### Question: How Are Accumulated Degree Days (ADD) Calculated?

Farmers, gardeners, and forensic entomologists use accumulated degree days (ADD) to predict when different stages of an insect's development will occur. Here's a simple method for calculating accumulated degree days.

### Answer:

There are several methods used to calculate accumulated degree days. For most purposes, a simple method using the average daily temperature will produce an acceptable result.

To calculate the accumulated degree days, take the minimum and maximum temperatures for the day, and divide by 2 to get the average temperature. If the result is greater than the threshold temperature, subtract the threshold temperature from the average to get the accumulated degree days for that 24 hour period. If the average temperature did not exceed the threshold temperature, then no degree days were accumulated for that time period.

Here's an example using the alfalfa weevil, which has a threshold of 48 ° F. On day one, the maximum temperature was 70 ° and the minimum temperature was 44 °. We add these numbers (70+44) and divide by 2 to get an average daily temperature of 57 ° F. Now we subtract the threshold temperature (57-48) to get the accumulated degree days for day one – 9 ADD.

On day two, the maximum temperature was 72 ° and the minimum temperature was again 44 ° F. The average temperature for this day is therefore 58 ° F. Subtracting the threshold temperature, we get 10 ADD for the second day.

For two days, then, the accumulated degree days total 19 – 9 ADD from day one, and 10 ADD from day two.