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Issue No. 5, Article 2/April 28, 2006

Black Cutworm Flights Diminish in Southern Illinois: Careful Scouting Still Warranted

Flights of black cutworm moths diminished across southern Illinois through the monitoring period that ended on April 25. Ron Hines, senior research specialist, Dixon Springs Agricultural Center, reported very low captures of moths in Fayette, Jefferson, Massac, Pope, Pulaski, and St. Clair counties. Farther north, in Coles and Cumberland counties, intense captures (nine or more moths caught over 1 to 2 days) were reported for the sampling period that ended on April 18. Because of the later planting than we've observed in several years, many fields have flourishing densities of weeds. These fields will serve as prime egg-laying targets for female black cutworm moths. Careful scouting of fields this spring is very important as corn plants begin to spike through the soil. Just this morning (April 26), Don Rhodes, agronomist with Burrus Hybrids, reported that black cutworm larvae, as well as dingy cutworm larvae, were being found in some cornfields northwest of the St. Louis vicinity, so vigilant scouting is in order. Following are some questions and answers designed to refresh our memories about the management of this insect pest. With the trend toward earlier and earlier planting, it has been quite a while since we've concerned ourselves with this early-season threat.

When should a producer begin to scout a field for cutworms? Plants within fields should be examined for pinhole feeding (very small holes) on leaves well before 300 heat units have accumulated beyond an intense capture. Waiting for 300 heat units before monitoring a field for cutworm injury is risky. Black cutworm larvae (fourth-larval instars) are generally able to begin cutting plants when 300 heat units (base 50°F) have been reached following an intense capture of moths. Use leaf feeding as an early warning system for potential cutting that may occur at a later date. By using the Degree-Day Calculator for black cutworms, you can project when 300 heat units (base 50°F) are likely to accrue for your area. Again, it's a good idea to begin looking for injury before 300 heat units have accumulated.

Will leaf feeding contribute to any yield loss, or is cutting the primary concern? Several years ago, a study conducted by the University of Illinois revealed that leaf feeding by black cutworms did not influence yields. In addition, while some plants cut at the soil surface produced ears, the yield was reduced 76% in these plots. Results from this research also revealed that plants cut below the soil surface did not produce ears. Ultimately, the location of the plant's growing point and the location of the cut (above or below the growing point) will have to be determined in deciding whether a plant is likely to survive and eventually produce an ear.

What fields are most at risk in developing economic infestations of black cutworms? Black cutworm moths are attracted to weedy areas in fields. The availability of actively growing (green) weeds creates very attractive ovipositional (egg-laying) sites for female moths. Soybean debris is more attractive than corn residue, and bare soil is unattractive for black cutworm oviposition. Growers report more instances of black cutworm attack in corn following soybeans than corn grown after corn, and the fine texture of soybean residue may be an important factor in influencing ovipositional behavior. Corn grown in rotation with wheat also is at greater risk for cutworms, especially if weeds were present during the egg-laying activities of female moths.

Fields such as this are targets for female black cutorms, Champaign County, April 24, 2006.

Are certain weeds more attractive to black cutworm moths? Yes. Fields that contain winter annual and perennial weeds prior to final tillage and planting are most at risk to an infestation of black cutworms. Fields with common chickweed, mouse-eared chickweed, bitter cress, shepherd's purse, yellow rocket, or pepper grass are likely candidates for the development of a black cutworm infestation.

How do delays in planting and tillage affect the potential for cutworm damage? Research conducted by William Showers (retired), USDA-ARS and Iowa State University, indicated that when tillage or herbicide applications were applied 8 or 14 days prior to planting, minimal cutting of corn seedlings occurred, presumably because cutworm larvae starved. If tillage operations and herbicide applications were delayed until 2 days prior to planting or made the same day of planting, more corn plants were cut. The researchers believed that corn plants emerged before significant numbers of black cutworms starved. Their assumption--cutworm larvae survived on plant debris (at least 10 days) until corn emergence occurred.

What are the economic thresholds for black cutworms in commercial corn and seed production fields? Is there a difference? Rescue treatments should be applied in commercial corn when 3% or more of the plants are cut and larvae are still present. In seed production fields, rescue treatments may be needed if 3% to 5% of the plants have leaf feeding and two or more cutworms are found per 100 plants. This suggested threshold is based on the assumption that for each 1% of the plants with leaf feeding, 3% to 5% of the plants may be subject to cutting at a later date.

If I've used a seed treatment (nicotinoid), do I need to scout or even be concerned about black cutworms? Yes. Scouting of cornfields, especially if you've used a low rate of Cruiser or Poncho, is warranted. In an article published in the Integrated Crop Management Newsletter (May 9, 2005), Marlin Rice, extension entomologist, offered the following comment on the control offered by nicotinoid seed treatments in Iowa State University trials: "However, preliminary research conducted at Iowa State last year and this year has found that the low rates of either Cruiser or Poncho were ineffective in preventing cutting by 4th-stage larvae. In contrast, the high rate of Poncho (Poncho 1250, and the only product tested so far at the high rate) has provided very good protection against cutworm injury."

There are many insecticides labeled for use as rescue treatments. The 2006 Illinois Agricultural Pest Management Handbook lists the following products for black cutworm control: Ambush 25W, Asana XL, Baythroid 2, Capture 2EC, Lorsban 4E, Mustang Max, Pounce 3.2 EC, Proaxis, and Warrior. All of these products are restricted-use insecticides and may be applied only by certified applicators. For use rates, consult either the Illinois Agricultural Pest Management Handbook or product labels.--Mike Gray

Mike Gray

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