Using Degree-Days to Predict Western Bean Cutworm Flights

Using Degree-Days to Predict Western Bean Cutworm Flights June 24, 2016

Introduction to Insect Degree-Day Models

One of the challenges to controlling insect pests in field crops is knowing when the damaging stage of the insect will be present in the field. Scouting for and effectively treating insects, if numbers are above the economic threshold, can be a challenge because the optimum time will vary from year to year. This is because insect development is dependent 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 therefore 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 in 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

For example given a minimum temperature of 50ºF, a maximum temperature of 85ºF, and a 38ºF lower threshold, degree-days for a given day might be calculated as

{(50 + 85)/2] – 38

[135/2] – 38 = 29.5 degree-days

Note:

  • 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.

Using Degree-Days to Predict Western Bean Cutworm Scouting

western bean cutworm
Figure 1. Western bean cutworm egg mass (L) and larva (R).
western bean cutworm
Figure 2. Western bean cutworm

UNL Entomologists Tom Hunt and Robert Wright, along with University of Minnesota researchers Roger Moon and Bill Hutchison and PhD student Anthony Hanson last year published a new model for the flight of western bean cutworm (WBC; Figure 1) that has improved predictions of cumulative moth flights and efficiency of field scouting. This model shows that the best method for predicting timing of WBC flight used simple degree-day calculations (see example above) beginning 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 (Fahrenheit) have accumulated. Field scouting to estimate egg density is recommended at this time.

Predicted Dates for 2016 Western Bean Cutworm Flights in Nebraska

Table 1. Using the new degree-day model and 2016 temperatures reported for the following locations, the predicted flight patterns for WBC in Nebraska are shown below. Keep in mind that scouting is recommended at 25% of the moth flight.
Location5%10%25%50%75%90%95%
Benkelman June 28 June 30 July 4 July 7 July 11 July 14 July 17
Big Springs July 5 July 7 July 11 July 15 July 18 July 22 July 25
Clay Center June 27 June 29 July 2 July 6 July 9 July 13 July 15
Concord July 4 July 7 July 10 July 13 July 17 July 21 July 24
Grand Island June 28 June 30 July 3 July 7 July 10 July 14 July 17
Hastings June 28 June 30 July 3 July 7 July 10 July 14 July 17
Holdrege July 4 July 6 July 9 July 13 July 16 July 20 July 23
Imperial July 7 July 10 July 13 July 16 July 20 July 24 July 26
Kearney July 3 July 5 July 8 July 12 July 16 July 19 July 22
Madrid July 5 July 8 July 11 July 15 July 18 July 22 July 25
McCook July 2 July 4 July 7 July 11 July 14 July 18 July 21
North Platte July 9 July 11 July 14 July 18 July 21 July 25 July 28
Ogallala July 6 July 9 July 12 July 16 July 19 July 23 July 26
Scottsbluff July 11 July 14 July 17 July 21 July 24 July 28 July 31

WBC Scouting & IPM

Western bean cutworm can be a devastating pest for corn and dry bean fields, particularly in the western half of Nebraska. Please refer to the Nebraska Extension NebGuide on this pest for more information and keep the following points in mind when considering scouting and treatment for western bean cutworm: 

  • Scouting for WBC in corn can be much easier using Nebraska Extension’s Speed Scouting free mobile app or spreadsheet.
  • Genetically engineered Bt corn expressing Cry1F (Herculex) proteins will suppress WBC feeding, but does 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 should occur and should also be used in combination with other monitoring methods. UNL black light trap reports from North Platte, Clay Center, and Concord are additional resources for monitoring western bean cutworm and other crop pests.