2018 U of MN Cooperative Black Cutworm Trapping Network Report #7

Week of May 26-June 1, 2018

This newsletter is available in a print-friendly pdf format: 2018 Black Cutworm Network Report #7

Table 1 shows counties that have reported significant trap captures (eight or more moths over two nights; indicating potential risk for economic damage to row crops) and dates. It also presents the degree-day calculations that predict egg hatch and beginning of leaf feeding, the start of the 4th instar (stage) larvae that are large enough to cut small corn plants, and when cutting stops and pupation begins. Because these calculations are based on air temperatures, they differ from temperatures the below ground residue are exposed to.

Corn plant with deadheartFigure 1. Corn plant with deadheart.

The warm weather created some slight adjustments to degree-days in this table. Once larvae reach the 6th instar and/or corn is larger, the risk of further damage starts to decrease. Larger larvae will cut fewer plants as they near the end of feeding as they pupate. Fewer large plants will be cut. Because of multiple flights, available food, and microclimate, there may be several sizes of larvae in the same field.

It is important to know what insect is feeding on your crop’s leaves or cutting stems. Beware of confusing non-damaging crane fly larvae with cutworms. Remember, some cutworm species do not cut below ground and, while they can kill small soybeans and other broadleaves cut below their above ground growing point, small corn, with its growing point safe below the injury, will recover. Images of some of these can be found in the fact sheet listed below. 

potato stem borer in cornFigure 2. The south end of a potato stem borer in corn.

Recently, I visited a field with non-Bt corn planted in a killed volunteer rye cover. In an area well out into the field (scouting hint), Jay Zielske found corn plants with “dead heart”. The growing point was injured and whorl was dead (Figure 1).  Black cutworm can cause a similar injury when they tunnel into corn too large to cut.

In some, but not all, of these plants, a caterpillar was found tunneling into the growing point below ground and upwards in the stem (Figure 2). Pending a better ID, the larvae appear to potato stem borers, relatives of hop vine borer. Similar to hop vine borer, insecticide is not likely to be effective once larvae tunneled into the plant.

In the same field, another type of damage that might be confused with cutworms was aggressive feeding on leaves of 4-leaf corn (Figure 3). This damage was caused by true armyworm (Figure 4). With less than two larvae/plant, the corn will recover and an insecticide application would have been money better spent elsewhere.

armyworm feeding damage on cornFigure 3. Armyworm feeding damage on small corn.

We are getting reports of black cutworm larvae, but, in most cases, crop injury has been low. 4-leaf and greater corn without any significant leaf feeding or cut plants can start coming off the scouting schedule - shift your focus to younger corn. If you’re doing a good job scouting, when a black cutworm infestation is economic, it is usually obvious.

Economic threshold for black cutworm can be found in the Extension fact sheet: https://www.extension.umn.edu/agriculture/corn/pest-management/black-cutworm/. This fact sheet provides information on what makes a field attractive to black cutworms, cutworm growth and development and tips on cutworm scouting and management. Pay close attention to the sections on scouting, cutworm development and economic thresholds as your corn scouting begins.

Previous black cutworm reports can be found at https://swroc.cfans.umn.edu/agricultural-programs/pest-management/black-cutworm-reporting-network.

true armyworm larvaFigure 4. True armyworm larva.

If you find black cutworm damage in your corn, soybean, sugarbeet and other crop scouting this spring, we would like to hear about them!

This spring, some trap locations caught many moths, some caught only a few or none. All these data are important. Yes, the negative captures reports are just as important as positives in determining where migrant moths may have dropped out.

A big thank you to all the cooperators for 2018! Thanks, A.J. Krusemark, Angie Peltier, Bernie Eichstadt, Bill Miller, Bob Braun, Brian Weller, Bruce Ness, Bryon & Kristy Kittleson, Carmen Fernholz, Chris Wessner, Cody Dobberstein, Curt Reese, Dale McMullen, Damon Traxler, Dan Haubrich, Dan Lemmon, Dave Nicolai, Dave Pomrenke, Dave Scheevel, Dave Schwartz, Derek Hansen, Dorian Gatchell, Duane Bakke, Ethan Quade, Evan Oberdieck, Gale Symens, Gary Prescher, Greg Haubrich, Kooper Cleveland, Jason Ertl, Jerome Lensing, Jodie Getting, Justin Golden, Justin Kennedy, Kerry Dittbenner, Kevin Adolf, Kevin Ballman, Larry Arentson, Lisa Behnken, Logan Wenzel, Lucas Sandager, Luke Peterson, Mark Bents, Mark Bernard, Mark Dietz, Mark Glady, Matt Prigge, Matt Salentiny, McKenna Murphy, Melinda Robertson, Nathan Wilber, Nathan Winter, Nick Pieske, Steve Commerford, SWROC/Travis, Ryan Thelemann, Tim Moline, and Tom Klader. Some cooperators had helpers checking and reporting trap captures. If I missed anyone in the list, I apologize.

5th instar black cutworm larvaFigure 5. 5th instar black cutworm larva. Photo submitted by Deepthroat.

- Bruce Potter (bpotter@umn.edu cell: (507) 276-1184) and Travis Vollmer

Table 1. Significant moth flight capture locations, dates and larval development predictions. These projections are guidelines only. Italicized dates are based on 30-year average temperatures. The rate of development is influenced by microclimate temperatures that are influenced by factors such as slope and aspect, residue and soil moisture.
County 2-night capture Biofix date Post-flight Degree-days as of May 29th Estimated BCW stage Estimated start leaf feeding1 Estimated start of cutting2 Projected end of cutting3
Blue Earth 8 3-May 504 6th instar 12-May 27-May 14-Jun
Dakota 8 8-May 429 5th instar 18-May 29-May 18-Jun
Faribault 10 8-May 443 6th instar 17-May 29-May 17-Jun
Jackson 11 8-May 415 5th instar 18-May 30-May 19-Jun
Jackson 8 19-May 306 3rd instar 26-May 6-Jun 24-Jun
Martin 20 2-May 505 6th instar 11-May 27-May 14-Jun
Martin 12 9-May 422 5th instar 18-May 30-May 18-Jun
McLeod 10 4-May 497 6th instar 12-May 26-May 14-Jun
Murray 9 2-May 511 6th instar 10-May 27-May 13-Jun
Nobles 27 1-May 499 6th instar 10-May 26-May 14-Jun
Olmsted 8 2-May 463 6th instar 10-May 28-May 16-Jun
Olmsted 13 21-May 259 2nd instar 27-May 8-Jun 27-Jun
Pipestone 9 4-May 503 6th instar 14-May 27-May 15-Jun
Redwood 8 3-May 504 6th instar 10-May 27-May 14-Jun
Rock 11 3-May 516 6th instar 10-May 27-May 13-Jun
Rock 8 9-May 434 6th instar 18-May 30-May 17-Jun
Rock 14 10-May 422 5th instar 19-May 30-May 18-Jun
Rock 21 11-May 417 5th instar 19-May 31-May 18-Jun
Steele 11 9-May 409 5th instar 19-May 30-May 19-Jun
Swift 9 7-May 434 6th instar 16-May 29-May 18-Jun

1Based on 90 degree-days (base 50oF) after significant flight (leaf feeding begins).

2Based on 312 degree-days (base50oF) from significant flight. 4th-6th instar larvae are large enough to cut corn. Small plants, e.g. sugarbeets, can be cut earlier.

3Based on >641 degree-days (base 50oF) after significant flight pupation.