Degree-days for Prediction of Western Bean Cutworm Flight

Degree-days for Prediction of Western Bean Cutworm Flight

REVISED: June 21, 2022 (originally published July 1, 2021)

Introduction to Insect Degree-day Models

The importance of knowing when an insect pest will cause damage in a field is essential to producers and crop consultants when making management decisions. Since insect development is greatly affected by temperature, the timing of insect emergence can be estimated accurately by using a combination of calendar dates and assessment of temperature variations from year to year.

The use of a degree-day model can help provide producers with accurate data. Degree-days measure the amount of heat between upper and lower thresholds to which an insect has been exposed. A model for the western bean cutworm (WBC) has been developed by entomologists from the University of Minnesota and the University of Nebraska.

Each degree-day model is different, but most models will subtract the lower threshold from the average temperature for each day. For example, in the model developed for WBC, if a certain day had a low of 55°F and a high of 65°F, and the model has a lower threshold of 38°F and an upper threshold of 75°F, then the degree-days would be as follows:


[120/2]-38 = 22°F degree-days

Note: If the average temperature is less than the lower threshold (38°F), then you should use the lower threshold as the average temperature. If the average temperature is greater than the upper threshold (75°F), then you should use the upper threshold prior to subtracting the lower threshold.

Western Bean Cutworm Degree-day Model

Entomologists from the University of Nebraska-Lincoln (Tom Hunt and Robert Wright), and the University of Minnesota (Roger Moon, Bill Hutchison and Anthony Hanson) developed the most recent degree-day model. The lower threshold is 38°F while the upper threshold is 75°F beginning on March 1. Once 2,577 degree-days Fahrenheit is reached, 25% of the WBC flight occurs and scouting of corn fields should begin. (Pheromone trapping in dry bean fields should begin the last week of June before any WBC have emerged.)

If an insecticide application is warranted, it should be made 10-21 days after peak flight in dry bean and when 95% of plants have tasseled in corn. For more information, see NebGuide G2013 by Seymour et al. 2010.

Table 1. Relationship between WBC flight and accumulated degree-days.
°Days F (±95% Confidence Interval)% WBC Flight
2,377 (1,843 - 3,066) 5
2,456 (2,147 - 2,809) 10
2,577 (2,416 - 2,749) 25
2,704 (2,577 - 2,838) 50
2,838 (2,660 - 3,027) 75
2,977 (2,603 - 3,046) 90
3,077 (2,385 - 3,968) 95

Predicted Dates of WBC Flight for Nebraska and Surrounding States in 2022

The estimated dates for WBC flight are similar to the year 2021. WBC moths prefer to lay their eggs on the corn plant when it is approaching tassel emergence, as the tassel will be used as the primary food source for newly hatched WBC larvae.

Another source for Degree-days in your area is UNL’s AgriTools mobile app for Apple or Android.

Recorded temperatures were taken from the High Plains Regional Climate Center (HPRCC) or the National Oceanic and Atmospheric Administration (NOAA) stations from March 1 to June 12. A 10-year (2012-2021) average was used in order to predict the dates following June 12. Additionally, if a weather station did not have data available for a specific year, a weather station that was no more than 10 miles away was used, and if a station was not present then an average of data from three surrounding stations was used. If data were missing for a few days, then two nearby stations were averaged.

It should be noted that there are two possibilities of error:

  1. Error in the predicted temperatures for the dates after June 12.
  2. Error within the model.

For example, the predicted 25% peak flight in Kearney, Nebraska was said to take place on July 8 in 2021, but the actual peak flight occurred on July 9. Degree-day accumulation can be found on the UNL’s Mesonet website (select “Western Bean Cutworm GDD” in the drop-down menu).

