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More Intense Captures of Black Cutworm Moths Have Occurred

April 12, 2002
Continued wet weather in most areas of Illinois will increase concern about black cutworms. The vegetation growing in fields, especially the winter annual weeds, are attractive egg-laying sites for incoming black cutworm females. Consequently, the potential for black cutworm problems increases when planting is delayed.

I discussed one "intense capture" (nine or more moths captured over a 1- to 2-day period) of black cutworm males in last week's Bulletin, but that was just the beginning. Other people are beginning to capture black cutworm moths in pheromone traps, and we want to keep you apprised of their findings. Randy Kenke, with Hamel Seed in Madison County, has established four pheromone traps to monitor black cutworm flights. In one trap he captured 15 black cutworm moths, from April 2 through April 5. In another trap he captured 10 moths, from April 6 through April 8. Jerry Harbour (Lincoln Land FS near Springfield in Sangamon County) captured 14 and 10 black cutworm moths on April 7 and 8, respectively. Steve Albrecht, with Illinois Valley Supply Company in Carrollton (Greene County), captured 15 black cutworm moths, between April 4 and April 8, then captured another 9 moths on April 9. I received a couple of other reports of black cutworm moth captures, as far north as Piatt County, but no other reports of intense captures.

It's likely that interest in black cutworms will continue to increase as the spring rolls on, so now is a good time to review what we know about black cutworms. Although they have been sporadic pests for the past several years, outbreaks can be devastating. Therefore, it's best to be prepared.

Description. The first life stage of black cutworms we encounter is the adult moth as it flies into Illinois from the southern states. The moths are robust, with a wingspan of approximately 1 1/2 inches. The basal two-thirds of the forewing is dark, and the outer third is much paler. Each forewing has a dagger-shaped marking at the outer edge of a kidney-shaped spot toward the middle of the wing. You'll find an excellent photograph of a black cutworm moth in the June 4, 2001, issue of Iowa State University's Integrated Crop Management. Other moths also identified in this article are found at the following URL on the Web: icm/2001/6-4-2001/moths.html.

Black cutworm adults.

Black cutworm larvae pass through six or seven instars, so their size varies with age. The average lengths (of the different instars) of the black to pale gray larvae are 1/4 inch (third instar), 1/2 inch (fourth instar), 1 inch (fifth instar), and 1 1/2 inches (sixth or seventh instar). The "skin" of a black cutworm larva is covered with convex granules, which can be seen readily under a microscope. The four knotlike tubercles (bumps, or protuberances) on each abdominal segment along the top of the back are unequal in size. Each pair of tubercles closest to the center of the body is two to three times smaller than the pair directly, and more widely spaced, behind them. Because of Marlin Rice's (Extension entomologist at Iowa State University) excellent photographic skills, the best photos of black cutworm larvae can be found in Integrated Crop Management. Check out different types of caterpillars in corn at 2000/5-8-2000/cutworm2000.html.

Life cycle. As most people know, black cutworms do not overwinter in most of the Midwest. Adults migrate from the Gulf States into the Midwest, primarily during April and May. Upon their arrival, mated females seek vegetation or debris on which to deposit their eggs (they prefer vegetation over debris). Damp, low-lying areas in fields that have not been tilled are particularly attractive for oviposition. Some of their preferred hosts of oviposition are curled dock, yellow rocket, velvetleaf, and rough pigweed, as well as winter annuals such as chickweed.

After hatching, the larvae feed on plants and grow through six or seven instars (as indicated previously). Depending on temperature, the larvae may feed for more than a month. Larvae pupate in the soil, and adults emerge about a week later to begin mating and laying eggs for a second generation. In Illinois, black cutworms complete two to three generations, although the first generation is the only one of concern to most corn growers.

Injury. Young instars feed on leaves of seedling corn, causing small shot holes to appear. Although this feeding injury is not economic, it's an early indication of the presence of black cutworm larvae. Fourth to seventh instars feed at night, cutting seedlings at or below the soil surface. Cutting of corn seedlings usually begins when approximately 300 degree-days (above a base temperature of 50F) have accumulated after an intense capture of moths. If the corn has grown beyond the stage when black cutworm larvae can cut the plants, larger larvae may drill into the base of a plant, often killing the growing point.

Corn seedling injured by early-instar black cutworm.

Corn seedling cut off above ground by fourth-instar black cutworm.

Management. Because the occurrence of black cutworms is difficult to predict, we still suggest the "wait and see" approach for managing this pest. In other words, instead of applying a preventive insecticide, timely scouting and application of an insecticide if needed usually make more economic sense in most fields. However, insecticides intended to prevent cutworm damage may be warranted in some fields (e.g., weedy fields before late planting, no-till fields).

If you intend to scout for cutworms and make control decisions after the corn seedlings are growing, be sure to scout early and frequently (probably every 2 to 3 days or so). Look for early signs of the presence of small instars (small shot holes in leaves), but don't overreact. Many early instars perish (environmental conditions, predators) before they can cause economic damage. If cutting is detected, an application of an insecticide may be warranted when 3% or more of the plants are cut below ground and larvae are present.

Insecticides registered for control of cutworms are listed in Table 2 and Table 3 (Table 2--preventive insecticides, Table 3--"rescue" insecticides). Be sure to follow all label directions and precautions.

We'll provide more information about scouting for and treating for black cutworms after corn has been planted and has begun to emerge. In the meantime, keep those reports of moth captures coming.--Kevin Steffey

Author: Kevin Steffey

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

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