Observations on Phenology

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Amy Stone (OSU Extension, Lucas County) posted an informative BYGL Alert earlier this season on Growing Degree Days (GDDs) [click this hotlink: https://bygl.osu.edu/node/1448 ].  She highlighted and described our Ohio State Phenology Calendar that was developed by Dan Herms (Davey Tree) and during his time with us at OSU [click this hotlink: https://www.oardc.ohio-state.edu/gdd/default.asp ].


Here's a quick review of the two components of the OSU Phenology Calendar:  GDDs and Plant Phenology.  GDDs are a mathematical measure of "heat units" accumulated over time, so they are commonly referred to as accumulated GDDs.


Accumulated GDDs are used to predict biological events that are governed by temperature such as the appearance of damaging stages of insect pests, or the bloom of certain plants.  This mathematical tool has been with us in one form or another since the 1950s.


The predictive value is much more accurate than using calendar dates because the seasonal appearance of important events can vary from year-to-year.  For example, over the past 17 years in southwest Ohio, the overwintered eggs of the eastern tent caterpillar moth (Malacosoma americanum) have hatched as early as March 17 and as late as April 9.  Knowing when the eggs are hatching is important if you're targeting early instar caterpillars using products based on the naturally occurring bacterium, Bacillus thuringiensis serotype kurstaki (Btk) which works best on small caterpillars.


Eastern Tent Caterpillar


What did people do before there were thermometers; before there were calendars; before we even had mathematics?  They used observations of one biological event to connect to another important event.  For example, my Native American great, great grandmother taught my family to "plant corn when oak leaves are the size of mouse ears."  My grandfather used a huge white oak (Quercus alba) at the end of one of our pastures to time planting sweet corn.


We were using the white oak as a phenological indicator plant.  "Phenology" is the practice of relating one recurring biological event to the timing of another event.  The term has its origins with the Greek word “phaino” meaning “to show,” or “to appear."  Of course, the connector between the events is temperature which is where phenology and GDDs cross paths.



Names Matter

I've found our OSU Phenology Calendar to be very useful for increasing my success in taking pictures of particular stages of insect development.  Instead of making multiple visits to various sites in the hope of catching eggs hatching or adults emerging, I use accumulated GDDs coupled with phenological indicators to guide my visits.  It's become a real time saver.


Of course, I've also found that names matter.  We used a white oak to time planting corn, not a red oak.  And it was a true white oak, not just a tree in the "white oak group."  I've found that the greatest challenge with successfully using phenological indicator plants is making certain we're observing the correct plant!


The first phenological event listed in the Phenology Calendar is the first bloom of silver maple (Acer saccharinum) at the accumulated GDD of 34.  Full bloom of red maple (A. rubrum) occurs at 75 GDD and is a good predictor of when overwintered white pine weevil (Pissodes strobi) females start moving from the duff beneath trees to main leaders where they feed and lay their eggs (84 GDD).  This is the "straight species;" not a red maple cultivar and certainly not a hybrid with silver maple.






Maple Bloom


White Pine Weevil


I also use the full bloom of red maple to get prepared for taking pictures of eastern tent caterpillars hatching from overwintered eggs (GDD 92).  I've refined the timing by also paying attention to the full bloom of Corneliancherry dogwood (Cornus mas) (GDD 98).  This "bracketing" strategy has never failed me.  Of course, note that I'm using a specific dogwood, not just "full bloom of dogwoods."




Corneliancherry Dogwood Bloom


Non-native boxwood leafminer (Monarthropalpus flavus) midge fly adults emerge at a GDD of 440; doublefile viburnum (Viburnum plicatum) reaches full bloom (444 GDD).  It's important to monitor the right viburnum!  Koreanspice viburnum (V. carlesi) reaches full bloom at 205 GDD.  While Koreanspice is a good phenological indicator plant for the first egg hatch of the non-native viburnum leaf beetle (Pyrrhalta viburni) (210 GDD), it blooms way too early to be a useful predictor of boxwood leafminer adult emergence.






Doublefile Viburnum


Boxwood Leafminer




Sequence Matters 

The Phenological Calendar is linear, just like astronomical calendars.  Astronomical calendars are governed by the unidirectional movements of our Earth spinning on its axis and orbiting the sun; the Earth never moves backward.  Thus, the chronological sequence of days and months never moves backward.  March always follows February and February 3rd follows February 2nd.  Unless you're Phil Connors.


