Reports of Intense Captures of Black Cutworm Adults

April 7, 2000
Even before planters begin rolling in the spring, we start to get reports of captures of black cutworm adults in pheromone traps that have been set up voluntarily by several people in Illinois. The first captures of these moths, as they fly into the Midwest from the states on the Gulf Coast, are interesting to note and are a signal for us to stay alert. Doug Gucker, academic hourly in the Piatt County Extension Unit, captured his first black cutworm adult on March 19. Dave Feltes, Extension IPM Educator at the Quad Cities Extension Center (Rock Island County); Jeff Hoffman with Piatt County Service Company in Bement; and Kevin Black with Cargill in Lexington (McLean County) all reported first captures as early as March 21 or 22 this year.

Pheromone trap for capturing black cutworm males.

More important to note, however, are "intense" captures of male black cutworms. Intense captures are defined as 8 to 9 moths captured in a pheromone trap in a 1- to 2-night period. Intense captures trigger our running a model that allows us to predict when black cutworm larvae will have grown large enough (fourth instars) to begin cutting corn seedlings. Cutworm larvae become fourth instars when approximately 300 heat units above a base developmental temperature of 50° F. Jeff Hoffman reported an intense capture (11 moths in two nights) as early as March 24 and 25 and two other intense captures on April 2 (12 moths) and April 3 (17 moths). Terry Behymer of Behymer Consulting in Mt. Sterling reported capturing nine black cutworm moths during April 1 and 2.

Will black cutworm pheromone traps capture other species of moths? Although the sexual pheromone used with the sticky traps is specific for black cutworms, other moths may inadvertently fly into the traps and get stuck. Therefore, it is very important to be able to identify black cutworm adults. 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 paler. The most notable characteristic is the dagger-shaped marking at the outer edge of a kidney-shaped spot toward the middle of the forewing. Moths that lack this characteristic are not black cutworms.

Profile of a black cutworm adult male (note dagger-shaped marking at theouter edge of the kidney-shaped spot).

What do intense captures of black cutworm moths mean? These captures, early in the season, mean that black cutworms have begun their flights for 2000 and that we can anticipate others arriving soon. As the adults fly into Illinois, they will seek oviposition sites, primarily fields in which winter annual weeds or other vegetation are growing. 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. Some favored weed hosts for oviposition include curled dock, yellow rocket, velvetleaf, rough pigweed, chickweed, and henbit. Adult females also will lay eggs on fine-textured debris, such as dead foxtail or soybean stems. Females rarely deposit eggs on living corn or soybean plants or on bare soil.

Early planting usually reduces the potential for infestations of black cutworms. If fields have been worked and weeds have been killed before adult black cutworms begin arriving in Illinois, the females will deposit their eggs elsewhere (e.g., in noncrop areas). On the other hand, fields "fuzzy" with small weeds are prime candidates for cutworm problems. As reports of black cutworm flights continue, you'll have to be the judge regarding the potential for individual fields as being oviposition sites for female black cutworms.

What happens after the females lay their eggs? After black cutworm larvae hatch from the eggs, they survive by feeding on growing weeds or crop or weed residue. After the weed hosts are killed with herbicides and the residue is tilled under, the larvae can survive for a brief time without feeding. When the corn seedlings emerge, and sometimes even before emergence, the cutworm larvae will begin feeding on the corn. Small cutworms (first through third instars) feed on leaf tissue, causing no economic damage. Larger cutworms (fourth through sixth or seventh instars) cut seedling off at or just below the soil surface. Plants cut off above the growing point survive but may not recover fully. Plants cut off below the growing point are killed. As a result, the plant population is reduced, sometimes dramatically.

Leaf feeding injury caused by early-instar black cutworm.

Black cutworm larva and cut corn seedling.

Can insecticides prevent cutworm damage? Yes, some insecticides applied before, during, or shortly after planting can prevent damage caused by black cutworms. However, the trick is to determine which fields will benefit from such a treatment. If cutworms are not present in a field to which an insecticide was applied, the cost of the treatment was an unnecessary expense. The time-honored method for managing black cutworms is to wait and see what happens before applying an insecticide. Black cutworm infestations can be observed during early and frequent scouting. If the amount of damage exceeds established economic thresholds, a "rescue" treatment can be applied.

However, fields planted without tillage, and particularly weedy fields, might be candidates for preventive insecticides. Table 1 lists the insecticides that can be applied as preventive treatments. In a future issue of the Bulletin, we will include the list of rescue treatments.

As reports of captures of black cutworms continue to come into our office, we will begin predicting dates on which you might expect cutting activity in your area.--Kevin Steffey

Author: Kevin Steffey