What are growing degree-days?
How are growing degree-days used?
How are growing degree-days measured?
What are
growing degree-days?
Growing degree-days (GDDs) are a measure of heat accumulation
during a growing season. Because many developmental events of
insects and plants are dependent on the accumulation of specific
quantities of heat, we can predict when these events should occur
during a growing season regardless of differences in temperatures
from year to year. This method is much better than using a calendar
for predicting such events because of the large differences in
temperatures that occur from one year to the next. GDD values
have been tabulated for many insect pests, but not all insect
developmental events are dependent on GDDs. For example, the emergence
of insects that overwinter in the adult stage is often dependent
on temperatures exceeding a certain critical value for several
consecutive days; think of this as the insect just needing to
warm up to "operating temperature." GDD-dependent events,
such as egg hatch for many insects, is different in that the developmental
process is incremental, with the biochemical and physiological
processes underlying development occurring a little bit at a time,
whenever temperatures are above some minimum temperature (called
the threshold temperature). What makes GDDs useful is that many
developmental events of insects requires the same amount of accumulated
thermal energy to be completed, whether that thermal energy comes
in large chunks (say, during a warm spring) or in drips and drabs
(during a cool, rainy spring). RETURN TO TOP OF PAGE
How are growing
degree-days used?
By comparing the accumulated GDDs during a growing season with
tabulated values for pests of interest, we can know better when
to expect certain pests to appear. This is very helpful in pest
management because it allows us to use our time more efficiently.
For example, if we know that a particular pest should appear after
1000 GDDs and we are currently at 500 GDDs, we can spend our time
on things other than scouting for that pest. GDD values have been
tabulated for many landscape pests. RETURN TO TOP OF PAGE
How are
growing degree-days measured?
Several methods can be used to measure growing degree-days, but
the most common method by far is the "high/low" method
or "max/min" method. The math involved is very simple.
We average the high and low temperature reading for a day, and
subtract the threshold temperature from the average. The commonly
used threshold temperature is 50F. So, if the high temperature
for the day is 55, and the low is 35, the average temperature
will be 45. Subtract 50 (the threshold) from 45, and the result
is -5. Because insect development does not reverse itself when
temperatures go below the threshold (they just "pause"
rather than "rewind"), the accumulated GDD total is
unchanged. If the high temperature for a day is 60 and the low
is 46, the average will be (60 + 46)/2 = 53, so we add 3 to the
GDD total. Because we are measuring the GDD totals for a season,
we need to agree on a starting point for the season. The commonly
accepted starting date in NY state is March 1. So, to measure
your own GDDs, start recording high and low daily temperatures
on March 1st, calculate the average, and subtract 50. If the result
is less than zero, we do nothing to the GDD total, but if the
result is positive, we add that to the accumulated GDD total.
There are ways to measure GDDs automatically. A number of companies manufacture weather stations that can measure GDDs for you, which has the advantage of measuring heat accumulation in your immediate vicinity. These units can be purchased for under $300. A number of newspapers and other media outlets post GDDs, and of course the NYS GDD Tracker follows GDDs for many locations throughout NY state. RETURN TO TOP OF PAGE
For additional information, you may want to consult the following links:
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