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Some Reports of Black Cutworm Larvae Feeding on Corn, and Moth Captures Continue

May 17, 2002
Not surprisingly, with storm fronts moving through Illinois with frequency, captures of adult black cutworms continue. Reports of "intense captures" (nine or more moths captured over a 1- to 2-day period) are common. (Refer to "The Hines Report,", for an example of continued captures of black cutworm adults.) We can project dates for the first signs of cutting by black cutworm larvae, but at this late date, suffice it to say that corn that has emerged should be scouted right now, regardless of location within the state. Corn planted from now on also will have to be scouted vigilantly after it emerges. The weeds in these not-yet-planted fields have been attractive egg-laying sites for gravid (pregnant) black cutworm females.

I have received a handful of reports of early signs of black cutworm injury to seedling corn--small pinholes in the leaves or notches chewed from leaf edges. (Don't confuse the latter symptom with injury caused by southern corn leaf beetles. Refer to the following article, "Southern Corn Leaf Beetle Injury Revisited.") Doug Kirkbride, with M & J Fertilizer in Pana, observed some shot-holing and leaf feeding by small black cutworms in a field in Christian County on May 8. At the time, he found only one larva (fourth instar) capable of cutting plants. By now, black cutworm larvae could be causing noticeable cutting damage in that field. On May 10, Pete Fandel, Woodford Extension unit educator in crop systems, reported "lots of early cutworm feeding but no cutting yet" in several fields in Woodford County where 95% of the corn had been planted. Cutting damage could be occurring there, as well.

In fields where corn has emerged and black cutworms are cutting plants off at or below ground level, a "rescue treatment" is warranted if 3% to 5% or more of the plants are cut below the growing point. Refer to issue no. 4 (April 19, 2002) of the Bulletin for scouting tips; refer to issue no. 3 (April 12, 2002) for suggested rescue insecticides (and remember to add Mustang [1.4 to 3 ounces per acre] to the list of products for cutworm control).

Among farmers who have not planted corn yet, many are considering application of an "insurance" insecticide tank-mixed with a burndown herbicide when the weather allows them to get back into the fields. I appreciate farmers' concerns about how they will deal with cutworms when they do get the chance to plant. I know that when planting resumes (begins in some places), farmers will be working dawn to dusk and maybe then some. Scouting will be the last thing on their minds. Scouts are in a tough spot because they can't cover enough acres to make everyone feel comfortable about scouting and rescue treatments.

To be honest, there is no stock response to this concern. There are advantages and disadvantages associated with this approach for cutworm management. The relatively inexpensive insecticides that can be used as preventive treatments are attractive to farmers concerned about cutworm damage. However, even the low cost of these applications is too much if cutworms are not present or have not survived. Although we have had intense moth flights this year, we have had these in the past with no follow-up outbreak. Although planting has been delayed and fields are "fuzzy" with weeds, we don't know whether young black cutworm larvae will survive all the moisture. They, too, are exposed to this incessant rain, and lots of rain can reduce cutworms' chances for survival.

If you are weighing the benefits and limitations of applying an insecticide to prevent cutworm damage, please consider all angles, including economic and environmental aspects. The pyrethroids are relatively cheap, and the peace of mind that might result from such "insurance" is priceless (to steal from a well-known television commercial). However, applying any insecticide without knowledge of the presence of cutworms violates some tenets of IPM. And the odds are pretty good that we will never know whether preventive insecticides paid off or not. Very few people, if any, will leave untreated check strips to find out whether the treatment was necessary. If cutworms don't cause a problem in any given field that is treated, we won't know whether the treatment worked or whether there weren't any cutworms there to begin with.

Making this decision is tough, and there are very few 100% surefire answers. Too bad the weather has placed us in this predicament.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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