University of Maryland Extension

Growing Degree Days: Getting Them and How to Use Them

Author: 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.

Smokebush in full bloomJapanese 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. 

Go to Weather Underground website and follow the directions listed below in this article.

Use the following information to calculate GDD for your location:

1. Enter your zip code (not all locations are included, check nearest weather station to your site) and hit enter
2. Click the “custom” tab/button below the date
3. Enter the start date below the word “from” (ex. Jan. 1) and the end date below the word “to” (current date)
4. Hit the get “history” button
5. Read your growing degree days (base 50) in the ‘Sum’ column (=Cummulatlive DD to date for the year)

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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