Using Your Landscape to Predict Pest Pressure

By Patrick Anderson

Forsythia blooms as crabgrass starts to germinate - pic by Joye`

Forsythia blooms predict crabgrass germination – pic by Joye`

Though it may seem hard to believe, the first flowers of spring will soon be upon us. It doesn’t take a horticultural aficionado to appreciate these brightly colored petaled reproductive organs of angiosperms. The yellows, whites, blues, reds, and purples are a welcomed site after a gray, cold winter. But, the emergence of certain plant species’ flowers can be the warning signs for the appearance of diabolical plant damaging pests.


The study of such environmental indicators is known as phenology. For those of you who want a more textbook definition, here it is: “phenology is the study of periodic plant and animal life cycle events and how these are influenced by seasonal and interannual variations in climate, as well as habitat factors (such as elevation).” (Wikipedia)

Monitoring the life cycle of plants (leaf expansion, flowering, color change, etc.) has been used for centuries in agriculture to determine the appropriate timing of when certain crops should be sown.  In fact, the first record of phenology dates back to 974 BC (University of Wisconsin).

Growing Degree Days

Complementing phenology are growing degree days (GDD).   GDD are a measure of heat accumulation used to predict plant and arthropod development rates, such as when flowers of a particular species may bloom, or when a particular insect will become active. GDD are calculated by taking the average of the daily maximum and minimum temperatures compared to a base temperature (usually 50°F).

                                         GDD      =        (daily high + daily low)/2     –   50

Growing degree day charts highlighting common tree/shrub pests have been developed by the Ohio State University and Cornell University to name a few. Here is a helpful article and chart to help you use GDD in your landscape. You can monitor Kentucky’s GDD here (Be sure to the “base” temperature to 50).

Black Locust flowers predict EAB activity - pic (edited) by Paige Filler
Black Locust flowers predict EAB activity – pic (edited) by Paige Filler

The use of phenology and GDD is an important monitoring tool for integrated pest management practices in the landscape. The activity of many plant damaging pests coordinates to either the flowering or leaf expansion of common shrubs and trees, some of which may be in your backyard.

A Few Examples

For example, when the hanging white racemes of black locust flowers appear (500 GDD +/-), the invasive emerald ash borer adults are beginning to emerge.  The first flight of the adult dogwood borer corresponds to about 2 weeks after peak dogwood bloom (850 GDD +/-). Eight weeks after the full leaf expansion of red maple (1000 GDD +/-) the most vulnerable stage of gloomy scale appears.  These are just a few examples of using phenological indicators to predict the emergence of tree damaging arthropod pests.

Limbwalker keeps track of GDD each week as knowing when insect pests are active allows Limbwalker’s Certified Arborists to make precise treatments, which limits the amount of products applied in landscapes.  This also reduces the chances of harming beneficial, or benign, critters that make our plants their home. Being in tune phenology and growing degree days can be invaluable when developing a plant health care plan for our landscapes.

If you have any questions about insects or other lawn and landscape pests, please contact Limbwalker at 502-634-0400.


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