Table 2. Predicted dates of WBC flight for locations in Nebraska, Colorado, Wyoming and Kansas.
Alliance, NE 42.17 -102.93 4072 22-Jul 24-Jul 27-Jul 31-Jul 4-Aug 9-Aug 12-Aug
Ainsworth, NE 42.57 -99.83 2500 15-Jul 18-Jul 21-Jul 24-Jul 28-Jul 1-Aug 4-Aug
Alda, NE 40.88 -98.52 1923 4-Jul 6-Jul 9-Jul 13-Jul 17-Jul 21-Jul 23-Jul
Arthur, NE 41.45 -101.72 3589 15-Jul 17-Jul 20-Jul 24-Jul 27-Jul 31-Jul 3-Aug
Axtell, NE 40.52 -99.05 2198 5-Jul 7-Jul 10-Jul 14-Jul 17-Jul 21-Jul 24-Jul
Big Springs, NE 41.15 -102.00 3701 13-Jul 16-Jul 19-Jul 22-Jul 26-Jul 30-Jul 2-Aug
Broadwater, NE 41.68 -102.87 4117 17-Jul 19-Jul 23-Jul 26-Jul 30-Jul 3-Aug 6-Aug
Brule, NE 41.02 -101.97 3474 12-Jul 14-Jul 18-Jul 21-Jul 25-Jul 29-Jul 1-Aug
Central City, NE 41.10 -98.05 1713 6-Jul 8-Jul 12-Jul 15-Jul 19-Jul 22-Jul 25-Jul
Clay Center, KS 39.42 -97.15 1273 26-Jun 29-Jun 2-Jul 5-Jul 9-Jul 13-Jul 15-Jul
Clay Center, NE 40.52 -98.05 1788 5-Jul 7-Jul 10-Jul 13-Jul 17-Jul 21-Jul 24-Jul
Columbus, NE 41.46 -97.33 1450 4-Jul 7-Jul 10-Jul 13-Jul 17-Jul 21-Jul 23-Jul
Concord, NE 42.38 -96.97 1545 12-Jul 14-Jul 18-Jul 21-Jul 25-Jul 29-Jul 1-Aug
Cozad, NE 40.97 -99.95 2562 9-Jul 11-Jun 15-Jul 19-Jul 22-Jul 26-Jul 29-Jul
Douglas, WY 42.8 -105.45 4892 2-Aug 5-Aug 9-Aug 13-Aug 17-Aug 22-Aug 25-Aug
Dunning, NE 41.9 -100.17 2782 14-Jul 16-Jul 19-Jul 23-Jul 26-Jul 31-Jul 3-Aug
Elgin, NE 41.93 -98.2 2064 10-Jul 13-Jul 16-Jul 19-Jul 23-Jul 27-Jul 30-Jul
Emmet, NE 42.47 -98.77 2011 13-Jul 15-Jul 19-Jul 22-Jul 26-Jul 30-Jul 2-Aug
Firth, NE 40.57 -96.62 1381 2-Jul 4-Jul 7-Jul 11-Jul 14-Jul 18-Jul 21-Jul
Fordyce, NE 42.75 -97.35 1460 11-Jul 13-Jul 16-Jul 20-Jul 23-Jul 27-Jul 30-Jul
Ft. Collins, CO 40.58 -105.15 1543 17-Jul 19-Jul 22-Jul 26-Jul 30-Jul 3-Aug 6-Aug
Garden City, KS 37.98 -100.82 2841 26-Jun 28-Jun 1-Jul 5-Jul 8-Jul 12-Jul 15-Jul
Gordon, NE 42.75 -102.18 3652 23-Jul 25-Jul 28-Jul 1-Aug 5-Aug 9-Aug 12-Aug
Gothenburg, NE 40.95 -100.20 2628 9-Jul 12-Jul 15-Jul 18-Jul 22-Jul 26-Jul 29-Jul
Grant, NE 40.85 -101.72 3402 12-Jul 14-Jul 17-Jul 21-Jul 24-Jul 28-Jul 31-Jul
Guide Rock, NE 40.08 -92.28 1742 30-Jun 2-Jul 5-Jul 9-Jul 12-Jul 16-Jul 19-Jul
Harvard, NE 40.57 -98.15 1824 6-Jul 8-Jul 11-Jul 15-Jul 19-Jul 22-Jul 25-Jul
Hayes Center, NE 40.55 -101.03 3064 8-Jul 10-Jul 14-Jul 17-Jul 21-Jul 25-Jul 27-Jul
Holdrege, NE 40.5 -99.37 2323 6-Jul 8-Jul 12-Jul 15-Jul 19-Jul 23-Jul 25-Jul
