2019 UMN Cooperative Black Cutworm Trapping Network Report #5

black cutworm moth captures throughout MN countiesFigure 1. Maximum two consecutive night black cutworm captures reported by county (May 4 -May 10).


Report #5 Week of May 4-10, 2019

For more information: Black Cutworm Reporting Network

May 16, 2019

Greetings:

Once again, black cutworm (BCW) moths rode the wind into southern MN from May 4-10 (Figure 1). Again, this week, some traps captured moths over four or more nights.

Significant captures occurred in Swift, Jackson, Watonwan, Faribault, Mower, Fillmore, and Olmstead. A trap in Dodge County was likely significant as well but the trap had not been checked for several days. The trap in Faribault County has been very active the past two weeks of trapping as well as one of the Fillmore County traps.

US weather forecast mapFigure 2. The forecast strong southerly flow of moisture indicates the potential for long distance insect and fungal spore transport.


Once again, black cutworm (BCW) moths rode the wind into southern MN from May 4-10 (Figure 1). Again, this week, some traps captured moths over four or more nights. Significant captures occurred in Swift, Jackson, Watonwan, Faribault, Mower, Fillmore, and Olmstead. A trap in Dodge County was likely significant as well but the trap had not been checked for several days. The trap in Faribault County has been very active the past two weeks of trapping as well as one of the Fillmore County traps.

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 14. 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.

Scouting for Black Cutworm (excerpt from Black Cutworm Facts)

early instar black cutwormFigure 3. Early (2nd) instar black cutworm and its leaf feeding on 1-leaf corn


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.

Be wary when lambsquarters and ragweed patches begin to disappear without the aid of an herbicide. Post emerge herbicide applications may cause cutworms to switch from feeding on weeds to corn.

Leaf feeding and missing or cut plants are not hard to see, but it is useful to find a few of the larvae that caused the damage and determine size and species. This can be frustrating so why bother?

Knowing the size of cutworm larvae will help determine the potential for future damage (Figures 3 & 4, Table 2). 

4th instar black cutwormFigure 4. A 4th instar black cutworm and corn plant cut at soil line.


Knowing which species is present is important to understand the extent of the threat. Black cutworms are more damaging to corn than some other species. For example, dingy cutworms are a very common cutworm species in Minnesota that feed at or above the soil surface. As a result, it does not cut corn below the growing point.

Cutworms are nocturnal. During the day, they hide under soil clods, crop residue, and loose soil, typically at the boundary between dry and moist soil. Cutworms will likely be deeper when soils are dry. Carefully look under pieces of residue and soil clods close to cut or injured plants. If you don’t find a cutworm near the base of an injured plant, look near a couple plants on either side in the row. Using a possum-like defense strategy, most cutworm species roll into a semicircle and remain motionless when disturbed. Unfortunately, increasing the difficulty of detection by crop scouts and other predators, most cutworm species, including black cutworms, are more or less soil colored.

Finding cutworms in high residue, cloddy or muddy conditions is especially difficult. With leaf feeding you are looking for very small larvae. Move to another area with injured plants if unsuccessful. Looking at this optimistically, you only need to find a few to make your treatment decision. Do not confuse headless, legless cranefly larvae with cutworms. All cutworm species have a distinct head capsule and three pairs of true legs near the front with fleshy abdominal prolegs at the back.

Take stand counts in areas of the field with damage and note the percentage of plants with leaf feeding and those cut. To help with your decision, you can flag areas of row within the field and return the next day to determine if damage is ongoing.

Cutworm infestations in small corn (3 leaf or less) require more aggressive management than large corn. In a worst-case scenario, large cutworms can cut corn and other plants before they have a chance to emerge. Do not give up on scouting too early. Late-instar black cutworms can kill up to 6-collar corn by burrowing into the growing point.

While not recommended as insurance treatments, corn rootworm and seed treatment insecticides labeled for cutworm can provide some control. Make sure cutworms are still present!

How to prioritize your black cutworm scouting efforts

  • Look at the timing of cutworm flights in Table1. Focus your initial efforts on cornfields that were not yet tilled and planted when the moths arrived. The end of cutting column provides a guideline for when larvae will begin to stop feeding.
  • Fields without fall tillage and ridge/strip till fields can be higher risk. The later a field was worked and planted with respect to moth arrival, the greater the potential for large cutworms to cause stand loss.  
  • In corn, focus scouting efforts on fields where a hybrid without a Herculex I or Viptera aboveground Bt trait were planted. The Handy Bt Trait Table shows which Bt trait packages have some efficacy on black cutworm.
  • Small corn is at greater risk for yield loss. Large corn (> 5 collar) is usually safe from black cutworm damage.
  • Very small cutworms of any species can cut sugarbeets. Pay close attention to emerging seedlings and oat cover crops for signs of damage.
  • Fields with a history of winter annual and early spring emerging weed (e.g. common lambsquarters) are higher risk. Cutworms are forced to concentrate on crop seedlings after  weeds or cover crops are killed.
  • Egg laying is often concentrated in lower and moister areas of the field.
  • If you start finding economic populations of black cutworm in these higher risk situations. It would be worth the effort to look at other fields in the area if you have not already done so. Bt hybrids and insecticides can be overwhelmed when faced with very high populations of large cutworms.

See the BCW reporting website for more information on thresholds and treatment.

During the spring, results of the 2019 trap network captures and predictions of BCW cutting will be posted weekly at: https://z.umn.edu/bcw-reporting. There will be additional alerts for any BCW high-risk areas picked up by the network!

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