Degree-Day Prediction of Adult Emergence of Photinus carolinus

The male display of Photinus carolinus, informally known as the synchronous firefly of the Smokies, attracts thousands of visitors to the Great Smoky Mountains National Park. On peak nights, masses of flying males emit their flash signal (4 - 11 flashes repeated every 12 - 13 seconds) and dark interval in synchrony. Activity lasts from 9:30 p.m. to midnight. Within days after peak activity, the population drops rapidly.

For both scientific and conservation management reasons, it is important to be able to predict the first emergence and peak nights of P. carolinus. In 2004, an estimated 25,000 people visited the Great Smoky Mountain National Park in 2004 to view the synchronous fireflies. Since 2005, the National Park Service has instituted conservation measures to protect the fireflies and their environment by prohibiting private vehicles, limiting crowd numbers, and providing guided tours. In the past, planning for these events, as well as planning by visiting scientists was difficult because peak display could occur any time between June 3 and June 21.
Lynn Faust, who has been observing these fireflies for more than 17 years, decided to see whether degree-days could be used to predict the emergence and peak activity of these fireflies. Often used to predict when plants like fruits and grasses will bloom, this technique measures the amount of heat a location receives starting at a predetermined date. Degree-days come in a variety of models, the most common being the maximum / minimum model. The formula for this model is:

DDi = [ (Tmax + Tmin) / 2 ] - Tbase

DDi = degree days for that day
Tmax = maximum temperature for that day
Tmin = minimum temperature for that day
Tbase = a predetermined standard temperature

Degree-days are calculated for each day, starting from a certain date, and added together to give total degree-days. Different models use different starting dates. The predetermined temperature (Tbase) can also vary in different models. The idea is to use a number of different models to calculate degree-days up to the event you are trying to predict and see if one of these models can accurately predict your event year after year.
The model that worked best for predicting four events of P. carolinus — first males seen, first females seen, good male activity (50% of the males are active), and peak male activity — was a modified version of degree-days, using two different formulas, depending on the minimum temperature (Tmin). In this model, degree-days were collected starting on March 1. The temperature (Tbase) was 50°F. If the minimum temperature for that day was below 50°, then 50° was substituted for Tmin. If the minimum temperature for that day was above 50°, then the minimum temperature was used for Tmin.
Using this formula,

• Males could be expected to emerge when the degree-days totaled 838.8;
• good male activity could be expected when the degree-days totaled 992.5;
• females could be expected to emerge when the degree-days totaled 1068.8; and
• peak male activity could be expected when the degree-days totaled 1094.2.

The results in this study led to the following conclusions:

1. The model described is simple enough and the data accessible enough to be of use to most people who are interested.
2. The degree-day model allows for more accurate prediction than using a calendar.
3. Degree-day values can be easily converted back to calendar dates for planning purposes and to communicate to the public and media the best dates for firefly viewing.