University of Illinois

No. 4/April 18, 1997

Black Cutworm Migration Update

Black cutworm moths were caught throughout much of Illinois for the monitoring period of April 5 to 11. Captures were very light with a few exceptions. An intense flight (nine or more moths caught over a 1- to 2 day period) was reported in Clark County (April 11) by H. Wilson Montgomery an entomologist with Arise Research and Discovery, Incorporated. Noel Troxclair, IPM Educator, Marion Extension Center, reported that an intense flight of moths occurred in Franklin County during the evening of April 13. Projected cutting dates provided by Robert Scott, Illinois State Water Survey, indicate that cutting of susceptible corn plants (1- to 4-leaf stage) could begin on the following dates for the given counties: Alexander and Randolph, May 2; Monroe, May 6; Franklin, May 10; and Clark, May 14. These projected cutting dates are based upon intense captures that occurred in these counties. Producers in nearby counties also should monitor their fields for cutworm injury within a similar time frame.

When should a producer begin to scout a field for cutworms? Plants within fields should be examined for pinhole (very small holes) feeding on leaves well before 300 heat units have accumulated beyond an intense capture. Waiting for 300 heat units to accumulate before monitoring a field for cutworm injury is risky business. Recall that black cutworm larvae (fourth larval instars) are generally able to begin cutting plants when 300 heat units (base 50F) 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.

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) must be determined in deciding if 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 from cutworms, especially if weeds were present during the egg-laying activities of female moths.

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. Those fields infested with common chickweed, mouse-eared chickweed, bitter cress, shepherd's purse, yellow rocket, and 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 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 as 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?
Extension entomologists suggest that rescue treatments should be applied in commercial corn when at least 3% 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 upon 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.

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