Skip to main content

Growing Degree Day (GDD) Calculations

(Source: UW-Madison "Vegetable Crop Entomology" - https://vegento.russell.wisc.edu/ipm/degree-day-modeling/)

Degree-days are a method for quantifying the amount of heat energy available in a given day for a cold-blooded (or poikilothermic) organism to grow and develop. Unlike warm-blooded animals, insects cannot regulate their body temperatures and so their rate of development is depended on environmental conditions – they develop more slowly when it’s cold, more quickly when it’s warmer, and may not develop at all below certain threshold temperatures. Different insects will have different growth rates and responses to environmental temperatures, so degree-day models and developmental thresholds must be matched to particular insect species. Fortunately, degree-day models have now been developed and tested for many common insect pests and are useful for predicting and monitoring insect development and risk to crops from insect pests.

Every degree-day model will have a base temperature or lower threshold (below which no development can occur for a particular insect), and optionally an upper threshold temperature (above which insect development does not accelerate, but remains at a maximum rate). In addition, the model will have a ‘biofix’, which is a date or event that triggers the start of degree-day accumulation. Daily degree-day values are accumulated after this point and developmental events are timed to specific accumulations of degree-days. Biofix dates are usually based on specific biological events such as planting dates, first trap catch, or first occurrence of a pest.

Simple Average Method

gdd = maxT + minT 2 - BaseT

Where:

  1. gdd = calculated growing degree days for the given day.
  2. maxT = maximum temperature recorded for the given day.
  3. minT = minimum temperature recorded for the given day.
  4. baseT = base temperature or lower threshold
  5. If maxT < baseT, gdd = 0
  6. If minT < baseT, minT = baseT
  7. If upper threshold:
    • If maxT > upper threshold, maxT = upper threshold

Sine Curve Method

This is the Baskerville-Emin calculation fitting temperatures to a sine curve and is more accurate that the simple average.

Here are the steps to calculate growing degree days using the Sine Curve method:

  1. If the maximum temperature for the day is less than the base temperature (lower threshold), GDD = 0
  2. If the minimum temperature is greater than the base temperature (lower threshold), then use the Simple Average described above.
  3. If the minimum temperature is less than the base temperature and the maximum temperature is greater than the base temperature, use the Sine Curve method:
    gdd = W * CosA - baseT - avgT * 3.142 - A 3.14
    Where:
    1. gdd = calculated growing degree days for the given day.
    2. maxT = maximum temperature recorded for the given day.
    3. minT = minimum temperature recorded for the given day.
    4. W = maxT - minT2
    5. baseT = base temperature or lower threshold
    6. avgT = maxT + minT2
    7. A = ArcsinbaseT - avgTW