What are Growing Degree Days?
Growing degree days (GDD’s) are a unit of measure used to calculate the amount of heat required, between a lower and upper threshold, for an organism (such as an insect) to develop into the next life stage. With insects, for example, GDD’s can help us to estimate when the eggs of a particular pest are going to hatch (and subsequently when larvae or immatures are going to begin feeding) or approximately when vulnerable stages of certain insects, such as the crawlers (or immatures) of certain scale insects will be present.
GDD’s are a much more accurate method of estimating insect growth and development and the timing of insect life cycles than using the calendar method of estimating insect activity. The calendar method is based on historical records or past experience when a certain insect is present in the landscape. This can be very different year-to-year, as we know some springs are warmer or cooler than others are. Therefore, basing management decisions off a specific calendar date might lead to incorrect timing because of “out of the ordinary” seasonal temperatures.
GDD’s may be thought of as accumulated heat units, as they are an accumulated product of time and temperature between developmental thresholds per day. Each insect species might require a different amount of accumulated heat or physiological time to develop from one life stage to another (egg to larva to pupa to adult in the case of insects with complete metamorphosis). A key piece of calculating growing degree days for a specific insect is the insect’s developmental threshold.
What is a Developmental Threshold?
For simplicity, we will focus on the lower developmental threshold of insects when calculating growing degree days. This is the temperature below which development of the insect does not occur (stops). This temperature is dependent upon the individual organism’s physiology (normal functioning) and does not change even when using different methods to calculate growing degree days.
Controlled laboratory and field experiments have been done to determine the developmental threshold temperature for certain different species of insects, but not all. Therefore, this information might not yet be available for certain pests of concern. To simplify the calculation of growing degree days, we will use a developmental threshold (baseline) temperature of 50°F. 50°F is used for insect and mite pests of woody ornamental plants in the Northeast as most of these plants initiate their growth between 45°F-55°F in our region. If an insect’s true developmental threshold is much farther from 50°F, the growing degree day estimates may be inaccurate, however using GDD’s has still been shown to be more accurate in estimating insect pest activity than simply relying on calendar dates.
How Do I Calculate Growing Degree Days?
A simple way of calculating growing degree days is sometimes referred to as the “Average Method”:
Average Daily Temperature
- Baseline Temperature (Developmental Threshold)
= Growing Degree Days Gained
Ignore any negative answers to this above equation, as insect growth will not reverse as a result. Here is an example:
April 1st High Temperature: 70º F
April 1st Low Temperature: 60º F
Average Daily Temperature for April 1st = (70º F + 60º F)/2 = 65º F in our example
Using the developmental threshold of 50º F, the growing degree day calculation would be:
Average Daily Temperature (65º F)
- Baseline Temperature (Developmental Threshold) (50º F)
= Growing Degree Days Gained (15 GDDs)
Each day’s total is added together if the GDD amount is positive (negative values are not subtracted). Growing degree days can be calculated starting on January 1st, or closer to the start of the growing season when accumulated heat units are more likely to occur above 50°F (such as March 1st in Massachusetts). When using the 50°F base, GDD’s in Massachusetts generally range from 0 GDD’s in the beginning of March to 500 GDD’s by the end of April-beginning of May to 1000 GDD’s between June and July and finally 3000 GDD’s by October.
Other, much more mathematically complicated methods of calculating growing degree days are available and are used by entomologists to more accurately track pests. A great explanation of these different methods of calculating growing degree days may be found at the following University of California web page: http://ipm.ucanr.edu/WEATHER/ddconcepts.html.
How Do I Use GDD’s in Management?
Tracking growing degree days throughout the growing season can help you to track insect pest activity and better time monitoring and management activities. This will allow you to use temperature to track the development of these insects and monitor for them close to when any detrimental or otherwise significant activity might occur based on their life cycle.
Using gypsy moth or Lymantria dispar as an example, egg hatch for this moth caterpillar is known to occur roughly between 90 and 100 GDD’s when using a base 50° F developmental threshold. For our example, consider that egg mass numbers from the previous season are present in the landscape in high numbers, defoliation was extensive in your area last year, and there is solid evidence this season that caterpillar numbers might be high again. You also have high value ornamental plants that are favored hosts for gypsy moth that you wish to protect.
By tracking growing degree days you can then plan to monitor (in the field) egg hatch once you locally accumulate 85-90 growing degree days and closely watch caterpillar development. Once hatched caterpillars begin dispersing to the canopies of their host trees and settle to feed, you will be prepared to apply biorational insecticides, according to your IPM plan, such as Bacillus thuringiensis Kurstaki (Btk) at the time caterpillars are first feeding and while caterpillars are still young (when they are between ¼ - ¾ inch in length). This will increase the likelihood of the success of your application. Btk only works on Lepidopteran (butterfly and moth) caterpillars and only when they ingest this bacteria-based toxin. It also works best on younger (smaller) caterpillars. If you had not been tracking growing degree days early in the season, you may not have noticed gypsy moth caterpillar feeding until the larvae were too large to treat with Btk.
Where Can I Find GDD’s for My Area?
UMass Landscape Message: https://ag.umass.edu/landscape/landscape-message
Network for Environmental and Weather Applications: http://newa.cornell.edu/index.php?page=growing-degree-days