Using Degree-Days To Predict Western Bean Cutworm Flight Times

 

One of the challenges with controlling insect pests in field crops is knowing when the damaging stage of the insect will be present in the field. Scouting for insects and making an effective treatment, if the pest is above the economic threshold, can be a challenge because the time when the insect is present and damaging a crop can vary from year to year. This variation occurs because insect development depends on variable weather conditions such as air and soil temperature. Important stages in insect life cycles, such as egg hatch, pupation, adult flight, and reproduction can be predicted based on environmental temperatures.

 

 

 

 

By using a degree-day model, you can account for the effect of temperature on insect development and approximate when certain insect development events will occur by measuring degree-days. Degree-days are units that measure how much heat an insect has been exposed to within an upper and lower temperature threshold. These temperature thresholds establish the range of temperatures that allow for insect growth and development. By studying insect development in the laboratory and the field, entomologists can determine these thresholds, as well as the number of degree-days that must be accumulated in order for a specific insect species to reach a certain stage in their development. These numbers will vary depending on the insect species.

 

One common way to measure degree-days is by taking the average temperature of each day and subtracting the lower threshold from that average:

 

[(Actual Minimum Temp + Actual Maximum Temp)/2] – Lower Threshold

 

 

A degree-day model will determine on which calendar date to start counting degree-days. After this date, degree-days from each day are accumulated to give the total cumulative degree-days.

 

 

Aiming to improve predictions of cumulative western bean cutworm (WBC) moth flights and efficiency of field scouting, UNL entomologists Thomas Hunt and Bob Wright, along with researchers from University of Minnesota, Roger Moon and Bill Hutchison, and Anthony Hanson, PhD student, developed a new model for the flight of western bean cutworm. It was published in 2015 in the Journal of Economic Entomology. Previously, extension entomologists, crop consultants, and growers throughout the Midwest had been using a model developed in 1979 by entomology master's student Tarik Ahmad and former University of Nebraska–Lincoln researchers.

 

Our new model improves upon this one greatly, and was developed and validated using data from black light traps in North Platte, Concord, Clay Center, and Aurora. Researchers found that the best model for predicting timing of western bean cutworm flight used simple degree-day calculations beginning on March 1, with a 38°F lower threshold and a 75°F upper threshold. The new cumulative flight model indicated that 25% of moth flight should be completed when 2,577 degree-days F have accumulated. Field scouting to estimate egg density is recommended at this time.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Western bean cutworm can be a devastating pest for corn and dry bean growers, particularly in western Nebraska. Please refer to the Nebraska Extension NebGuide, Western Bean Cutworm in Corn and Dry Beans, for more information.  Also consider these points related to scouting and treatment of western bean cutworm: