Monitoring Growing Degree Days and Plant Phenology

Timing

The growing use of less persistent, more environmentally benign pesticides along with increased use of alternative management strategies and the rising costs of labor has magnified the importance of accurate timing in pest management. Effective plant protection and efficient time management is dependent on our ability to predict pest activity.

There are several ways to predict when monitoring should begin or when pests are vulnerable to treatment. The calendar, growing degree days (GDD), and plant phenology methods are the three most commonly used for insects.

Calendar

The calendar method is based on past experience and the historical record and is expressed as an approximate date. For example, Gypsy Moth egg hatch occurs in Massachusetts somewhere between late April and late May. As each spring in New England is unique and the season progresses differently in different areas, scheduling treatments by the calendar method alone can result in poor control, wasting both material and labor time.

Growing Degree Days(GDD)

Insects are cold-blooded animals whose activity and development is controlled by the temperature of the surrounding environment. It has long been recognized that growth could be measured indirectly by tracking temperature over time once the lower (baseline) and upper threshold temperatures for a particular insect were known. This would allow us to predict events in an insect's life cycle during the season by measuring growth in terms of temperature over time. While the concept of GDD has been around for many years, the baseline threshold temperatures are known for only a relatively few insect species. Currently, 50°F is used as a standard baseline for all insect and mite pests of woody plants. This was chosen as plant growth in the northeast is thought to start between 45 °F and 55 °F. Obviously, the farther an insect or mite's true baseline is from 50, the less accurate these range numbers are. However, in most cases it is proving to be much more accurate than the calendar method.

Example:

Average daily temperature - Baseline temperature=Growing Degree Days gained. (Negative numbers are ignored as growth does not go backwards.)

If the high temperature for April 1 was 70°F and the low was 60°F, then the average temperature for April 1 was 65 °F:

Using a baseline temperature of 50°F, then the accumulation for April 1 would be 15 Growing Degree Days (GDD):

As you add up each day's GDD, you create a Growth Unit calendar for the season. We know Gypsy Moth egg hatch occurs between 90 and 100 GDD. By the calendar, this can be anywhere from late April to late May, a range of some 30 days. If we follow Growing Degree Days as they approach 90, we can predict egg hatch within a few days.

The daily average temperature is readily available from weather stations, newspapers etc. or easy to record using a high-low thermometer, thermograph, or a Biophenometer.

Approximate GDD Scale for Massachusetts

Plant Phenology

Plant growth also responds to accumulating heat units to some degree. Bud swell, leaf emergence, flowering, fruiting, and other growth stages can be correlated to the growth stages of some insects and mites. Continuing with our example of Gypsy Moth egg hatch, this is said to occur about the time Amelanchier (Shadbush) is in bloom. However, as day length and other environmental factors can affect specific events in a plants life cycle and different cultivars frequently have different bloom periods, these correlation's are less precise than using GDD but more accurate than using calendar dates. As landscapers and nursery workers can easily observe bloom and other plant events as they perform their normal routines, this is an attractive method for basing monitoring and management. Plant phenology and GDD information relative to Massachusetts' plants and insects has been researched and is incorporated into fact sheets, newsletters and the Insects, Diseases, and Weeds of Shade Trees and Woody Ornamentals guide. In Massachusetts, weedy GDD accumulations and current plant bloom are available through the Landscape Message System. As with relying on a calendar approach, caution should be exercised when using GDD and phenology. Both are meant as an aid to monitoring, not as a substitute for visual confirmation.

Gary J. Couch, UMASS Extension Educator