an insecticide applied as a preventive treatment cannot be economically or environmentally justified when a rescue treatment can provide equal or better control.

Concern 2: Black cutworm migration and egg laying are spotty. Black cutworm adults (moths) are migratory and fly into Iowa from southern states. Captures of male black cutworms in pheromone traps tell us when they arrive, but the trap captures do not tell us in what fields females lay their eggs, how many eggs they laid, what the cutting potential is, or whether the moths stayed within the county where individuals were trapped. In fact, the moths may continue their migration the following night into Illinois, Minnesota, or Wisconsin. For moths that do remain in Iowa, they may not lay eggs in all fields. Previous crop residue (soybean is preferred over corn), tillage, soil moisture, and winter annual weed growth all affect where moths will lay eggs.

Concern 3: Potential for cutworm damage is low. The last serious cutworm outbreak occurred in Iowa was 1984. Recent information strongly suggests that the threat of black cutworm damage on a large scale is overrated. The probability of black cutworm damage is very low in any field, particularly if the field is free of broadleaf weeds during April and early May.

Concern 4: Insecticide cost is expensive. These insecticides are not cheap. For the low- and high-end label rates, cost could range from $5.00 to $17.00 per acre, depending on the product used and dealer incentives.

Concern 5: Insecticide performance guarantees. One company may guarantee that its insecticide provides control of cutworms when applied as a preemergence treatment, whereas another company may state that with one preplant application for cutworms, you have one less problem to worry about. Do not be lulled into a false sense of security with insecticide guarantees or claims. Any guarantee or claim is subject to the condition that the field must be scouted for insect damage. Just because an insecticide was applied to the field at planting does not preclude the possibility of crop injury by the insects later in the season.

Several years ago, our trap catches allowed us to predict that black cutworm cutting would begin between May 18 (southeastern Iowa) and May 25 (northern Iowa). The first cutting was reported in eastern Iowa on May 22. This first cutting date was nearly 5 weeks after the first corn was planted and this is an extremely long time to expect an at-planting insecticide to still be effective.

Alternative to insecticides. There is a better alternative to black cutworm management than buying unnecessary insecticide and increasing on-farm input costs. This alternative is to have the fields scouted when first cutting is expected.

Then you should scout the field, look for early signs of injury, and determine whether the economic threshold has been reached. Based on your findings, insecticide can be applied if it is really needed. Remember that black cutworm females may not lay eggs in your field and that it is to your advantage to use insecticides wisely and economically.

This article originally appeared on pages 63-64 of the IC-488 (7) -- May 6, 2002 issue.

Updated 05/05/2002 - 1:00pm