University of Maryland Extension

How to Use the Pest Predictive Calendar

Beginning in early spring, this table predicts the sequential phenological events of plants and pest insects and mites, and the cumulative growing degree days that correlate with each phenological event. Columns one and two list the plant or pest insect and mite by common name and taxonomic name, respectively. Column three is the phenological event of the plant (flowering stage) or pest (target stage for control). Column four is the cumulative number of growing degree days (GDD).  Cumulative degree-days were calculated using a base temperature of 50°F and a starting date of January 1st. Insect and mite information is in bold. The fifth column provides links to detailed information (ex. fact sheets or articles) on or images of the specific pest or plant.  GDD and plant phenological indicators (PPI) are used to predict when the susceptible stage of a pest will be active (pest phenology). If the phenological event is: “adult” this means 1st adult activity; “adult emergence” as with clearwing borers such as lilac borer or lesser peach tree borer this means 1st adult emergence and 10 to 14 days from that date protective bark sprays should be applied to the host trees; and “egg hatch” means for caterpillars first instars are active and for scale insects that first crawler activity began. 

Using Eastern tent caterpillar (ETC) as an example, scroll down the table until you see ETC and note that egg hatch is the phenological event of interest. Control measures should target early instar caterpillars. Look at which plants, and their bloom phenology (first or full bloom), are just before or just after ETC on the table.  For example, border forsythia in full bloom or spicebush at first bloom. When these plants bloom in the landscape or nursery, you can predict ETC eggs are hatching and time control measures to treat the early instar caterpillars.  Alternatively, the table states ETC eggs hatch when 62 GDD have accumulated. Determine the GDD for your area (see below for how to determine GDD) and when your area is close to 62 GDD you can predict ETC eggs are hatching and time control measures. Be sure to monitor for young caterpillars to assess if ETC eggs have hatched in your local micro-climate and implement control measures only if warranted. 

The table can also be used to predict what pests will be active in the near future.  For example, determine the GDD for your area every week and/or what plants from the table are in bloom.  Scroll down the table to the section where the GDD are near what they are for your area or the plant phenology is the same as what is happening in the field.  You can predict that the insects and mites that are in the same area of the table will be active (ex. similar GDD).  By looking ahead on the table you can predict what will likely be active in the upcoming days. This allows for more efficient monitoring and timing of control measures.  For example, if there were ~275 GDD accumulated in your nursery or dwarf azalea were in first bloom, you should be monitoring for activity of spruce spider mite (adults and nymphs), and pine needle scale crawlers as indicated by the table.

To determine GDD for a specific location

Go to website and follow the directions listed below in this article.

Use the following information to calculate GDD for your site:

Select your location from the mapModel Category: All models   
Select Degree-day calculator Thresholds in: Fahrenheit °F    Lower: 50    Upper: 95
Calculation type: simple average/growing dds  
Start: Jan 1

Note that this Pest Predictive Calendar is based on biological and climatic activity from specific locations.  Microclimates can vary, therefore; the GDD in the table should provide a close, but not necessarily exact, estimation on the timing of the reported phenological events.  Information in this table is based on 2 or more years of data sampling in Maryland.  Our UME team will continue to add pest species, flowering plant phenology and GDD to the Pest Predictive Calendar to increase its usefulness.

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