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:

[(65+55)/2]-38

[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.

Municipality	Coordinates	Elev.	5%	10%	25%	50%	75%	90%	95%
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

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.

Figure 2. Western bean cutworm egg mass. (Photo by J Cluever)

Figure 3. Western bean cutworm larva. (Photo by S. Paula-Moraes, UNL)

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.

Location	Coordinates	Predicted 25% Flight for 2021	Actual 25% Flight for 2021	Difference (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

Other Available Resources

• Western bean cutworm speed scouting app for Apple or Android. 
• Western bean cutworm spreadsheet.
• Other extension apps on topics ranging from insect control to irrigation management to finance.
• Light trap reports for Clay Center, Mead and North Platte.