Holyoke, CO 40.48 -102.10 3694 14-Jul 16-Jul 19-Jul 23-Jul 27-Jul 31-Jul 3-Aug
Idalia, CO 39.72 -102.32 3930 11-Jul 13-Jul 16-Jul 19-Jul 23-Jul 27-Jul 30-Jul
Kearney, NE 40.72 -99.02 2129 5-Jul 7-Jul 10-Jul 14-Jul 17-Jul 21-Jul 24-Jul
Keystone, NE 41.18 -101.65 1033 10-Jul 12-Jul 16-Jul 19-Jul 23-Jul 27-Jul 29-Jul
Lamar, CO 37.97 -102.60 3734 27-Jun 29-Jun 3-Jul 6-Jul 10-Jul 13-Jul 16-Jul
Lexington, NE 40.72 -99.75 2385 7-Jul 9-Jul 13-Jul 16-Jul 20-Jul 23-Jul 26-Jul
Lingle, WY 42.13 -104.38 4186 21-Jul 23-Jul 26-Jul 30-Jul 3-Aug 7-Aug 10-Aug
Lincoln, NE 40.72 -96.62 1394 30-Jun 2-Jul 6-Jul 9-Jul 13-Jul 17-Jul 19-Jul
Manhattan, KS 39.2 -96.6 1050 24-Jun 26-Jun 29-Jun 3-Jul 6-Jul 10-Jul 13-Jul
Memphis, NE 41.15 -96.42 1145 6-Jul 8-Jul 11-Jul 14-Jul 18-Jul 22-Jul 25-Jul
Merna, NE 41.45 -99.78 2641 13-Jul 15-Jul 19-Jul 22-Jul 26-Jul 30-Jul 2-Aug
Mitchell, NE 41.95 -103.70 4114 17-Jul 19-Jul 22-Jul 26-Jul 30-Jul 3-Aug 6-Aug
Mullen, NE 42.45 -100.90 3091 16-Jul 18-Jul 21-Jul 25-Jul 28-Jul 2-Aug 5-Aug
Naper, NE 42.83 -99.27 2001 15-Jul 17-Jul 20-Jul 24-Jul 28-Jul 1-Aug 4-Aug
Nebraska City, NE 40.70 -95.90 1079 2-Jul 4-Jul 8-Jul 11-Jul 15-Jul 18-Jul 21-Jul
North Platte, NE 41.08 -100.78 2841 10-Jul 12-Jul 16-Jul 19-Jul 23-Jul 26-Jul 29-Jul
Oakland, NE 41.83 -96.55 1348 9-Jul 11-Jul 14-Jul 18-Jul 21-Jul 25-Jul 28-Jul
Ord, NE 41.62 -98.95 2060 12-Jul 14-Jul 17-Jul 21-Jul 24-Jul 28-Jul 31-Jul
Oshkosh, NE 41.48 -102.35 3763 13-Jul 16-Jul 19-Jul 22-Jul 26-Jul 30-Jul 2-Aug
Powell, WY 44.7764 -108.7590 4372 31-Jul 2-Aug 6-Aug 10-Aug 15-Aug 20-Aug 23-Aug
Riverton, WY 43.0307 -108.3743 4955 25-Jul 28-Jul 31-Jul 6-Aug 8-Aug 12-Aug 15-Aug
Ragan, NE 40.32 -99.38 2300 6-Jul 8-Jul 12-Jul 15-Jul 19-Jul 23-Jul 25-Jul
Scandia, KS 39.82 -97.85 1480 3-Jul 5-Jul 8-Jul 12-Jul 15-Jul 19-Jul 22-Jul
Scottsbluff, NE 41.88 -103.68 3934 16-Jul 19-Jul 22-Jul 25-Jul 29-Jul 2-Aug 5-Aug
Sidney, NE 41.22 -103.03 4331 19-Jul 21-Jul 24-Jul 28-Jul 1-Aug 5-Aug 8-Aug
Smithfield, NE 40.57 -99.70 2530 8-Jul 10-Jul 13-Jul 17-Jul 20-Jul 24-Jul 27-Jul
Sparks, NE 43.00 -100.20 2520 18-Jul 20-Jul 24-Jul 27-Jul 31-Jul 4-Aug 7-Aug
St Francis, KS 39.62 -101.82 3661 10-Jul 12-Jul 15-Jul 19-Jul 22-Jul 26-Jul 29-Jul
York, NE 40.87 -97.63 1637 5-Jul 7-Jul 10-Jul 14-Jul 17-Jul 21-Jul 24-Jul
Washington, KS 39.77 -97.07 1368 28-Jun 30-Jun 4-Jul 7-Jul 11-Jul 14-Jul 17-Jul
Nebraska corn progress 2022
Figure 1. Nebraska corn progress for 2022.

