How Are Accumulated Degree Days (ADD) Calculated?

A man holding alfalfa weevil larvae
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Entomologists and agriculturalists study insects and plants to learn about our world. These scientists may try to use a species to improve human life, protect us from dangerous organisms, or even answer questions and solve problems. Crime scene insects are just one example of how helpful forensic entomology and similar fields of study can be. One way to get a better look at the development stages of a plant or insect in order to understand them more deeply is to calculate degree days.

What Are Accumulated Degree Days?

Degree days are a projection of organism development. They are a unit that represents the amount of time that an insect or other organism spends at a temperature above its lower development threshold and below its upper development threshold. If an insect spends 24 hours one degree above its lower development threshold or the temperature beneath which its development ceases, then one-degree day has been accumulated. The higher the temperature, the more degree days acquired for that period.

How ADD Are Used

Accumulated degree days, or ADD, can be used to determine whether the total heat requirement for a stage of development has been met for an organism or predict whether it will be reached. Farmers, gardeners, and forensic entomologists also use accumulated degree days to predict insect or plant development and success. These calculations can help scientists to understand the life of an organism by providing a helpful estimation of the total effect that temperature and time have on that organism.

Every organism requires a predetermined number of days spent within its optimal temperature range for development in order to complete a stage of growth. Studying accumulated degree days offers a glimpse into the imperceptible growth of a plant or insect and this unit requires only a few simple calculations to obtain. Here's a simple method for calculating accumulated degree days.

How to Calculate ADD

There are several methods that can be used to calculate accumulated degree days. For most purposes, a simple method using the average daily temperature will produce an acceptable result.

To calculate the accumulated degree days, take the minimum and maximum temperatures for the day and divide by 2 to get the average or mean temperature. If the result is greater than the threshold temperature, or the base temperature for development, subtract the threshold temperature from the average to get the accumulated degree days for that 24-hour period. If the average temperature did not exceed the threshold temperature, then no degree days were accumulated for that time period.

Example Calculations

Here are some example calculations for the alfalfa weevil, which has a threshold temperature of 48 degrees F, over the course of two days.

Day One: The first day, the maximum temperature was 70 degrees F and the minimum temperature was 44 degrees F. We add these numbers (70 + 44) and divide by 2 to get an average daily temperature of 57 degrees F. Subtract the threshold temperature from this average (57 - 48) to find the accumulated degree days for day one—the answer is 9 ADD.

Day Two: The maximum temperature was 72 degrees F on day two and the minimum temperature was 44 degrees F again. The average temperature for this day was then 58 degrees F. Subtracting the threshold temperature from 58, we get 10 ADD for the second day.

Total: The total accumulated degree days is equal to 19, 9 ADD from day one and 10 ADD from day two.