By John E. Woodmansee
Agriculture/Natural Resources Extension Educator, Purdue Extension Whitley County

COLUMBIA CITY — One measurement that agronomists and entomologists frequently use is Growing Degree Days. Sometimes referred to as “Heat Units,” Growing Degree Days closely correspond with crop growth and development, as well as the growth and development of insect pests. If that is the case, how well do you understand GDD, and how can it become more useful to you in your management decisions?

John Woodmansee

In the extension publication, “Growing Season Characteristics and Requirements in the Corn Belt,” the authors explain GDD. “Growing-degree days are systems used to classify the maturity of corn hybrids. GDD result from summation of mean (average) daily temperature.”

The GDD concept assumes that:

* There is a value or base temperature below which plants do not grow or grow very slowly.

* The rate of growth increases as temperature increases above a base temperature.

* Plant growth and development are more closely related to daily temperature mean accumulations above a base value in the absence of other limiting conditions.

“A base temperature of 40°F (5°C) is commonly used for cool-season crops such as wheat, oats, and canning peas while a higher temperature of 50°F (10°C) is used for warm-season crops such as field corn, sorghum, and sweet corn,” they said. “The GDD are determined by subtracting the base temperature from the daily average temperature.”

For example, if the average daily temperature for corn planted on May 1 is 56 F, a 56-50 (degree representation) = 6 GDD. Let’s say that on July 4, the average temperature is warmer, 74 F, then 74-50 (degree representation) = 24 GDD. In terms of degree days, the rate of corn growth on July 4 would be 24, and when divided by six, it would be four times as great as at the cooler temperature on May 1.

The Midwest Regional Climate Center explains that Modified GDDs are used for crops like corn. “Modified Growing Degree Days are an adaptation of the traditional Growing Degree Days used to more accurately track the development of crops, such as corn. In MGDDs, daily high and low temperatures are adjusted before calculating the average: if the daily high exceeds 86 F, it is capped at 86 F, and if the daily low is below 50 F, it is raised to 50 F. These adjustments reflect the temperatures most favorable for crop growth, assuming that development slows significantly outside of this range.”

For insects, calculations are similar, but daily base and maximum temperatures may be different than crops.

An insect many farmers dealt with in forages this spring was the alfalfa weevil, so we’ll use it as an example. Entomologists have studied the development of insects and have learned that their growth and development closely follow GDD (or “heat units”), but the base temperature for alfalfa weevil in these calculations is 48 F. Below this temperature, the insects are not very active.

In northern Indiana, Purdue entomologists recommend beginning sampling at 250 heat units. They recommend that farmers reevaluate in seven to 10 days using the appropriate HU (at roughly 300 HU), or treat immediately with a residual insecticide if three or more larvae are noted per stem and the percent tip feeding is above 50%. At 400 HU, treat immediately with a residual insecticide if tip feeding is 50% or higher. Recommendations proceed on, but hopefully you now have a clearer picture of growing-degree days, or heat units, and how they can help you with insect pest management.

To recap, the average temperature minus the established base temperature equals GDD for that day and that crop or insect. The daily average is calculated by adding the maximum and minimum temperatures for that day and dividing by two2. GDD accumulations have a starting date, and GDDs add up from that date to the present date. Different crops and insects may have different base and maximum temperatures, and calculating MGDD will utilize these limits.

Calculating GDD by hand and tracking accumulations would be cumbersome. Fortunately, our Indiana State Climate Office has maps that track GDD accumulations in various locales from various starting dates. Find them at ag.purdue.edu/indiana-state-climate/growing-degree-day-climate-maps/. Access MRCC data at mrcc.purdue.edu/, which includes a Corn GDD tool and Modified GDD maps.

Lastly, Purdue climatologist Austin Pearson wrote about GDD and MGDD in a more detailed article he wrote for the Purdue Pest & Crop Newsletter. Find it at extension.entm.purdue.edu/newsletters/pestandcrop/issue/2025-12/.