How to determine soil temperatures to monitor when periodical cicadas should start to emerge.
Periodical cicadas usually start to emerge in early to mid May when the soil temperature reaches 64ºF. In 2017, a few cicadas emerged as early April 26, but large emergences began on May 16.This is almost two weeks earlier than occurred before 1950.
You can determine the soil temperature using a temperature probe, or you can estimate the soil temperature from weather data. Research placing temperature sensors at cicada depths at the emergence years found that it is possible to use the high and low temperatures for the preceding three days to obtain an estimate of the soil temperature.
To estimate soil temperatures, you need the daily high and low temperatures for the previous three days. From that you can determine the average daily temperature by adding the high and the low temperatures together and dividing by two.
Next, calculate the two-day and the three-day running temperature averages.
Finally, find the average of the two-day and three-day running average, and that will provide you with an estimate of the soil temperature at the cicada depths.
Example: To determine the soil temperature for Thursday, obtain the high and low temps for Wednesday, Tuesday and Monday. Lets say the temperatures for Wednesday were 71º for the high and 44º for the low, Tuesday was 66º and 39º and Monday was 63º and 41º.
The average temperatures for Wednesday was 57.5º, Tuesday 52.5º and Monday 52º.
The three-day running average will be 54.0º and the two day running average will be 55º.
The soil temperature estimate will be 54.5ºF.
Using long range forecasts will permit you to estimate soil temperatures up to ten days in advance. Of course, these estimates will only be a good as the weather forecast, but they can be helpful in short term planning for cicada emergences. Note also, that studies indicate that exposed south facing cicada areas will warm up before the more shaded areas. Thus the above soil temperature model is more predictive for the start of the major emergence, not when the first few cicadas will emerge from the ground.