Insects

Degree Days

Degree days are a measurement of heat accumulation. They are calculated on a daily basis (thus the "days" part of "degree days") and then added up every day, accumulating over a given time period. Often people accumulate degree days starting from January 1st, but other dates or events can serve as the starting date. The date or event from which degree days begin accumulating is known as the "biofix."

A biofix is a biological marker that initiates the beginning of degree day (i.e. heat unit) calculation used to forecast temperature-dependent insect developmental stages. For insects, it is usually the calendar date of a particular pest event, such as peak moth flight in spring.

Degree days are used in entomology (among other fields) because insects develop incrementally depending on temperature, with development occurring faster under warm temperature conditions. The progression through an insect's life cycle can be estimated using easy to collect air temperature data and then converting that data into degree days. (See below for where to get temperature data.) In the conversion, the temperature data is compared to an insect's lower developmental threshold—the temperature above which the insect can develop. This threshold is also called the base temperature. Each species of insect has a particular base temperature.

How to Calculate Degree Days

  1. Beginning January 1 or the biofix date, the first step to calculating degree days involves determining the average daily temperature. This can be computed by adding the daily low temperature to the daily high temperature and then dividing this amount by two.
  2. Next, the insect's base temperature is subtracted from the value computed for the average daily temperature. If the average daily temperature is below the threshold for development for the insect in question, then you accumulate zero degree days that day. This calculation gives you a daily value of growing degree days. Remember, degree days are a measurement of thermal units available to an insect—you can never have negative growing degree days because there are either enough thermal units for development (a positive number of degree days), or not enough for development (zero degree days).
  3. This process is repeated each day beginning from a start date (either January 1st or the biofix date), with the calculations for daily accumulations being summed over time to give you total growing degree days.
Degree day equation

More information on degree days can be found on the University of California-Davis website.

Where to Get Temperature Data

A degree day calculator can be accessed on the Wisconsin-Minnesota Cooperative Extension Agricultural Weather Page here. This website will calculate total Fahrenheit growing degree days for you. The steps below explain in detail how to calculate growing degree days for your specific location.

  1. Enter the latitude and longitude of your location using the map provided.
  2. Choose your degree day calculation parameters
    1. Choose your biofix. This is the day from which you want to begin calculating degree days.
      • For Monitoring Option 1: the biofix is January 1st.
      • For Monitoring Option 2: the biofix is the date you till your field.
      • For Monitoring Option 3: the biofix is the date you trap the most adult flies.
    2. Choose your end date. The website end date will default to the most recent date for which weather station data are available.
    3. For "Degree day method" check the "Simple" method.
    4. For "Lower threshold temperature (Degrees F)" enter in 39.
    5. For "Upper threshold temperature (Degrees F)" leave blank.
  3. Choose your data output format.
    • If you check the circle for "Seven day summary page" the value on the bottom right corner of the displayed output will be the total accumulation of Fahrenheit degree days beginning from the biofix start date.
    • If you check the circle for "As text, for selected time period, ready for downloading" the value on the bottom right corner of the displayed output will be the total accumulation of Fahrenheit degree days beginning from the biofix start date.
  4. Click the submit button.

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