Black Cutworm Decisions in Corn and a Word About Soybean
Eileen Cullen, Extension Entomologist
Black cutworm larvae and plant cutting have been occurring in V5 or smaller corn since early June with economic infestation reported from Manitowoc, Sheboygan and other counties. Bryan Jensen had a good article in the April 26, Vol. 19(6) WCM issue alerting readers about Where to Check for Black Cutworms and Armyworms.
This week, calls are coming in from agronomists and county agents in Clark and Marathon counties about larvae approximately one-inch in length in V4 to V6 cornfields. We are finding similar sized black cutworm in corn in Columbia County at the Arlington Agricultural Research Station. Black cutworm infestations are not limited to these counties and the prime time to scout for this pest was during corn emergence until the V5 growth stage.
Black cutworm larvae begin to cut plants when they reach 4th instar (approximately ½-inch in length) and plants are vulnerable to cutting until the V5 stage. Fields of smaller corn, late or replanted, are still vulnerable to cutting and are candidates for treatment if the economic threshold of 3% cut or damaged plants and black cutworm larvae are confirmed. (Scouting. larval ID and treatment recommendations are shown for <V5 corn on the UW Extension video Black Cutworm in Corn).
Current dry soil conditions are causing the larvae to remain below ground in conventional tillage and beneath residue in no-till. Larvae will still feed on V4 to V5 corn beneath the soil surface burrowing into the corn stem below ground. You will notice this as wilted center whorl leaves on the plant.
Treatment is not economical for cornfields larger than V5 because larvae can no longer cut the plants and insecticide efficacy will be limited as the material cannot contact larvae beneath the soil.
4th (left) and 6th (right) instar black cutworm larvae.
(Photo: Roger Schmidt, UW NPM Program)
As for black cutworm in soybean, there is no threshold for cutworm in soybeans. While uncommon, larvae can occasionally be a concern in soybean. Brian Lang, Iowa State University Extension Agronomist, summed it up well in his recent newsletter:
The cutworm moths are attracted to no-till and/or weedy or poorly drained areas whether its corn or soybean fields. In soybean, if the weeds are then controlled, the only green material left to feed on are soybean plants. There is no threshold regarding cutworm in soybeans. It’s just a common sense assessment of infestation vs. viable population you want to keep, versus cost of control. Smaller cutworm are a greater threat than larger ones.