University of Illinois

No. 8/May 15, 1998

Black Cutworm Danage and Recovery of Corn:
Leaf Feeding Common

Dave Mowers, Mowers Soil Testing Plus, Inc., indicated that black cutworm "pinhole" leaf feeding and small larvae are common in many cornfields in Henry, Knox, Marshall, Peoria, and Stark counties. Dave also reported that larger dingy and claybacked cutworms are being found in many of the same cornfields. Don't hesitate to begin scouting fields in earnest for cutworms and leaf feeding. Also, please refer to several of the previous issues of this Bulletin for projected cutting dates. For southern areas of the state that experienced intense flights in mid-April, cutting in some fields is under way. In northern Illinois, intense captures of black cutworm moths in mid-April should yield cutting infestations of larvae by next week. With the expected very warm temperatures that are forecast for late this week, larval development will be accelerated. Bottom line--cutting is occurring in southern Illinois and is not far behind for most areas of Illinois.

In 1989 and 1990, an experiment was conducted at the University of Illinois Foundation Shaw Farm, Urbana, to examine the influence of pyrethroid and organophosphate soil-insecticide treatments on cutworm damage and the recovery of corn plants. The experiment involved artificially infesting black cutworm larvae into test arenas created by placing steel barriers around a given number of corn plants (17 to 19 plants). Ten fourth-instar larvae were placed within each barrier 10 days after planting. Hand-weeding kept all the arenas free from weeds during the experiments.

The pyrethroids used in our studies included Ambush 2E (permethrin), Capture 2EC (bifenthrin), Force 1.5G (tefluthrin), and three experimental pyrethroid compounds. The organophosphate insecticides included Counter 15G (terbufos), Dyfonate II 15G, 15CG, 20G (fonofos), Holdem 20G (ethoprop + phorate), Lorsban 15G (chlorpyrifos), and two experimentalcompounds. Aastar 15G (flucythrinate + phorate) also was included in the experiment. Some of these compounds may be very familiar to you, others may not; keep in mind this experiment was conducted nearly a decade ago.

Rather than concentrate on how well or poorly any single insecticide protected plants, the authors of the study chose to focus on the performance of rescue versus planting-time treatments and the overall effectiveness of the two classes of products. The results of the study were summarized as follows:

"Overall, pyrethroids used as rescue treatments were more effective than the organophosphates or pyrethroids applied at planting time. In plots receiving pyrethroid rescue treatments, approximately 88% of the plants were not damaged, compared with 59 and 67% in the planting-time pyrethroid- or organophosphate-treated plots, respectively. Leaf feedingwas significantly lower in plots receiving organophosphate insecticides than in those receiving pyrethroid insecticides at planting time. In addition, plots treated with pyrethroid insecticides at planting time had a higher number of plants with whorl feeding (9.7%) than did those treated with the organophosphate insecticides (4.3%) at planting time. Black cutworm larvae might have been repelled by pyrethroid insecticides applied to the soil and may have moved upwards on the plant, causing extensive leaf feeding and whorl damage. The percentage of plants that were cut at the soil surface tended to be higher in the organophosphatetreated plots than in the pyrethroid-treated plots at planting time (21.8 vs 13.4%)."

Leaf feeding by black cutworm larvae did not significantly affect yields. Yields were reduced significantly when cutting occurred, even if plants were cut above the growing point. Cutting at the soil surface resulted in an average yield reduction of 76 percent (most of these plants were likely cut below the growing point). For a more detailed analysis of this paper, refer to Oloumi-Sadeghi, H., et al. (1992). Crop Protection 11: 323-328.

Mike Gray ( Extension Entomology, (217)333-6652