One way to think of degree days is by imagining what your frozen hands feel like if you spent too many minutes outside on a subzero day without gloves (not recommended!). When you come inside, your hands are stiff and numb and they are hard to move.

If you run your hands under icy cold water, it will take a long time to warm them up…upt o 10 minutes.

If you run your hands under warm water, they will begin moving in about a minute or so.

Now if we are living through an extended cold spring, and if you are an insect who is “expected” to emerge around May 15, you may not emerge until May 22 or May 26 or – depending on how cold the spring is – until June 3. Likewise, if you have an early warm spring, you might emerge a week or two earlier than May 15.

Because our weather does not stay the same temperature every day, insects develop at different rates over time.

Degree days (DD) are a way of calculating how many units of heat add up over time. Knowing how to use insect degree days can help growers plan when to take actions in their field that target certain stages of their life cycle.

The Formula: Simple method

During an atypically warm – or cold – spring, the simple degree day formula offers a more precise way for predicting insect emergence. To calculate insect degree days using the simple method:

Take the MAX Temp plus the MIN Temp, divide them by 2 and subtract the insect-of-concern’s base temperature, or the minimum temperature it needs to function or move. The simplified version goes something like this: (TMAX + TMIN) / 2 – 50.

Using the simple Degree Day model gives growers a more accurate sense of when to expect certain plant feeding insects at the beginning, middle, and end of the growing season.

– Why growers use degree day models

If, after you subtract 50 (many insects have a threshold temperature of 50 degrees F), you have any positive number above zero, that’s the number of “degree day heat units” that your insect has accumulated.

Example

March 1 — 42 (max) — 30 (min) = 72 / 2 = 36 – 50 = -14 (+ 0 DD)

March 2 — 45 (max) — 33 (min) = 78 / 2 = 39 -50 = -11 (+ 0 DD)

March 3 — 54 (max — 42 (min) = 96 / 2 = 48 – 50 = -2 (+0 DD)

March 4 — 62 (max) — 51 (min) = 113 / 2 = 56.5 – 50 = (+6.5 DD)

So, in the four dates above, March 4 is the first day that insect DD are adding up. If you are a cabbage flea beetle, you will emerge when your DD adds up to 150-250 DD.

Source

https://www.weather.gov/media/mpx/Climate/MSP/mar2022.pdf