Understanding Degree Days

Updated: March 1, 2023

Key points

Degree Days (DD), also termed Growing Degree Days (GDD), measure how much warmth above a certain threshold has accumulated since the beginning of the year. Formulas are used to calculate DD values, which are based on the average temperature each day.
This data is used to track how cumulative warmth contributes to insect development, which is highly dependent on temperature. The warmer the average temperature each day, the faster the increase in degree day values over time.
Temperature accumulations are a more accurate predictor of insect maturation than calendar dates. This makes insect monitoring more efficient and management techniques more effective. DD measurements are an increasingly important tool as climate change impacts pest populations.

What are degree days?

Degree Days are a measure of temperature accumulation over the course of the growing season, spring through autumn. The value resets to zero at the start of each year and only starts increasing once the average daily temperature exceeds a certain threshold, also referred to as the base temperature, which is often 50℉.
The warmer the average temperature each day, the faster the increase in degree day values. The DD number is cumulative; it increases with each passing day.

Why are degree days important?

Degree day tracking helps to accurately predict insect activity. Depending on calendar dates for pest monitoring and control is unreliable if temperature patterns trend above or below average, changing the pace of insect development. In the event of unusual weather, if you only use calendar benchmarks, you could miss the most effective (or only effective) pesticide application window.
Some pests have a greater vulnerability to pesticides at specific points in their life cycle. Often, the targeted life cycle event will be an egg hatch or adult emergence, during which plant monitoring and treatment will help interrupt pest reproduction. This allows you to manage a pest outbreak before plant damage becomes evident, which is especially valuable when the host plant does not easily recuperate from insect feeding.
Predicting when to expect a key developmental stage is critical in the timing of pesticide applications. By using degree day predictions for a specific pest, you can monitor when to expect peak vulnerability based on recent temperatures and can make preventative pesticide applications only when they will have the greatest impact. This both saves time and reduces pesticide use.

Examples of how to use degree day information

Entomologists document degree day values for important stages in a pest insect’s life cycle. For example, the crawler emergence period for cottony camellia scale is expected to begin around 649 DD. When you see DD values for your area nearing this point (e.g., 600 DD) it’s time to start inspecting host plants for crawlers.
Reported DD values for specific pests are meant to be guidelines for when action might be needed. Nuances of microclimate, even over small areas of a single plant or garden, result in varied DD accumulations. Do not use them to dictate the timing of pesticide use without also verifying a pest’s identity and that it is in the targeted developmental stage.

1) Non-native or Invasive Insects

Using insect DD data collected by researchers outside of the mid-Atlantic is valuable when monitoring for pests new to Maryland. These may be invasive species or native southern species expanding their range further north.
For instance, if an insect is relatively new to Maryland but more established and well-studied in the southern U.S., rather than converting key calendar dates of pest development from their climate to ours, we can use DD instead to predict its life cycle for our area. This is especially important in monitoring for highly damaging pests, where focused management can prevent their establishment.
Crapemyrtle bark scale, for example, was first found in southern states in 2004. It was detected in several Maryland locations in 2020. Life cycle and DD data from both the southern U.S. and its native range in Asia helps to focus monitoring activity in MD.

2) Climate Change

Changing weather patterns due to climate change is another reason DD tracking is important since pest activity will likely shift to earlier dates over time.
Warming could also allow them to produce an additional generation, resulting in more opportunity for plant damage.
While important calendar dates for the developmental pace of the insect will change over time, the DD values for specific lifecycle events should remain fairly constant.

There are several web-based tools such as University of Maryland Extension IPMNet where you can look up DD values for your area.

As a point of reference, central Maryland generally starts to accumulate degree days in earnest in March, though values can very slowly begin increasing in January in a mild year. By the end of October, when most pests are becoming inactive and DD impacts wane, the cumulative values range from about 3300 to 4400 DD across the state.

DD data example

For data collected at the College Park weather station in 2021, using the parameters given in the link above, here are values illustrating the pace of changes in DD accumulations at different points in the growing season:

Date	DD Value	Date	DD Value	Date	DD Value
March 26	0.3	May 26	581.9	July 26	2212.4
March 27	0.9	May 27	600.6	July 27	2242.6
March 28	1.9	May 28	619.7	July 28	2272.8
March 29	3.3	May 29	639.2	July 29	2302.9
March 30	5.0	May 30	658.9	July 30	2333.0
March 31	7.1	May 31	679.0	July 31	2363.1
April 1	8.8	June 1	700.3	Aug 1	2393.3

Related tool: plant phenological indicators

Temperature plays an equally important role in the progression of plant development. If you do not have DD information available, you can use stages of plant growth to help reference key parts of an insect’s life cycle when they coincide. These events are called Plant Phenological Indicators (PPIs).
Phenology relates to seasonal changes in a plant’s life cycle, such as the timing of bloom or spring leaf emergence. This data is used like DD data, helping to target the timing of pest monitoring and the application of any needed pesticides.
As an example, the PPI for cottony camellia scale crawler emergence is when yellowwood trees (Cladrastis kentukea) have reached full bloom and before Japanese snowbell (Styrax japonicus) trees have reached full bloom. You’ll know you should start monitoring for crawlers of this species when yellowwood flower buds are just starting to open, or when Japanese snowbell flower buds start to appear.
Most PPIs refer to bloom cycles, and they will reference commonly-grown trees or shrubs when possible. In any case, the key stage of growth will be easily observable.
Growth references include “first bloom” (when buds start to open), or “peak bloom” or “full bloom” (when most of the buds are open).

white flowers of a yellowwood tree — Yellowwood tree in bloom
Photo: Miri Talabac

blooms of a Japanese snowbell tree — Japanese snowbell in bloom
Photo: Miri Talabac

Related information

Soft Scale on Trees and Shrubs

Armored (hard) Scale on Trees and Shrubs

Additional resources

Using Nature’s Signals to Manage Landscape Pests | Penn State Extension

Degree Day Calculation | University of Wisconsin-Madison

Degree Day Maps & Phenology Forecasts | USA National Phenology Network

Author: Miri Talabac, Horticulturist & Coordinator, HGIC 2022