Guide to Degree Day Models

Degree days represent the amount of heat necessary for a cold-blooded organism to grow or develop. Most insects and plants only grow when temperatures are above a certain base temperature (or threshold). Below this temperature, development slows or comes to a stop. Unlike humans, insects cannot regulate their body temperatures. Instead, insects rely on the environment to regulate their body temperature, meaning an insect will develop faster in warmer temperatures and slower in cooler temperatures. Different insect species have different base temperatures and development thresholds; therefore, degree day models must be tailored to each pest species. To account for species variability, degree day models will include multiple base temperatures to select from (see table below for pest development thresholds).

If a degree day model is not available, then degree days may be calculated for each day with the following equation:

For example, codling moth has a base developmental threshold of 50°F. If the high temperature for the day is 75°F, and the low temperature for the day is 55°F, then the degree days accumulated on that day will be 15 DD50. To calculate the degree days accumulated for this one day, we will use the following equation:

A BioFix is the date or event that marks the beginning of degree day accumulation. Daily degree day values past BioFix will estimate the timing of an insect pest’s developmental events, such as 20% egg hatch. For example, a grower puts a codling moth trap in her orchard. She captures no codling moth on May 16^th. A week later (May 23^rd), she captures two codling moth. We cannot set a BioFix for these captures since the week before had zero trap captures. On May 30^th, she captures one codling moth in her trap. At this point, she has two consecutive weeks of codling moth trap captures. To set the BioFix, we will use the first date of the two consecutive weeks (May 23^rd). Using a degree day model, we can look at the number of accumulated degree days for May 23^rd. For this example, let us say that the model shows 350 DD (Base 50°F) accumulated between January 1^st and May 23^rd. We can now plan our management strategy. For codling moth, 20% egg hatch occurs 250 – 350 DD50 Post BioFix. This means that we will apply an ovicide when the farm’s weather station reads that 600 – 700 DD50 have passed.

References

https://www.youtube.com/watch?v=RNYevBVIhYQ

https://enviroweather.msu.edu/weathermodels/weathersummary

https://treefruit.wsu.edu/crop-protection/opm/dd-models/#:~:text=Phenology%20models%2C%20also%20known%20as,temperature%20where%20it%20is%20found

https://ipm.ucanr.edu/WEATHER/ddconcepts.html

https://vegento.russell.wisc.edu/ipm/degree-day-modeling/