Calculating Degree Days in the Garden
Often when discussing the trials that accompany vegetable gardening, gardeners will mention issues they’ve historically had with various insects in their garden. Be they squash bugs, squash vine borers, cucumber beetles, aphids, Japanese beetles, or other garden foes, nearly all gardeners have tales of how their once-pristine crops came to be ravaged by an unexpected onslaught of insects. What many gardeners may not realize, however, is that there’s actually a way to predict when these insect onslaughts will happen. The concept of “degree day” counting, an integrated pest management practice, involves observing a particular insect pest, noting their life stages, and then calculating the number of times in the year when the temperature threshold for their growth has been surpassed.
Sound complicated? Perhaps a little, but it doesn’t have to be.
Insects are cold-blooded, which means their growth and development depend entirely on external temperatures. Each insect species has a specific minimum temperature, known as its “base temperature,” below which it won’t develop. Once the temperature exceeds this threshold, the insect begins to grow, and the degree days start to accumulate. Degree days are not the same as actual days; they are a simple calculation that tracks accumulated heat over time, helping predict when pests will move through important lifecycle stages, such as laying eggs or feeding.
To calculate degree days, you first need to know the base temperature for the pest you’re tracking. Squash vine borers, for example, have a base temperature of 50°F. With the daily high and low temperatures for your area, the formula for degree days is straightforward. Take the average of the day’s high and low temperatures, then subtract the base temperature. The resulting number is the number of degree days for that day (again, not the actual number of days).
For instance, if the high for a day is 80°F and the low is 60°F, the day’s calculation would be: ((80 + 60) / 2) - 50 = 20-degree days. From here, you simply add up the degree days over time, keeping track until you reach the threshold at which the pests will start to emerge or become active. Daily highs and lows can be seen in typical weather reports.
Looking again at the squash vine borer, these pests typically emerge once 900-to-1,000-degree days have accumulated. In warmer seasons, where temperatures often exceed 80°F, you might accumulate 20–30-degree days each day. Thus, reaching the insect’s threshold for action may not take all that long —with only a few weeks passing before it is time to take preventive steps.
The preventative step you take will then depend on the insect, the life stage it’s in, and the control measures you wish to implement. Row covers may be the solution in some cases, while other situations may call for scouting for adult moths, incorporating organic or cultural controls, or deciding when to apply pesticides if needed.
By using degree day counting, you’re no longer guessing when pests might show up. It’s a proactive approach that helps you stay ahead of them, ultimately leading to a more effective, healthier, and productive growing season. If you have questions, reach out to your local extension garden hotline or entomologist.
by Anthony Reardon, Horticulture Agent, Small Farms, 2024