University of Maryland Extension

Growing Degree Days: Why and how to use them, and where to find them

Paula Shrewsbury, Department of Entomology, University of Maryland


Insects (and mites) are cold blooded organisms and therefore their development rate is directly related to temperature – as temperatures get warmer insects develop faster.  In addition, most insects have a lower and upper temperature development threshold, below or above which, respectively, they do not develop. Growing degree days (GDD or DD) are a measure of the “heat units” (related to temperature and the amount of time per day that an insect spends actively growing) that accumulate over time. Therefore, by keeping track of the number of DD that accumulate over time we can predict activity of different insects and their susceptible life stage(s) (target life stage for control measures). For example, when crawler activity of a specific scale insect is likely to occur could be predicted.

Japanese Maple Scale:

One of the phenological indicators for the crawler stage of the first generation is when Cotinus coggygria is in bloom (Degree days at start of this crawler period is 806, and they are 1144 at the peak)

Where can you find / how do you get DD accumulation data?  There are several brands of “technology” that can be purchased and placed in a specific location that record temperature and calculate DD. There is often software associated with this technology that allows you to download the data to a computer. We do this in our research and have used temperature monitors made by Hobo and Watch Dog, but there are others too (no specific endorsement meant here). DD accumulations are reported in various extension publications (ex. the IPM Alert Newsletter). Weather data can be accessed from various sources (NOAA, Underground Weather, Weather Channel) which provide DD or the temperature data that allows you to calculate DD.  

In the IPM Alert Newsletters we have used weather data from NOAA and However, at various times, issues arise with various websites. Currently (as of March 2016), we are using the Online Phenology and Degree-Day Models (link is external) site. Use the following information to calculate GDD for your site:

Select your location from the map
Model Category: All models
Select Degree-day calculator
Thresholds in: Fahrenheit F
Lower: 50       Upper: 95
Calculation type: simple average/growing dds
Start: Jan 1

Degree day (DD) accumulations provide a tool for more accurately predicting activity of insects and the life stages you want to target for management.

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