Black Cutworm Feeding on Corn – Out of the Woods Yet?
Eileen Cullen, Extension Entomologist
Black cutworm (BCW) larvae are in the larger stages (4th to 6th instar) and corn in the V4-V6 stages from what I have seen in the field and reports received this week from consultants, growers, and UW-Extension agriculture agents in Dane, Dodge, Jefferson, Jackson and Vernon counties. Reports are ranging from 3% cut plants to 6-8%, 10-12% or more (30 to 40% in fields that were not scouted previously). Please see earlier WCM articles from this season for details on BCW Black Cutworm Damage Potential for Corn (WCM Vol. 18, No. 8) and Black Cutworm Feeding on Bt Technology (WCM Vol. 18, No. 13).
On Friday June 10th, I visited a field with Bill Stangel, CCA Soil Solutions Consulting, in Dodge County. I post some photos here from our field visit with tips on how to gauge how much longer early V stage corn plants are at risk if you have BCW larvae and stand loss in the field. The particular field pictured in this post was at 3% cutting on June 10th.
The most important aspect of cutworm control is careful field scouting as soon as plants begin to emerge. Obviously, corn field scouting should have been underway for the last two to three weeks. In typical years, BCW is only an occasional pest, but this year the heavy moth flights have given us a large larval population that has reached and exceeded the economic threshold of 5% cut plants. (note: with high corn prices, a more conservative BCW threshold is approximately 3% cut plants).
The question now is … how much longer will the larvae feed and can they still cut plants?
Stage of corn plant, original plant population, and BCW larval stage should all be considered. BCW larvae are in the cutting stage (capable of cutting plants) from 4th through 7th instar. One cutworm is capable of cutting several plants. Generally, the cutting stage larvae are between 0.5 inch and 1.5 inch when full grown. However, body length is not always an accurate indicator of how “old” a BCW is. The best technique is to measure the insect’s head capsule (just the head, not the pronotum_first segment_ directly behind it) between the shaded areas under the head capsule width column in Table 2-9 below from UWEX Publication A3646, Pest Management in Wisconsin Field Crops 2011 (Pg. 63).
For example, a sixth instar BCW larva will feed for approximately 14 days. In later crop stages, by V4, large larvae may not be able to cut plants. Instead, they will burrow into the corn plant, below ground level, and result in “wilted whorl” or “dead heart” symptom where newly emerging whorl leaves wilt.
At this point in the field, you should determine if BCW larvae are 6th instar or smaller, and if corn plants can still be cut. By V5-V6, the plants should be out of the woods. By 7th instar, BCW will only feed for a few more days before pupating. There are three BCW generations per year, but it is only the first generation which is active in May and June that causes damage to field corn.