Degree-days for Prediction of Western Bean Cutworm Flight

June 29, 2023

The very first western bean cutworm (WBC) moths have been caught in the black light trap at Clay Center, Nebraska; WBC have not yet been reported from Mead or North Platte (https://entomology.unl.edu/fldcrops/lightrap). It is time to start thinking about scheduling for scouting for this important pest of corn and dry beans. WBC moths prefer to lay their eggs on corn plants when they are approaching tassel emergence, as the tassel is the preferred food source for newly hatched WBC larvae. A match between the flight of the moths and the late whorl to early tassel stage of corn can result in high levels of infestation.

Predicted Dates of WBC Flight for Nebraska in 2023

Overall, the estimated dates for WBC flight in 2023 are similar to those from 2021-2022, although some areas have predicted earlier flights compared to last year (such as Ainsworth and Concord, Nebraska) while other areas have predicted later flights (such as North Platte and Hayes Center, Nebraska) (Table 1).

**Table 1.** Predicted dates of WBC flight for select locations in Nebraska.
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	8-Aug	11-Aug
Ainsworth, NE	42.57 -99.83	2500	10-Jul	12-Jul	15-Jul	18-Jul	22-Jul	26-Jul	28-Jul
Arthur, NE	41.45 -101.72	3589	12-Jul	14-Jul	18-Jul	21-Jul	25-Jul	28-Jul	31-Jul
Clay Center, NE	40.52 -98.05	1788	5-Jul	7-Jul	10-Jul	14-Jul	17-Jul	21-Jul	24-Jul
Columbus, NE	41.46 -97.33	1450	2-Jul	4-Jul	7-Jul	11-Jul	14-Jul	18-Jul	21-Jul
Concord, NE	42.38 -96.97	1545	5-Jul	7-Jul	11-Jul	14-Jul	18-Jul	22-Jul	25-Jul
Gothenburg, NE	40.95 -100.20	2628	6-Jul	8-Jul	11-Jul	15-Jul	18-Jul	22-Jul	25-Jul
Grand Island, NE	40.96, -98.31	1843	1-Jul	3-Jul	6-Jul	9-Jul	13-Jul	17-Jul	20-Jul
Hayes Center, NE	40.55 -101.03	3064	11-Jul	14-Jul	17-Jul	20-Jul	24-Jul	28-Jul	30-Jul
Holdrege, NE	40.5 -99.37	2323	7-Jul	10-Jul	13-Jul	16-Jul	20-Jul	24-Jul	26-Jul
Imperial, NE	40.50, -101.65	3294	10-Jul	12-Jul	16-Jul	19-Jul	23-Jul	26-Jul	29-Jul
Kearney, NE	40.72 -99.02	2129	6-Jul	8-Jul	11-Jul	15-Jul	18-Jul	22-Jul	25-Jul
Lincoln, NE	40.84, -96.76	1170	27-Jun	29-Jun	2-Jul	6-Jul	9-Jul	13-Jul	16-Jul
North Platte, NE	41.08 -100.78	2841	13-Jul	16-Jul	19-Jul	22-Jul	26-Jul	30-Jul	2-Aug
Ogallala, NE	41.12 -101.72	3230	10-Jul	12-Jul	15-Jul	19-Jul	22-Jul	26-Jul	29-Jul
Scottsbluff, NE	41.88 -103.68	3934	15-Jul	17-Jul	20-Jul	24-Jul	27-Jul	31-Jul	3-Aug
Sidney, NE	41.22 -103.03	4331	20-Jul	22-Jul	26-Jul	29-Jul	2-Aug	6-Aug	8-Aug
York, NE	40.87 -97.63	1637	2-Jul	4-Jul	8-Jul	11-Jul	15-Jul	19-Jul	21-Jul

To produce the table above, recorded temperatures were taken from the High Plains Regional Climate Center (HPRCC) from March 1 to June 26. A 10-year (2013-2022) average was used in order to predict the average temperature for dates after June 26.

Another option for seeing moth flight predictions in your area is to use UNL’s AgriTools mobile app for Apple or Android, which will provide predictions similar to the table below by pulling temperature data from weather stations around your location.

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 2. 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

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 five to 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 1). 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 2). 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 3). WBC moths are about three-fourths of an inch long by 1.5 inches wide.

WBC adult larvae — **Figure 2.** Western bean cutworm larva. (Photo by S. Paula-Moraes, UNL)

WBC adult moth — **Figure 3.** 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.