July 2, 2015

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

• If the average temperature [(Minimum Temp + Maximum Temp)/2] is lower than the Lower Threshold, change the average temperature to the Lower Threshold before subtracting the Lower Threshold.
• If the average temperature [(Minimum Temp + Maximum Temp)/2] is higher than the Upper Threshold, change the average temperature to the Upper Threshold before subtracting the 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.

A New Degree-Day Model for Western Bean Cutworm

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.

Predicted Dates for 2015 Western Bean Cutworm Flights in Nebraska

Table 1. Using the new degree-day model and 2015 temperatures reported for the following locations, the predicted flight patterns for western bean cutworm in Nebraska are shown below. Scouting is recommended at 25% of the moth flight.
	Percent of Total Western Bean Cutworm Flight
Location	5%	10%	25%	50%	75%	90%	95%
Benkelman	July 5	July 8	July 12	July 16	July 21	July 25	July 29
Big Springs	July 10	July 13	July 17	July 21	July 26	July 31	Aug. 3
Clay Center	June 30	July 3	July 7	July  11	July 16	July 20	July 24
Concord	July 1	July 4	July 8	July 13	July 17	July 22	July 25
Grant	July 5	July 8	July 12	July 17	July 21	July 26	July 29
Hastings	June 29	July 3	July 7	July 11	July 15	July 20	July 24
Holdrege	July 3	July 6	July 10	July 14	July 18	July 23	July 26
Imperial	July 3	July 7	July 11	July 16	July 20	July 25	July 28
McCook	June 22	June 25	June 29	July 4	July 8	July 13	July 16
North Platte	July 9	July 12	July 15	July 20	July 24	July 29	July 31
Ogallala	July 6	July 9	July 13	July 17	July 22	July 27	July 30
Scottsbluff	July 8	July 11	July 15	July 19	July 24	July 28	July 31

 

Scouting and Integrated Pest Management of Western Bean Cutworm

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:

• Nebraska Extension's Speed Scouting free mobile app or spreadsheet can make scouting for WBC easier.
• Genetically engineered Bt corn expressing Cry1F (Herculex) proteins will suppress WBC feeding, but will not eliminate all caterpillars. (See 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.

Degree-day models can help predict when scouting needs to occur, but should also be used in combination with other monitoring methods. Data from black light traps used to monitor western bean cutworm and other crop pests in North Platte, Clay Center, and Concord can be found at http://entomology.unl.edu/fldcrops/lightrap.shtml.

Julie Peterson
Extension Entomologist, West Central Research and Extension Center
Westen Archibald
Masters Student, Entomology
Jeff Bradshaw
Extension Entomologist, Panhandle Research and Extension Center
Débora Montezano
PhD Student, Entomology
Robert Wright
Extension Entomologist, Lincoln