Pest alert: Black cutworm

This has been an active spring for black cutworm (BCW) flights into MN. Pheromone trap captures indicating moth flights large enough to pose risk to corn, sugarbeets, and other crops have occurred through the past few weeks (Figure 1) and are continuing. The southern tiers of counties have seen the most activity but traps located in more northern areas have seen issues as well. The NWS precipitation forecast (Figure 2) shows conditions likely favorable for insect migration from the south. Unfortunately, these same conditions will be unfavorable for fieldwork. I would expect to see another influx of BCW moths with these systems. Other migrant crop pests including true armyworm, potato leafhopper and cereal aphids may show as well. 

Unfortunately, these immigration events coincide with a late planting season in much of Minnesota. This increases the risk of economic injury to crop seedlings.  maps of black cutworm moth captures throughout MN 

How to prioritize black cutworm scouting efforts

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 (Base 50ºF) have accumulated after moth arrival. This means that scouting of emerged corn should be underway before June 1. Sugarbeets and other crops where the growing point is above ground can experience stand loss before then.

  • Look at the timing of cutworm flights in Table1. Focus your initial efforts on cornfields that were not yet been 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 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.
black cutworm development tableTable 1.
US weather forecast mapFigure 2.

Results of the 2019 UMN Cooperative Black Cutworm Reporting Network captures and predictions of BCW cutting are posted weekly at: https://z.umn.edu/bcw-reporting. Information on cutworm ID, scouting thresholds and treatment options can be found in black cutworm facts there and https://extension.umn.edu/corn-pest-management/black-cutworm.

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