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Understanding Growing Degree Days

Adult Japanese beetle on hardy hibiscus leaves
Adult Japanese beetle on hardy hibiscus leaves. Their appearance and feeding occurs between GDD 950-2150.

Understanding Growing Degree DaysA tool to predict insect emergence and development; increasing pesticide effectivenss and efficiency

Insects are cold-blooded creatures, relying on external temperatures to advance in growth and development. Knowing when insects will reach a specific development stage, and are most susceptible to control, is difficult for several reasons. First, weather varies from year to year. Unusually cool spring weather slows insect development, making correct pesticide timing later than it would be in a warm year.

Second, there is great variation in weather between locations. Typically eastern Nebraska is 2-4 weeks ahead of western Nebraska in both plant and insect development. But even within the same county, urban locations are typically warmer with faster plant and insect development, than rural locations.   


When pesticide applications are needed, all this variation makes is difficult to pinpoint the best timing. Use of growing degree days is a tool landscape managers use to identify the best time for their location, enabling them to make highly effective pesticide applications and minimize total pesticide needed for good control.

What Are Growing Degree Days?
Growing degree days (GGD) are a measure of accumulated heat. A heat value is assigned to each day based on temperature. Each day’s heat value is added together to give an estimate of insect growth stage. One degree day results when the average day’s temperature is one degree over the insect’s threshold temperature.

Begin with the insect pest needing control and determine its base or threshold temperature threshold - the temperature below which no growth or development occurs.  Many horticulture and forestry insect GDD calculations are made using the base temperature of 50° F, although some cool season insects may be active at 38° F or 43° F. Below is a short list of common landscape insects and their GDD. For a more complete list, visit Michigan State University GGD of Landscape Insects or GGD of Conifer Insects.  
 

Insect Pest Life StageGDD50
American plum borer   adult flight and egg laying  245-440
2nd generation  1375-1500 
Bagworm  caterpillar emergence  600-900 
Banded ash clearwing    adult emergence 1800-2200
Bronze birch borer    adults; eggs; new grubs 400-600
Codling moth   1st generation control stage 250 
2nd generation control stage   1250
Eastern tent caterpillar    egg hatch 45-100
tents apparent  150 
pupation  450
Emerald ash borer   1st adult emergence  400-500
peak adult activity  1000-1200
Euonymus scale  egg hatch – 2nd generation  1050-1900 
European pine sawfly  1st larvae  100-195 
Fall webworm    egg hatch 850-900
caterpillars feeding   1200-1800
tents become apparent  1850-2050 
Greater peach tree borer    adult emergence 575-710
Honeylocust spider mite    egg hatch 220-250
Japanese beetle  adults emerge and feed  950-2150
Lesser peach tree borer  adult flight  350-375 
Lilac borer    adult flight 325-350
Magnolia scale    egg hatch  1925-1950
Mimosa webworm   1st generation egg hatch 850-900 
Oystershell scale   egg hatch  350-500
Pine needle scale  1st generation control stage  400-500 
2nd generation control stage  1500 
Spruce spider mite    1st egg hatch 150-175
Zimmerman pine moth   1st larvae  25-100
adult flight   1200


Insects also have an upper temperature threshold value, 86° F, above which no significant increase in insect growth rates occurs. 

Calculating GGD
When temperatures fall between the base and upper temperature threshold, the easiest way to calculate GDD is to add the low temperature and high temperature for the day, then divide by 2. Subtract the threshold temperature from the insect.

For example, if the day’s minimum temperature was 55° F and the high was 75° F, the average temperature is (55+75)/2=65° F. If the insect’s threshold temperature is 50° F, then 15 degree days accumulated in that 24-hour period. 

Remember - 1 GGD results when the average day’s temperature is 1 degree over the insect’s threshold temperature. So if the day’s average temperature is 15 degrees above the threshold temperature, then 15 GDDs have accumulated in that day.  

When temperatures are below or above the insect’s threshold values - GGD is calculated as below using 50° F as the base threshold and 86° F as the upper threshold.   

GGD = (Temp max + Temp min) / 2- Temp min  

Temp max is the actual daily high temperature or is set to the upper threshold value, 86° F, when the day’s temperature goes above this value. Temp min is the daily low temperature or is set to the base threshold value, 50° F, when the day’s temperature goes below this value.   

Example 1 - Temperature drops below the insect’s lower temperature threshold, 50° F. Day’s low temperature 30° F and high 65° F.
Calculation - ((65 + 50)/2) - 50= 7.5 GGD 

Example 2 - Temperature goes above the insect’s high temperature threshold, 86° F.  Day’s low temperature 68° F and high 95° F.
Calculation - ((86+68)/2) - 50 = 27 GGD 

Each monthly HortUpdate will list the GDD for Lincoln, NE and a list of is also available for Omaha, NE .

For your location, use a minimum/maximum thermometer to keep track of each day’s temperatures and keep a running tally of GDDs to determine the exact time for control of your customer's pests. Or visit the High Plains Regional Climate Center to see if there is a weather station in your area keeping track of GDD.

  • Under Product Selection choose "Daily Degree Days".
  • Next, under Options Selections, click on the calendar icon next to End Date and choose "Year to Date". Degree Date Type should be "Growing Degree Days" and Base "50". 
  • Under Station/Area Selection enter a city name or zip code for your area. 
  • Choose a Station listed to see if they are recording GDD. 
Sarah Browning
Sarah Browning
Extension Educator, Horticulture & Urban Agriculture
Sarah focusses on environmental horticulture, fruit & vegetable production and food safety. Working with the general public and commercial green industry professionals, her major program goals include conserving water, protecting water quality, promoting local food production and protecting human health.

Contact Sarah at:
Lancaster County Extension
444 Cherrycreek Rd Ste A
Lincoln NE 68528-1591
402-441-7180