In the kitchen, recipes tell chefs when to add an egg or turn down the heat under the potatoes. Having a predetermined list of what to do and when to do it makes meals come out perfect and with all the side dishes done at the same time.
In a way, degree-day information and biological calendars can help lawn care operators (LCOs) manage plant care in much the same way that chefs manage their kitchens – by knowing what effect certain environmental “ingredients” will have on the landscape. The only problem is that understanding what degree-days are and how to calculate them is much more complicated than knowing that flour, eggs, sugar and milk make a cake. Thankfully, at the Central Environmental Nursery Trade Show (CENTS) held in Columbus, Ohio, Jan. 26-28, Dan Herms showed several attendees that using degree-day information is easy as pie.
DEGREE-DAYS DEFINED. “Pest management is tied directly to pest life cycles,” said Herms, associate professor of entomology, Ohio State University, Columbus, Ohio. “Because of this, properly timing pest management efforts is everything – especially when pesticides are being regulated more and more. Degree-day information can help us predict pest development so we can make more culturally-based pest management decisions and make those decision sooner.”
By definition, a degree-day (also known as growing degree-days, thermal units or heating units) is the amount of heat accumulated above a specified base temperature over a 24-hour day. Indeed, tracking temperatures is key when it comes to pest life cycles because of what Herms calls “temperature-dependent development.” “Because insects emerge sooner and plants bloom sooner in warmer seasons than they do in cooler seasons, keeping track of temperature and weather patterns is the first step in accumulating degree-day information,” Herms said. He noted that, in Ohio, weather stations around the state are linked to the Ohio Agricultural Research & Development Center (OADRC) where degree-day data for the state is collected. LCOs who access the OARDC can look up degree-day information for their area and find out when certain pests are active.
But how is that data calculated? Herms explained that a base temperature must first be specified in order for degree-days to have meaning. Ideally, this temperature is lower than the “lower temperature threshold,” or LTT, required for certain pests to develop. For example, entomologists know that gypsy moths hatch in mid-April with an LTT of 48 degrees Fahrenheit. From that point, development of gypsy moths will occur only as the temperature rises. “This lower temperature threshold is known for only a small number of species,” Herms noted, “but we do know that 50 degrees Fahrenheit is a good approximation, so we frequently use that as our base temperature.”
CRUNCHING NUMBERS. Herms explained three different methods that can be used to calculate degree-days, using the base temperature (50 F), as well as the maximum and minimum temperatures for a given day.
1. Average Method. This method simply takes the average temperature for the day and subtracts the base temperature from it. On a day with a high of 65 F and a low of 45 F, the math would look like this:
Average temp = (max + min)/2
Example = (65 + 45)/2 = 55
Degree-days = average temp – base temp
Example = 55 – 5 = 5 degree-days
2. Modified Average Method. When the minimum temperature drops below the base temperature, it is possible to substitute the base temperature for the minimum temperature. “This method is generally more accurate than the Average Method because pests don’t get younger as the temperature drops,” Herms said. “In our example the base temperature is 50 F, but the minimum temperature is 45 F. In that five-degree difference, there is no development occurring, so we can substitute the base temperature in the equation.”
Average temp = (65 + 50)/2 = 57.5
Degree-days = 57.5 – 50 = 7.5
3. Sine Wave Method. Much more complicated than the Average and Modified Average methods, the Sine Wave Method uses a sine equation to calculate the number of degree-days above the base temperature. While there is a slight increase in accuracy, Herms noted that the results from this method are often the same or very similar to those achieved with the Average Method when the minimum temperature stays above the base temperature.
DEGREE DAYS, DAY BY DAY. In many cases, degree-days are counted cumulatively, meaning that the degree-day data for a given day is added to the total from the previous days. Usually, this is done over the course of a year, beginning on Jan. 1, but Herms noted that, when looking up degree-day information, LCOs should identify both the base temperature used to calculate the data, as well as the accumulation starting date for cumulative degree-days.
Still, degree-day numbers mean nothing unless they are assigned to something. This is where Biological calendars come into play. “Because both plant and insect development is temperature dependent, the flowering sequence of plants can be used as a biological calendar to time pest management decisions,” Herms said. Part of the biological calendar data recorded by the OADRC between 1997 and 2002 looks like this (DD = Degree-Days):
Referencing this calendar, Herms explained that, more often than not, phonological events follow a distinct pattern. For instance, the Eastern Tent Caterpillar always reaches egg hatch just before the Norway Maples reach first bloom, but after the Border Forsythia first blooms. Moreover, LCOs following the calendar will know that when their cumulative degree-day data reaches the high 80s, Eastern Tent Caterpillar will soon emerge.
While the actual days of certain events will change slightly from year to year, as in the above calendar, “if you simply note what is blooming when you observe a pest, you can duplicate that timing in future years,” Herms said. “Additionally, you can customize the calendar to include pests and plants that are specific to your area. Even one year of observations can be useful.”
The author is associate editor of Lawn & Landscape magazine and can be reached at lspiers@lawnandlandscape.com.