It’s that time of year, the weather is warming, plants are greening up, and many of us are in the garden.

The warmer temperatures also bring the return of insect and disease pests. How can we keep one step ahead of these unwanted visitors?

A tool that can help is Growing Degree Days (or GDD for short). So, let’s get acquainted with GDD. To do this, we will work through a bit of math, and then I will show you how to use GDD without the math.

Concept and math behind GDD

GDD is a metric for the accumulated heat experienced by pest organisms starting at a specified date (often March 1). The idea is that garden pests develop slowly at cool temperatures and more rapidly at warm temperatures.

The simplest way to calculate GDD is by using a temperature averaging formula for each day:

The base temperature is typically 50°F. If the formula result is negative, then the contribution for the day is zero. This contribution is added to a running total from prior days. The result is the cumulative GDD on that day. The table provides example results from this method.

As the season progresses and temperatures climb, GDD increases. This is shown by the plot of GDD for three consecutive growing seasons. Notice that GDD differs from year to year, increasing to larger values in years having more warm days.

The plot indicates that 2024 was warmer than the other years. This is important because pests appear earlier in warmer years.

Practical use of GDD

To use GDD to manage garden pests, first keep track of the GDD for your area. You can perform the calculations described above, or you can check out the resources below. Second, know the GDD for a given pest’s appearance. Here are three examples:

• Adult Japanese Beetles chew leaves at a GDD between 900 and 1200. Last year, that happened in late June.

• Magnolia Scale feeds on the sap of magnolia trees, creating a sticky mess beneath the canopy. The crawler stage is most readily managed. It begins at a GDD of 1950. Last year, that happened in late July.

• Squash Vine Borer is a pest of summer squash. Adult moths emerge from the soil near where plants were impacted last year. These moths lay eggs at a GDD between 900 and 1000. The eggs hatch into larvae that bore into stems to feed, causing plant wilting and death.

When will these pests appear in 2026? Rather than guess, stay up to date on the 2026 GDD values and know when to be on the lookout and assess control strategies.

To use GDD in your garden, check out the Plant Health Care Report published by the Morton Arboretum: https://mortonarb.org/about-arboretum/plant-health-care-report/.

This weekly report provides GDD, pest activity, pest identification, and management options. For pest specific GDD values, visit this University of Wisconsin Extension webpage: https://hort.extension.wisc.edu/degree-days-for-common-insect-pests/.

Try the GDD models from Oregon State University:: https://uspest.org/dd/model_app.

Susan Somers is a certified master gardener volunteer with University of Illinois Extension serving DuPage, Kane, and Kendall counties.