Scouting and Management of Western Bean Cutworm

WBC eggs can be found on the upper surfaces of corn leaves, and the lower surfaces of dry bean leaves. Eggs are found in masses ranging from 5-200, with an average of 50-85. When first laid, the eggs will be white and gradually darken during the five- to seven-day development (Figure 2). Freshly hatched larvae will first eat the eggshells before consuming the host plant. Older larvae can be identified by two dark-colored rectangular spots directly behind their head (Figure 3). Adult moths are identifiable by a white band on the anterior margin on the forewing followed by a white circle halfway down the wing and a white crescent located two-thirds down the wing (Figure 4). WBC moths are about three-fourths of an inch long by 1.5 inches wide.

WBC eggs
Figure 2. Western bean cutworm egg mass. (Photo by J Cluever)
WBC adult larvae
Figure 3. Western bean cutworm larva. (Photo by S. Paula-Moraes, UNL)
WBC adult moth
Figure 4. Western bean cutworm adult moth. (Photo by J Peterson, UNL)

It is more difficult to scout dry bean fields for WBC, but pheromone traps can be used to provide an estimate of a possible infestation and should be installed prior to the predicted 5% flight date. If fewer than 700 moths are caught up to peak, there is low risk; if more than 700 but fewer than 1,000 moths are caught then there is moderate risk; and if there are more than 1,000 moths caught then the risk if high.

Table 3. Degree to which 2021’s predictions were off compared to actual flights.
LocationCoordinatesPredicted 25% Flight for 2021Actual 25% Flight for 2021Difference (Days)
Alliance, NE 42.17 -102.93 25-Jul 25-Jul 0
Arthur, NE 41.45 -101.72 20-Jul 22-Jul 2
Big Springs, NE 41.15 -102.00 17-Jul 18-Jul 1
Concord, NE 42.38 -96.97 12-Jul 14-Jul 2
Douglas, WY 42.8 -105.45 1-Aug 1-Aug 0
Emmet, NE 42.47 -98.77 13-Jul 15-Jul 2
Ft. Collins, CO 40.58 -105.15 25-Jul 25-Jul 0
Grant, NE 40.85 -101.72 17-Jul 18-Jul 1
Holyoke, CO 40.48 -102.10 18-Jul 19-Jul 1
Idalia, CO 39.72 -102.32 14-Jul 16-Jul 2
Kearney, NE 40.72 -99.02 8-Jul 9-Jul 1
Lingle, WY 42.13 -104.38 26-Jul 25-Jul -1
Lincoln, NE 40.72 -96.62 3-Jul 1-Jul -2
Manhattan, KS 39.2 -96.6 29-Jun 29-Jun 0
North Platte, NE 41.08 -100.78 16-Jul 17-Jul 1
Scottsbluff, NE 41.88 -103.68 21-Jul 22-Jul 1

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