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The Garden Scoop

Using Phenology for Ornamental Pest Control

Sick tree

The timing of spring can vary each year based on weather, but there are always ques in nature that line up in similar fashion year after year.  While climatic conditions on exact calendar dates my vary, the sequence of emerging biota is consistent and based a long evolutionary history which has carefully lined up everything from blooming plants to hatching insects and birds. 

What is phenology?

Phenology is the study of cyclical patterns in the natural world that link climate, plants and animal life.  Through the study of this sequence of events, occurrences in the natural world can be predicted seasonally with better accuracy than calendar date alone. 

Historically, phenology has long been used to make decisions in plant management.  Our ancestors did not have the technology we enjoy today, yet they were able to effectively manage many agricultural crops often relying on cues from nature.

Whether historically accurate or not, many sources report the use of oak leaves to time corn planting in the Americas.  In this example, the proper timing for planting occurred when oak leaves were “the size of mouse ears”.  Reportedly, Native Americans taught this relationship to early American settlers who used this knowledge to time crop planting. 

In nature, many of these synchronous phenological relationships were necessary for existence.  For example, caterpillar emergence of many species must be timed to coincide with succulent, tender new growth of host plants. If caterpillars emerge too late, mature leaves may be too tough and leathery for the tiny caterpillars to consume quick enough to fuel the excessive growth they will experience in a relatively short period of time.

Blooming catalpa trees are indicator plants for bagworm crawler emergence.
Blooming catalpa trees are indicator plants for bagworm crawler emergence.

Phenology and pest control

One fascinating and highly effective use of phenological relationships relates to pest management.  For many pests, both plant and insect, the timing of control measures is matched to the most vulnerable life stage of the pest.  This careful timing can make treatments more efficient or, in some cases, may be necessary to achieve control. 

Bagworm control on ornamental plants is a good example of the use of phenology to meet a critical treatment window.  These damaging, leaf-feeding insects must be controlled as larvae, called crawlers. Control treatments simply are not effective once they mature and enter their protective bag structure, unless you are willing to hand-pick each bag which may not be practical on larger plants. 



Each spring, as bagworm crawlers emerge there are several plant ques that can be observed to time control.  The peak bloom of both catalpa (Catalpa speciosa) and Japanese tree lilac (Syringa reticulata) coincide with crawler emergences.  In this same phenological timeframe, smooth hydrangea (Hydrangea arborescence) will be in early bloom and Allegheny serviceberry (Amelanchier laevis) will have an initial portion of its fruit crop ripe with less mature, unripe fruits on the plant as well.

One or all of these phenological indicators can be used to time control treatment of bagworms.  In this case, proper timing is essential for effective control and crawlers are not always easy to observe, or at least not as easy to observe as canopies of flowers or ripening fruits. 

In addition, observation of phenological indicators can be used to better time treatments with respect to site specific conditions.   There is considerable variability between rural and urban locations in central Illinois.  I have commonly observed flower emergence of many species up to one week earlier in urban areas when compared to rural locations.  Since indicator plants at a given location report exposure to site-specific conditions via their phenological timeline, when present, they can be used to better assess treatment timing at that location.

Other Resources

 “Coincide: The Orton System of Pest Management” was written by Donald A. Orton, who spent his career documenting phenology of both pests and indicator plants.  Using his extensive dataset of observations in Illinois, Mr. Orton compiled a manual which uses phenology of common ornamental plants to time control treatments of various pests.

Currently, this manual is out of print making it somewhat hard to come by unless you know someone with a copy.  The Funk ACES library on University of Illinois campus and other local libraries have copies on the shelf or used copies can sometimes be found online. I highly recommend this reference as the dataset behind it is unparalleled and Mr. Orton’s system of pest management is a unique way to more efficiently control pests using seasonal ques that are all around us. 

Growing Degree Days (GDD's)

Growing Degree Days (GDD’s) are a unit of measure that expresses accumulated heat over the growing season, as opposed to a time unit measurement as its name implies.  The relationship between GDD’s and phenology has been defined for various plants, insects and other biota.   

While indicator plants are a real-time measure of GDD’s based on their phase of development in spring, GDD’s can be calculated using weather data as well.  The Illinois State Water Survey’s Water and Atmospheric Resources Monitoring Program (WARM) has an excellent, data-filled website.  The WARM website contains current and historic weather data and features a Growing Degree Day Calculator for Illinois

The Morton Arboretum publishes a trove of Plant Health Care Reports each growing season.  These indispensable reports include weekly GDD’s calculations for various locations around the state as well as timely reporting on disease and insect pests present in northeastern Illinois at the time of publication.  Reports are viewable at the link above or you may subscribe to email alerts by contacting Sharon Yiesla at

GDD calculations for plants and insects

Michigan State published a nice table of GDD for Forecasting Landscape Insect Life Stages.  Other tabs available on the website above explore GDD’s in more detail and list conifer pests development according to GDD’s. 

Cornell University lists GDD values for various common landscape plants by stage of development thought out the growing season.  Their table lists GDD dates for beginning bloom, full bloom and end of bloom periods.

The IPM of New Hampshire website includes an insect development timing chart which lists various landscape pests and the typically timing of their development by calendar date in New Hampshire, but also lists GDD’s for each pest which can easily be related to Illinois GDD’s.