The Cicadapocalypse – Part 2

How do 13-year cicadas know when it’s time to emerge? Much of Missouri is inundated right now with noisy 13-year cicadas (the 17-year cicadas are mostly to the north and east of Missouri). BIG QUESTION: How do the cicadas know 13 years have passed? Seriously, most of them emerge within a few weeks of each other, after living underground in isolated darkness and silence for exactly 13 years. It’s mind blowing.

First, let’s look at WHY cicadas emerge all at once. This answer is simple—they do it to avoid being eaten before they have a chance to mate. Cicadas are not only defenseless, they are tasty and nutritious. Other animals snarf them up like a Las Vegas buffet. Well, cicadas need to emerge at the same time so that they can find each other as adults (so they can mate). The problem is, emerging at the same time creates a huge feeding frenzy among their predators. The solution? Emerge in such massive, incomprehensible numbers that predators cannot possibly eat them all. This is why we have periodical cicadas, a group of nine species of cicadas (out of more than 3,000 species worldwide... the others are annual cicadas) that time their emergence so that there are simply too many for predators to eat. This behavior is a rare and amazing adaptation.

Okay, back to the question of how periodical cicadas know when exactly 13 (or 17) years have passed. As it turns out, we still don’t understand the whole story. We DO know that cicadas emerge when the soil temperature (at 12 to 18 inches deep) reaches about 64ºF in the spring. But that doesn’t explain why 13-year cicadas refuse to obey that urge every spring until the 13th year of their life.

The leading hypothesis suggests cicadas are using tree chemistry to mark the years. For 13 (or 17) years, cicada nymphs live underground, slurping up the juices from tree roots. Each spring, when the trees produce flowers, the juices become rich with amino acids. The cicadas, of course, can detect this chemical change. Here’s proof: researchers took some 17-year cicadas that were 15 years old. The researchers altered their trees so the trees would flower twice in one year. As a result, the cicadas emerged a year early (at 16 years instead of 17 years). Proof that the cicadas count the number of times the trees flower, based on the surge of amino acids in the root juice.

However, we still don’t know how the cicadas TALLY these annual events. Do they put little chalk marks on the wall of their burrow? Doubtful, but there has to be some explanation, right?

Gotta love mysteries like this!

Photo Credit:
13-year cicada - Stan C. Smith