Insects rely on warm temperatures to grow, and thus their emergence and life cycle progression can be tracked by tracking the "heating units" that accumulate each day. This is referred to as growing degree days.
Growing degree days (GDD) are calculated by average the high and low temperature for the day and subtracting 50 (base 50). The positive remainder is added to the cumulative total of GDD. For more information on GDD and how to calculate the go to: http://www.gardening.cornell.edu/weather/gdd.html
The life cycle stages of many of our insect pests have GDD equivalents. For instance we know that the time to treat for forest tent caterpillars is between 192 and 363 GDD and eastern tent caterpillar 90 to 190 GDD. Before then, they shouldn't be out and after they are not really susceptible.
We can also use GDD for monitoring. A great example is the Emerald Ash Borer; whose adults emerge at 450-500 GDD. You've all seen those purple sticky traps used for monitoring EAB. Timing their deployment so they are out before 450 GDD is crucial.
Fortunately, you don't have to keep track of GDD yourself. You can use this handy reference from the CE Network for Environmental Weather Applications: http://newa.cornell.edu/index.php?page=growing-degree-days Just choose the station nearest you. If you're especially concerned about EAB, you can monitor this NRCC website: http://www.nrcc.cornell.edu/eab/ mapping GDD just for EAB.
Moreover, you're not by a computer or you can't remember the website....nature has another way of letting us know when to expect pests- flowers. The critical pest life stages have been correlated with native/common blooming plants, so all we need to remember is the signal flower (or Plant Phenological Indicator, PPI). Our forest tent caterpillars match up with redbud and Tatarian honeysuckle; eastern tents with Japanese flowering quince (which is looking stunning here); and emerald ash borer with black locust.
You can look up GDD and PPI for your favorite insect pests at http://ipmguidelines.org/ Just click on the appropriate category.
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