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 Field Reports: Dingy Cutworm in Corn, Look for Black Cutworm Cutting Minimize
Location: BlogsWisconsin Crop ManagerInsects and Mites    
Posted by: WCM Staff 5/26/2005 1:21 PM
Several reports have come in this week on Dingy cutworm larvae in seedling corn fields. Several reports have come in this week on Dingy cutworm larvae in seedling corn fields. These corn fields are just emerging, or up to the first true leaf stage. Common to all these consultant calls is that the fields of concern are of sandy soil type.

Field Reports: Dingy Cutworm in Corn,

 

Look for Black Cutworm Cutting

 

Eileen Cullen, Extension Entomologist UW Entomology Department

Several reports have come in this week on Dingy cutworm larvae in seedling corn fields. Thank you to Paul Sturgis (watching fields with dingy cutworm presence near Mauston), Jerry Vacek (Advantage Ag, near Sparta, WI), Greg Kerr (Kerr Agronomics, field in Dunn Co.), and Dave Cole (about 60 acres in a Columbia Co. field). These corn fields are just emerging, or up to the first true leaf stage. Common to all these consultant calls is that the fields of concern are of sandy soil type.
While black cutworm moths migrate to WI from southeastern US in spring, laying eggs after they arrive, Dingy cutworm overwinters in the Midwest as small, partially grown larvae. Since Dingy cutworm are already present in the soil in spring, they are usually the first to be noticed in corn fields. Dingy cutworm larvae are pale gray to reddish brown with mottled pigmentation. A key trait in Dingy versus black cutworm larval identification is that on each segment of the Dingy cutworm the characteristic paired tubercles (small dots) are both the same size, as compared to black cutworms in which one tubercle is larger than the other. Dingy cutworms reach approximately 1 inch length when mature, and there is one generation per year.
Dingy cutworms are primarily leaf feeders, and rarely cut plants as the black cutworm does. Compared to black cutworm Dingy cutworms represent less of an economic threat and do not require the same degree of control. Generally, the recommendation is to watch fields with Dingy cutworm, if damage reaches or exceeds the 5% plant damage level set for black cutworm, an insecticide treatment may be warranted, but only if excessive Dingy cutworm feeding accompanies poor growing conditions. As the weather warms up corn plants will pick up the pace toward the V5 stage (five fully developed leaves with visible collars) after which plants withstand cutworm feeding much better. For example, Black cutworms are less able to cut the V5 plant, and you can stop scouting for cutworms in general (Dingy or black) once plants have reached the V5 growth stage.
It is possible to have both Dingy and Black Cutworm in the field. As we’ve discussed in previous Wisconsin Crop Manager insect columns this spring, although black cutworm pheromone traps catches and black light collection have been low up to this point, cutting potential still exists. Sufficient degree days (310 DD) have elapsed since the first concentrated pheromone trap captures of male black cutworm moths reported by WI DATCP in late April that we know black cutworm egg hatch has occurred.
If you are finding leaf feeding and/or cut plants or nearly cut plants laying on their side in seedling corn fields, dig through the soil surrounding the damaged plants to collect larvae and identify as Dingy or black cutworm. Flag plants in the damaged area so that you can monitor the stands over several days to determine whether damage is progressing to an economic level. Monitoring this progression, or lack thereof, is just as important with Dingy cutworm which represent less of an economic threat (e.g. avoid unnecessary insecticide applications), as it is for the occasional, but potentially economic stand loss due to black cutworm damage where you may be justified in applying insecticide to protect stand.
Black cutworm scouting and economic threshold information was discussed in the Vol. 12, No. 10, May 12 WCM issue at http://ipcm.wisc.edu/wcm/pdfs/2005/CullenMay11.pdf.
 
In summary, based on a sample of 250 corn plants, 50 plants at five locations throughout the field, the economic threshold for black cutworm is reached if 5% of the plants show cutworm damage and cutworm larvae are 6th instar or smaller. Refer back to the May 12th issue for a discussion of the dynamic relationship between cutworm larval size and plant growth stage, as well as a link to insecticides labeled for cutworm should treatment be economically justified.
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