Monitoring: Growing Degree Days and Plant Phenology

Timing
The growing use of less persistent, more environmentally benign pesticides, increased use of alternative management strategies and the rising costs of labor have all magnified the importance of accurate timing in pest management. Effective plant protection and efficient time management are dependent on our ability to predict pest activity. There are several ways to predict when pests are vulnerable to treatment or when monitoring for pest activity should begin. The calendar, calculation of growing degree days (GDD), and correlation of pest deveopment with plant phenology are the three most commonly used methods for insects and mites.

Calendar
The calendar method is based on following the historical record and past experience 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 enable predictions of 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 standard was chosen because 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, the GDD method 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°and the low was 60° then the average temperature for April 1 was 65° F.

When a baseline temperature of 50° is used, the accumulation for April 1 is 15 growing degree days.

(GDD): 65 - 50 = 15 GDD.

As each day's GDD are added to the total, a growth unit calendar for the season is created. Gypsy moth egg hatch is known to occur between 90 and 100 GDD. By the calendar, this can be anywhere from late April to late May, a range of some 30 days. In contrast, if growing degree days are closely observed as they approach 90, egg hatch can be predicted within a few days.

The daily average temperature is readily available from weather stations or newspapers, or is 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 the 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 correlations 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 are incorporated into fact sheets and newsletters. Weekly GDD accumulations and current plant bloom are available through the UMass Extension Landscape Nursery & Urban Forestry program website at umassgreennfo.org. 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.

Selected IPM Articles:
Bacillus thuringiensis (B.t.)
Beneficial Nematodes
Bio-Rational Pesticides
Current Growing Degree Day Accummulations (link to UMassGreenInfo.org - Landscape Message)
Fundamentals of An Insect and Mite IPM Program
Horticultural Oils
Managing Insects Using Superior or Horticultural Oil
Monitoring: Growing Degree Days and Plant Phenology
Monitoring and Management Checklists