IPM Methods: Determining Treatment Timing Using Degree Days and Insect Phenology Models
Degree days (DD) are a measurement of heat units over time, calculated from daily maximum and minimum temperatures. Degree days are used to predict insect life cycles, and in turn, time insecticide treatments to those life cycles. Insects are exothermic (“cold-blooded”) and their body temperature and growth are affected by their surrounding temperature. Every insect requires a consistent amount of heat accumulation to reach certain life stages, such as egg hatch or adult flight. Degree days interpret that heat accumulation.
Using Degree Day Models
The minimum temperature at which insects first start to develop is called the “lower developmental threshold”, or baseline. The maximum temperature at which insects stop developing is called the “upper developmental threshold,” or cutoff. The lower and upper thresholds vary among species, and have been determined for many tree fruit pests.
These values are used in calculating species-specific degree days. Entomologists have studied biological development over time (phenology) of several fruit insect pests, discovering exact degree day values that correlate to key physiological events, such as egg hatch or adult flight. This predictive information is known as an insect phenology model. Insect models are useful in timing insecticide treatment because the entire life cycle (or key events) of the insect is known.
The minimum temperature at which insects first start to develop is called the “lower developmental threshold”, or baseline. The maximum temperature at which insects stop developing is called the “upper developmental threshold,” or cutoff. The lower and upper thresholds vary among species, and have been determined for many tree fruit pests. These values are used in calculating species-specific degree days.
Entomologists have studied biological development over time (phenology) of several fruit insect pests, discovering exact degree day values that correlate to key physiological events, such as egg hatch or adult flight. This predictive information is known as an insect phenology model. Insect models are useful in timing insecticide treatment because the entire life cycle (or key events) of the insect is known.
Insect Model Information for Certain Fruit Pests
| Insect | Lower threshold | Upper threshold | Start accumulating DD | Calculation Method |
|---|---|---|---|---|
| codling moth |
50 | 88 | biofix | single sine |
| peach twig borer |
50 | 88 | biofix | single sine |
| greater peachtree borer |
50 | 87 | March 1 | single sine |
| western cherry fruit fly |
41 | none | March 1 | single sine |
| walnut husk fly |
41 | none | March 1 | single sine |
| pear psylla | 41 | none | Jan. 1 | double sine |
| European red mite |
51 | none | March 1 | single sine |
| oblique- banded leafroller |
43 | 85 | biofix | single sine |
| San Jose scale |
51 | 90 | use codling moth biofix |
single sine |
Calculations
In general, degree days can be calculated using a simple formula for the average daily temperature, calculated from the daily maximum and minimum temperatures, minus the baseline (lower developmental threshold):
[(daily maximum temperature + daily minimum temperature)/2] – baseline temperature.
For example, a day where the high is 72˚F and the low is 44˚F would accumulate 8 degree days using 50˚F as the baseline:
[(72 + 44)/2] – 50 = 8.
The sine wave method yields a more precise calculation. This method still uses the daily minimum, maximum, and baseline temperatures (lower threshold), but also incorporates the upper threshold into the calculation. It is based on the assumption that temperatures of a 24-hour day follow a sine wave curve. The number of degree days is then calculated as the area under this curve within the lower and upper temperature thresholds.
With more precise temperature sensors, the degree day total for a single day is calculated from max/min temperatures recorded hourly or even every minute. No matter the precision, the calculated value is added to the prior value and so on, resulting in an accumulated number from a set starting point. The set starting point can be a fixed date (which would be January 1 in the Intermountain West) or an event such as the date of first moth flight, called biofix (which is determined by using pheromone traps). For an average growing season in northern Utah, areas will accumulate approximately 2500-3500 degree days (with a baseline of 50˚F).
Treatment timing is useful by forecasting degree day values for a given location, using either forecasted daily highs and lows, or 30-year average highs and lows. This information is only an approximation of a future event, but is highly useful in planning.
Obtaining Degree Days
There are a variety of ways to acquire degree days, from dataloggers, online calculators, or printed newsletters from your local Extension service.
- Biophenometers are instruments that calculate degree days every few minutes and are highly accurate. Many brands allow you to manually input the target pest’s upper and lower thresholds.
- University Extension newsletters:
- USU IPM pest email advisories provide accumulated and forecasted degree days for a variety of sites across northern Utah, and are delivered via email weekly.
- The Western Colorado Research Center provides general fruit information and contact information on their fruit page website.
- Idaho participates in the Pacific Northwest and Treasure Valley Pest Alert Network (register here), providing fruit tree and small fruit pest advisories.
- Utah TRAPs (Timing Resource and Alert for Pests) is an online degree day calculator for a variety of locations in northern Utah. It is also available as a mobile app.
- Idaho information can be found here: Idaho information.
- For Montana, pest advisories and other information are available at Missoula County Weed District and Extension, MSU Ravalli County Extension, and MSU's Western Agricultural Research Center.