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Using Degree-Day Models to Predict Western Bean Cutworm Flights

June 24, 2019

How Will Slow Crop Progress Affect Western Bean Cutworm in 2019?

The timing of western bean cutworm (WBC) moth flights in relation to corn growth stage has an important impact on the survival of young larvae, and subsequently the amount of damage they can do to developing corn ears. Moths prefer to lay their eggs on late whorl to early tassel stage corn plants so that the young larvae will have fresh tassel tissue to feed upon. Survival of newly hatched WBC is the highest if they are able to feed on freshly emerged tassel prior to moving down into the developing ears. Survival is lowest if corn plants are still in the vegetative stage and there is no reproductive tissue available when the larvae hatch. Therefore, with late planting, slow emergence, and cooler summer temperatures affecting crop progress (Figure 1), you may hope that one positive side effect would be less damage from WBC in 2019. Unfortunately, that may not be the case.

Chart showing season-long crop progress & condition  2015-2019
Figure 1. Crop progress and conditions for corn from the USDA National Agricultural Statistics Service indicating predicted timing for corn growth stages (2019).

The same cooler temperatures that have affected crop progress can also lead to slower development of WBC. Insect development is dependent on variable weather conditions such as air and soil temperature. Using a degree-day model developed by UNL Entomologists Tom Hunt and Robert Wright, along with University of Minnesota researchers, we can predict when WBC moths will emerge to begin mating and laying eggs in corn and dry bean fields. Additional information on how this model uses temperatures to calculate degree-day accumulations and WBC development can be found in this 2018 CropWatch article.

Predicted Dates for 2019 Western Bean Cutworm Flights in Nebraska and Surrounding States

Our predictions for 2019 locations across Nebraska, Colorado, and Wyoming (Table 1) indicate that moth flight will be delayed compared to previous years. For example, North Platte shows that 25% moth flight will be on approximately July 23, 2019, whereas this level was reached on July 18 in 2018 and July 14 in 2017 and 2016. However, WBC should not be ignored over the next month: degree-day models can help predict when scouting should occur but should also be used in combination with other monitoring methods. Setting up pheromone traps or monitoring UNL’s black light trap reports from North Platte, Clay Center, Concord, and Mead are additional resources. More information on these resources can be found in the bulleted list at the end of this article.

Western Bean Cutworm Scouting and Integrated Management

Western bean cutworm can be a devastating pest for corn and dry bean fields, particularly in the western half of Nebraska. Please refer to the Nebraska Extension NebGuide on this pest for more information and keep the following points in mind when considering scouting and treatment for western bean cutworm:

  • Scouting for WBC in corn can be much easier using Nebraska Extension’s Speed Scouting free mobile app or spreadsheet.
  • Genetically engineered Bt corn expressing Cry1F (Herculex) proteins may provide some suppression of WBC feeding, but should not be relied upon for control. (See Nebraska Perspective on Efficacy of Cry1F Bt Corn Against Western Bean Cutworm.) VIP3A proteins are still highly effective. (See the updated Handy Bt Trait Table for those trait packages that protect against WBC.)
  • For corn that does not have Bt traits that protect against WBC, the treatment threshold is 5%-8% of corn plants with eggs or larvae.
  • For scouting western bean cutworm in dry bean, pheromone traps are the easiest method. Note the NebGuide linked above for methods and thresholds for western bean cutworm in dry bean. In order to get accurate counts, it is important to set pheromone traps in late June or early July before moths take flight.

See previous CropWatch articles on Western Bean Cutworm at

Source: University of Nebraska CropWatch