Growing Degree Days and swarm collection

In Better Beekeeping, Kim Flottum described Growing Degree Days (GDD) and how this measure of heat accumulation predicts plant blooming. A growing degree day is a formula that reflects temperatures above 50°F which is the most common base temperature.

For example, red maples (Acer rubrum) begin to bloom when they reach a GDD threshold of 30–50 GDD (base 50°F). I started thinking that there was a relationship between GDD and swarms. I returned to Kim’s GDD page over several years struggling to relate GDD and collecting swarms - my favorite aspect of beekeeping.

At first glance, GDD calculator rules and heat accumulation bookkeeping results might look abstract - you are not alone.

Atlanta, GA	Atlanta, GA
First Swarm Date -	Growing Degree Days
Date from David Marshall	(GDD BASE 50 °F) data from NOAA GDD calculator
2-Mar-22	173
25-Feb-23	312
1-Mar-24	176

Explanation of the Atlanta first swarm date versus Atlanta GDD table

Thanks to MABA’s swarm commander, David Marshall, I have three years of swarm collection data and combined these three first swarm dates with NOAA’s GDD plot. GDD measured on the first swarm day shows that 2023 GDD has a surprisingly large value compared to 2022 and 2024. In the table, two of these 3 data points (2022 and 2024) are extremely similar, but I nevertheless connected all points with a straight line. Wow, a relationship between GDD and swarm collection seems within reach!

Explanation of the Accumulated GDD graph

• Smooth brown line is an average of several decades of Atlanta’s GDD calculations
• Blue line is Atlanta’s historically highest GDD calculation (2017)
• Red line is Atlanta’s historically lowest GDD calculation (1915)

Black line passes through Atlanta’s first swarm dates of 2022, 2023, and 2024. For simplicity, Atlanta’s GDD curves for these three years are not shown, but those curves lie between the blue and brown curves. I hypothesize that years with GDD curves closer to the blue curve also have their first swarm date earlier in the year - as much as eight days earlier in the Atlanta area compared to an “average” year.

Explanation of NOAA’s historical last freeze day

Georgia includes a wide range of historically average last freeze dates, which implies that your local bee club GDD curves do not match my experience in Atlanta. Please do not hesitate to share your club’s swarm collection dates with me - I promise to treat your data with respect and acknowledgment.
In summary, it’s not too early to make a plan to find your swarm traps (bait hives) in winter storage and spruce up that trap (paint, frames, and lure) before the start of the 2025 swarm season. Keep an eye on your local GDD curve. In hindsight, Atlanta’s 2023 higher-than-average GDD trend was detectable in late January.