2019 UMN Cooperative Black Cutworm Trapping Network Report #6

Report #6 Week of May 11-17, 2019

For more information: Black Cutworm Reporting Network

May 23, 2019

MN map showing black cutworm moth captures by countyFigure 1. Maximum two consecutive night black cutworm captures reported by county.

Greetings:

Again, some black cutworm (BCW) moth flights entered Minnesota airspace during May 11-17 (Figure 1). Significant captures occurred in Stevens, Swift, Lac Qui Parle, Murray, Jackson, Nobles and Blue Earth.

There were some very large captures this week. Two traps in Jackson County were significant with one trap capturing 31 moths over a 5-night period. Murray County had 27 moths May 11-12. This has been a very active year for black cutworm, likely the largest since the current network began operating in 2013.

I suspect that there has also been some significant BCW moth migration into the counties north of Stevens and Polk this year. If you have crops located in these counties, I would use the same degree-day projections for Stevens County to help time scouting efforts.

Cool weather has slowed corn development and the development of BCW larvae. The very cool early spring means that larval cohorts from the individual migrations from April 22 onward should be relatively close in development. Remember, cutworm development can be influenced by soil moisture, crop residue and field topography.

Economic infestations of black cutworm are relatively rare in Minnesota but that is little consolation if you happen to have one in your field. What is concerning is the large number of acres that remained unworked and unplanted when these flights have arrived.

Table 1 shows degree-days for BCW larval development and risk for corn damage for the significant captures so far this spring. Based on the earliest flights, scout corn before 300 degree-days after moth arrival or June 1.

black cutworm development tableTable 1. Estimated black cutworm development for significant moth captures as of May 22. Current and forecast degree-days are generated from the U2U Corn Growing Degree-Day application at the Midwest Regional Climate Center https://mrcc.illinois.edu/U2U/gdd/ using current year and NWS forecast temperatures.

Identifying black and other cutworms in MN (excerpt from Black Cutworm Facts)

Black cutworms are not the only cutworm species than can injure crops in Minnesota. As corn (and other row crops) germinate and begin to emerge they can be attacked by several species of cutworms. Table 2 lists some of the species that might be found in Minnesota corn fields. Most cutworm species overwinter in Minnesota as eggs or larvae. Black cutworms cannot winter here and migrate into the state each spring. There is good evidence that similar to many agriculturalists, some black cutworm moths migrate south to warmer climates in the fall.

cutworm species that can be found on MN corn

While we can project cutting dates for the black cutworm, corn should be scouted for other cutworm species as soon as it emerges. Because cutworms that overwinter, particularly those that winter as larvae, begin development before migrant black cutworms arrive, they are ready to feed on corn early. Often, the first corn leaf feeding observed in the spring is from overwintered dingy cutworm larvae.

black cutworm curled upFigure 2. Black cutworm. Note the defensive C-shape. Find the larva's head, the true legs and prolegs. Examine the tubercles, the small black bumps on each segment near the cutworm's back. The unequal size, with the rear tubercle much larger is characteristic of black cutworms. The overall appearance is a black and "greasy".


Certain species prefer particular habitats (Table 2). For example, sandhill cutworms are found in sandy soils and several species tend to be problems in crops planted into sod. Dingy cutworms are often abundant when corn is planted after alfalfa or fields that were weedy the previous year.

Species identification is important to determine damage potential. Small larvae of all species feed on weeds and leaves and cannot cut corn. Dingy, redbacked, spotted, and variegated cutworms are primarily leaf feeders feeding at or above the soil surface. Consequently, these climbing cutworms usually cut corn above the soil line and growing point and the corn plant recovers. 

However, unlike the climbing cutworms, the larvae of some cutworm species (e.g. glassy, sandhill, darksided, claybacked, and black) tend to feed below ground at or below the growing point. This potential for its feeding to kill corn plants makes black cutworm a threat. When larger larvae tunnel into the growing point, corn as large as five or six collars can be killed.

With a bit of practice, the black cutworms are easily distinguished from many other pictures by the size of paired black bumps (tubercles) on the upper edges of each segment. These tubercles are unequal in size on the black cutworm (Figure 2).

crane fly larvaeFigure 3. Crane fly larvae. Not a cutworm or a pest. Note the lack of a head capsule, legs and the head area tapered to a point. Photo: Ed Lehman


Other Minnesota insects that cause damage to larger corn and might be confused with cutworm include the hop vine borer and common stalk borer.

Particularly when scouting mucky, high organic soils, be aware of crane fly larvae. These large fly larvae can be mistaken for cutworms but they do not have a head capsule or legs (Figure 3). 

Images of several cutworm species can be found in Black Cutworm Facts as is information on black cutworm thresholds and management for corn. 

Cutworm pests of crops on the Canadian prairies is a very good reference for cutworms in the northern plains.

During the spring, results of the 2019 trap network captures and predictions of BCW cutting will be posted weekly at website: https://z.umn.edu/bcw-reporting.  

This project is supported, in part, by the farm families of Minnesota and their corn check-off investment.

Until next week,

Bruce Potter and Travis Vollmer