Likewise, the Phenological Calendar does not move backward.  Just as March follows February, the predicted sequence of phenological events remains the same from year-to-year.  The full bloom of red maple (75 GDD) never happens after eastern tent caterpillars hatch from eggs (GDD 92).  If you think you've seen this, you were looking at the wrong maple.


The events depicted in the OSU Phenological Calendar are governed by temperature which is quantified using GDDs and qualified using plant bloom.  However, a cold day doesn't mean we lose some accumulated GDDs for the same reason that it doesn't cause flowers to draw back into flower buds.


On the other hand, temperatures do affect speed.  Hot temperatures are like pressing a car's accelerator pedal and cold temperatures are like hitting the brake. The rate of accumulated GDDs can speed up and slow down.  A warm spring means GDDs accumulate faster compared to a cool spring; plants bloom earlier.  In COVID-19 parlance, the curve steepens.


This is where the comparison to an astronomical calendar falls apart.  The chronological sequencing of phenological events remains the same from year-to-year, but the speed at which they arrive can vary considerably from year-to-year.


However, localized micro-climates can muddy the waters.  I've sometimes seen differences in the timing of phenological events in Greater Cincinnati between the hilltops and nearby river valleys.  The plant flowering sequence within a given location was consistent with the Phenological Calendar, but the arrival of bloom events across the locations were different.  Of course, it's always important to remember that names matter.




This brings up a wrinkle with using our OSU Phenological Calendar.  You can find your local GDD by typing your zip code into a field on the Calendar website.  However, if you click on the "Weather" tab at the top of the website page, you'll see the CFAES Weather Stations aren't located in post offices. 




Does this matter?  In terms of absolute GDD accuracy, yes.  However, it matters less if you consider the value of predictive mathematical models.  During Ohio COVID-19 updates, Dr. Amy Acton (Director, Ohio Department of Health) has been demonstrating the effective use of mathematical models to predict an outcome.  Although Ohio has not yet "flattened the curve," Dr. Acton has been presenting a gradually flattening curve based on extrapolations derived from a mathematical model.


The same approach applies to our Phenological Calendar.  For example, I'm based in Cincinnati and the two closest CFAES Weather Stations used to calculate GDDs for my location are in Clark and Pike Counties.  They aren't close by, but the temperature data is coupled with isothermal data in the mathematical model to predict what is happening in Cincinnati.


Of course, no mathematical model is ever perfect.  However, the results provide useful touchstone information for monitoring with the timing being further refined using phenological indicator plants.  I use GDDs to provide a "heads-up" on what's about to happen; I start looking at my personal calendar to plan for my drives.  However, I use phenological indicator plants to tell me when I should get in my car and drive and to confirm that my arrival at a location will be rewarded.


Finally, accumulated GDDs should be used to schedule your appointments for monitoring and phenological indicator plants used to confirm what you're expecting to observe.  However, neither of these predictive tools should be used to schedule insecticidal treatments.


Properly timed insecticide treatments should be based on monitoring.  You need to assess what's happening with your own eyes, not what appears on a computer screen.  Second, insecticides should only be used if pest populations threaten the overall health of a tree.  There was a time when it was common to base applications on calendar dates.  Thankfully, the days of blindly applying insecticides to woody ornamentals based solely on calendar dates are gone.


However, if you look closely at our OSU Phenological Calendar, you will find there is one notable exception regarding using the Calendar to time an insecticide application.  You will see that the first bloom of black cherry (Prunus serotina) (GDD 368), which is when flower buds are first showing color, is followed by "Optimal time to spray for gypsy moth" (GDD 370).  Also, note that the full bloom of Ohio buckeye (Aesculus glabra) occurs at GDD 374; a nice example of "bracketing" for an event.






I recently talked with Dan Herms about this and he said the specific gypsy moth (Lymantria dispar) recommendation came after many years of working with the Ohio Department of Agriculture to better time aerial suppression applications targeting the caterpillars.  Aerial applications require a considerable amount of logistical planning.


Spray Plane


The Calendar shows that gypsy moth eggs hatch at a GDD of 192.  Eastern redbud (Cercis canadensis) has long been a dependable indicator plant for egg hatch with first bloom occurring at 191.  However, this is far too early for suppression applications.






Gypsy Moth


Dan noted that the exception to the rule was only added after multiple years of data consistently showed black cherry to be a very accurate predictor of when gypsy moth caterpillars arrive at a developmental stage that is most susceptible to the suppression products.  The important operative words are "multiple years of data."