The Black Cutworm Watch Begins

April 9, 1999

Among the soil-inhabiting insects that cause headaches for corn growers, black cutworms probably are the most feared. When black cutworm larvae occur in large numbers, the damage they cause to cornfields can be devastating. Many growers, particularly growers in southern counties, protect their fields with applications of insecticides intended to prevent black cutworm injury. However, we typically recommend the wait-and-see approach before recommending action against black cutworms. Because black cutworms do not overwinter in Illinois, we never know from year to year whether their densities will be large enough to result in economic damage. Therefore, we strongly encourage diligent scouting of cornfields when the seedlings emerge, and application of insecticides only when 3 percent or more of the seedlings are cut below ground. Nevertheless, preventive treatments may be appropriate in no-till or reduced-tillage systems where vegetation is plentiful during the adults' egg-laying period (more on this later). The insecticides listed in Table 1 are preventive treatments that can be considered in such situations.

Are there ways to anticipate the occurrence of black cutworms? Because of the unpredictability of infestations of black cutworms, a plan of scouting for black cutworms and applying a rescue treatment (postemergence) only when necessary can save many growers money. If people are aware of cutworm activity in an area, they can prepare themselves for the potential appearance of the pest. In the past, we supervised a network of black cutworm pheromone traps to monitor flights of adults in the spring as they flew into Illinois from southern states. However, because of diminished resources, we have not been able to maintain this network. Therefore, we rely on reports from people who have placed pheromone traps in different areas of the state. If you are checking one or more of these traps for black cutworm adults, we would be grateful to receive the information so we can pass it on to others.

We already have received some reports of captures of male black cutworm moths in pheromone traps. Carlyle Mueller, Monroe County, began checking for black cutworm moths on March 17 and has found as many as four adult males on one date (March 17). He found one or two moths per night on six other dates through April 2. Doug Gucker at the Piatt County Extension Unit and David Feltes, Extension Educator/IPM at the Quad Cities Extension Center, both captured one black cutworm moth on April 2. Jim Morrison, Extension Educator/Crop Systems at the Rockford Extension Center, captured two moths in one trap and three moths in another on April 4. Mike Roegge, Extension Unit Educator/Crop Systems at the Adams/Brown Extension Unit, captured nine moths during the weekend of March 27. If he captured these moths in a 1- or 2-day period, Mike's capture qualifies as an "intense capture." After an intense capture of black cutworm moths, we can predict when black cutworm larvae have developed to fourth instars with the ability to cut plants (accumulation of 300 heat units above a base temperature of 50 deg F). When we receive more information about captures of black cutworm adults in pheromone traps from different areas of Illinois, we will provide heat-unit accumulations in these areas.

If you are examining pheromone traps for black cutworms, you may encounter more than one species of moth. The synthesized sexual pheromone used with the traps is supposed to attract only black cutworm males. However, other moths occasionally accidentally fly into the traps and get stuck on the sticky substance. Therefore, it is important that you are able to identify black cutworm moths. The adult moth has a wingspan of 1 1/8 to 1 5/8 inches. The basal two-thirds of the forewing is dark, and the outer third is much paler. One of the easiest characteristics to see is the dagger-shaped marking at the outer edge of a kidney-shaped spot toward the middle of the forewing (see photo, above). Moths that lack this characteristic are not black cutworms.

So what is the life cycle of black cutworms in Illinois, and what types of fields are most at risk? As indicated previously, the black cutworm does not overwinter in the Midwest, except occasionally in the Mississippi delta region of southeastern Missouri. Adults migrate from the Gulf states and arrive in the Corn Belt within 2 to 4 nights. This annual immigration is heaviest during April and May. Females deposit eggs singly or in groups (as many as 30) on pasture and fencerow grasses and on low, densely growing weeds and debris in fields that have not been tilled. Damp, low-lying areas in fields that have not been tilled are particularly attractive for oviposition. Females firmly attach eggs to densely growing plants such as curled dock, yellow rocket, velvetleaf, and rough pigweed, or to fine-textured plant debris such as dead foxtail or soybean stems. Females rarely deposit eggs on living soybean or corn plants. Typically, if weeds are destroyed early and corn is planted relatively early, black cutworms do not pose much of a threat. However, if planting is delayed and cutworms have a chance to develop on weeds before the weeds are destroyed, the potential for cutworm damage increases.

Do black cutworms have any natural enemies? The most important parasitoids that attack older black cutworm larvae are the fly Bonnetia comta and the tiny wasp Meteorus vulgaris. Predation by ground beetle larvae and adults (Scarites substriatus, Pterostrichus chalcites, P. lucublandus, and Harpalus pennsylvanicus) also may reduce densities of black cutworms. However, these natural enemies often do not keep densities of black cutworm larvae below economic levels.

We will offer some more commentary about rescue treatments for cutworm control in future issues of the Bulletin. Again, if you obtain any information about captures of black cutworm adults in pheromone traps, please pass it along.--Kevin Steffey, Mike Gray

Author: Mike Gray Kevin Steffey