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
March, April, May, June |
July |
August, September |
October |
|
0 |
500 |
1000 |
1500 |
2000 |
2500 |
3000 |
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 Landscape
Message. 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.
Current pesticide and integrated pest management recommendations
can be found in
UMass Extension's Management Guide for Woody Ornamentals.
Additional
Information:
Bacillus thuringiensis (B.t.) ![](https://webarchive.library.unt.edu/eot2008/20090116083548im_/http://www.umassgreeninfo.org/images/iconpdf_sm.gif)
Beneficial Nematodes ![](https://webarchive.library.unt.edu/eot2008/20090116083548im_/http://www.umassgreeninfo.org/images/iconpdf_sm.gif)
Bio-Rational
Pesticides
Current Growing
Degree Day Accumulations
Filing a School IPM Plan in Massachusetts ![Adobe PDF icon](https://webarchive.library.unt.edu/eot2008/20090116083548im_/http://www.umassgreeninfo.org/images/iconpdf_sm.gif)
Fundamentals of An Insect and Mite
IPM Program
Horticultural Oils ![](https://webarchive.library.unt.edu/eot2008/20090116083548im_/http://www.umassgreeninfo.org/images/iconpdf_sm.gif)
Managing Insects Using Superior or Horticultural
Oils ![](https://webarchive.library.unt.edu/eot2008/20090116083548im_/http://www.umassgreeninfo.org/images/iconpdf_sm.gif)
Monitoring: Growing Degree Days and
Plant Phenology
Monitoring and Management Checklists
New Insect Products and
How They Work ![](https://webarchive.library.unt.edu/eot2008/20090116083548im_/http://www.umassgreeninfo.org/images/iconpdf_sm.gif)
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