What are Grower Degree Days?

What are Grower Degree Days?

Plant and insect development is temperature dependent, not calendar dependent.  After the winter dormancy period, temperatures rise and plants and insects develop.  Grower Degree Days are a measure of accumulated heat.  Growers and landscape technicians can use the Grower Degree Day (GDD) measurement of heat to predict which insects will be active at which point.  For example, at 90 GDD, Eastern Tent Caterpillars are active.  This is the time to scout for Eastern Tent Caterpillars and to make management decisions.

Phenology is the study of periodic occurrences in nature and their relation to weather.  The study of phenology is made by observation.  When does Eastern Redbud bloom?  When do gypsy moth eggs hatch?  Because the development of plants and insects is related to the accumulation of heat, these questions can be answered in GDD units.  {When does Eastern Redbud bloom? = 90GDD; When do gypsy moth eggs hatch? = 90GDD}  However, grower can skip the GDD calculations by using straight phenology.  When the Eastern Redbud is blooming, it is time to scout for gypsy moth larva (caterpillar).  

By using a GDD and phenology pest management system combined with scouting, growers and landscape technicians may improve pesticide application efficacy and reduce pesticide usage when compared with a calendar-based pest management system. 


How are Grower Degree Days Calculated?

In a 24-hour period, the minimum and maximum temperatures are recorded.  Then the following formula is used to calculate the GDD for that time period:


       Temp Min + Temp Max   - Base Temperature =GDD


What is the base temperature?   It is the temperature below which plants and insects do not develop.  Base temperatures vary by species.  For some species of plants and insects, the base temperature is 30 degrees F, for others it might be 50 degrees F.  We will use a base temperature of 50 in our calculations, because the majority of the pests that we are looking at do not develop below 50 degrees F. 


For Example, if the high was 65 and the low was 48:


 48 + 65   - 50 =6.5 GDD for that day



We began measuring GDD in mid March in Erie County.  The GDD for each day are added together, giving a cumulative GDD measurement.  This cumulative measurement is the measurement of accumulated heat for the growing season. 


Here's the technology we use to record Grower Degree Days:




The weather tracker monitors the daily highs and lows for us, then calculates the Grower Degree Days.  The information will be updated on this site every Monday and Thursday; April - October.  The Penn State Cooperative Extension in Northwest Pennsylvania currently has weather trackers in Armstrong, Crawford, Erie, Venango and Warren Counties.  Because there is a great deal of variation throughout the region, you may wish to calculate the GDD at your location.  A simple Min/Max Thermometer is all you need to calculate GDD. 



Mike Masiuk, a Penn State Cooperative Extension Educator in Allegheny County has collected Grower Degree Days in locations in Southwestern Pennsylvania and has developed a very useful website.  http://woodypests.cas.psu.edu/PestActivity/index.html

Grower Degree Day information has been collected by, Dr. Dan Herms of Ohio State University/OARDC and by Dr. Warren T, Johnson of Cornell University.  They have collected data for more than twenty years each, and their research will be the basis of the phenology and insect activity predictions/expectations in this blog.

Dr Herm's Data: http://ohioline.osu.edu/sc157/sc157_16.html

Dr Johnson's Data: http://counties.cce.cornell.edu/suffolk/grownet/ipm/gdd.html


Penn State Publication: Creating Healthy Landscapes, Using Nature's Signals to Manage Landscape Pests http://sepaipm.cas.psu.edu/factsheets/chl-9.htm


The book, Coincide, The Orton System of Pest Management by Donald A. Orton is a very useful book that uses phenology of plants to predict insect pest activity.  Information on ordering this book is at http://woodypestguide.cas.psu.edu/44.htm


The USA National Phenology Network (USA-NPN) exists to facilitate collection and dissemination of phenological data to support global change research.  It can be accessed at www.npn.uwm.edu

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