September
2017

 

By
Tim Gibb
 
Professor of
Entomology
Purdue University

 

 

 

09-08-17

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Cicada chorus - the waiting strategy


Dear Dr. Tim,

I have lived in our rural home in southern IN for 16 years now but have never experienced tree locusts like this year. The noise is overwhelming. Locals tell me they are the 17-yr. locust. Whatever you call 'em, they are deafening. It sounds like they are all screaming at the top of their lungs and it is about to drive me crazy. I am also told that they live underground for 17 years before they emerge. Could that be true, and if so, then why? And why are they so upset that they scream so?

Sincerely,
Deaf in Southern Indiana

Dear Deaf,

I will try to write very loudly so that you can hear me over the noise. Your questions are very good and I will answer them as best I can.

Why the waiting and screaming? That is the question.

It does seem like waiting and screaming is what these locusts are all about. It is true that they wait for 17 years and then come out of the ground only to scream their heads off.

As children, we used to play a game called Grey Wolf – you may have played it as well. It is the one where the designated wolf hides in the dark and waits for all of the others to come find it. The whole fun of the game is when it is your turn to be Grey Wolf – finding a really unexpected place to hide – waiting in silence until the searchers are very, very close then at the last second jump out – cry "Grey Wolf" at the top of your lungs and watch as all the little, frightened searchers scatter.

The thrill in the Grey Wolf game is in the wait. Timing is everything. If the wolf jumps out of a hiding spot either too early or too late, it spoils the scare. But if the timing is perfect, there can be nothing better. Timing is critical.

I remember playing the game with all of my cousins after Thanksgiving dinner at my grandmother's house one year. It was my turn to be Grey Wolf and I found the perfect hiding spot – in an upstairs bathroom inside a wicker clothes hamper. It was the perfect size for me. I closed the bathroom door, climbed inside the hamper, reached over and turned the light off, hunkered down in the hamper, and with the lid closed on top of me, I began the all-important wait for the other kids to find me.

After what seemed like forever, I finally heard the door open, and through the cracks in the wicker saw the light turn on. They were getting close. There was some rustling, searching noises. My waiting was about to pay off. They were close, but not quite close enough. Give them just a couple more seconds. Wait for it wait for it ... NOW! I threw open the lid, jumped to my feet, and as loud as I could I yelled "Grrreeyyy Woooolfff!"

Only to find Aunt Alice (Grandma's sister) sitting there. As you can imagine, she was NOT what I expected to see, and apparently, I was NOT what she expected to see either. She screamed, I screamed, then we both screamed together in chorus – kind of like the scenario you describe with your locusts.

They wait and wait and wait – actually 17 years – until they finally emerge, only to engage in a chorus of combined screaming.

Your neighbors are correct and they probably know about 17-year locusts because they experienced them 17 years ago. If you bought your home 16 years ago, you just missed them by one year.

The common name for this insect is locust, but its proper name is cicada. We have quite a number of cicadas throughout the country and most appear on an annual basis, but the 17-year cicada is unique because it occurs only after every 17 years. We refer to these as "periodical" cicadas to differentiate them from the "annual" cicadas that emerge every year. So, the locusts you are describing are technically 17-year periodical cicadas.

Periodical cicadas are about 1 1/2 inches long, have large buggy red eyes, black bodies, orange-colored legs and 4 glassy, transparent wings that have a network of reddish-orange veins. By comparison, their larger cousins, the annual cicadas, have an even more robust shape, black eyes, a whitish underbelly, and distinct green coloration on the head, thorax and wing veins.

periodical cicada

periodical cicada

annual cicada

annual cicada

It is true that 17-year periodical cicadas live underground in their immature nymphal form for exactly 204 months, or 17 years, before they crawl up out of the soil, break out of their nymphal skin, and climb up into trees to begin the obnoxiously shrill noise that you call a scream. Seasonally, periodical cicadas usually emerge in the springtime while annuals cicadas come out later in the summer.

annual cicada breaking out of its nymphal skin

annual cicada breaking out of its nymphal skin

So, now let's tackle your first question. Why the wait? Why do periodical cicadas wait 17 years before emerging? The short answer is "We have no idea," and the long answer is very similar. Some think that waiting is a way that they can confuse predators. After all, what predator is going to wait around 17 years for a meal, no matter how good it might taste? Most predators don't live long enough to even put the next emergence on their calendars.

We do know that it is important for cicadas to emerge together as a large group to help them avoid predators. So the advantage of waiting 17 years to emerge and emerging all together, may actually be linked. This is how.

Imagine that you are a nice, big, juicy cicada and you emerge in a forest all by yourself to begin screaming at the top of your lungs. In most forests, there are more than just a few potential predators such as birds, squirrels, raccoons, possums or other hungry sorts lurking around, and chances are that you will be found and eaten.

On the other hand, if you and millions of your brother and sister cicadas emerge together, the predators cannot possibly eat all of you – there are just too many, and therefore you stand a better chance of survival. Sure, you throw a couple of your siblings under the bus, but hey, if there are millions and millions of them, it is no big loss.

We call this survival strategy "predator satiation." The predators may gorge themselves to the point of being barely able to walk home, and may even do this for several days in a row, but over time they cannot keep up with the huge numbers of emerging cicadas. So, if you must persist in the screaming, emerge as a whole choir, because you stand a reasonably good chance of surviving.

But remember the Grey Wolf game – timing is everything. If you are a cicada, don't be the first to emerge. Emerging too early or too late may be fatal. Make certain that you make your appearance in the middle of the group. Then scream all you want to. Chances are that the predators will find one of your siblings before they find you.

Which brings us to the question about the screaming. Some entomologists postulate that the loud, incessant, shrill screaming that cicadas are famously known for may also act as a predator deterrent. It may be overwhelming. Individually, one cicada can be quite loud, but when in a group, (some estimates are 1.5 million per acre) periodical cicada noises can be quite deafening.

If the noise is driving YOU crazy, just think about the poor predators that have to live in the woods with them. They might go bonkers and leave. While this theory of why they are so loud may be a bit of a stretch, it is the best that I can come up with and worth noodling on.

Cicadas make their sounds by way of a pair of ribbed, membranes called tymbals, located at the base of their abdomens. As muscles alternately contract and release this membrane back and forth, it vibrates, creating the high-pitched sounds that cicadas are known for.

Audiologist types have measured this sound in close range to be in excess of 100 and even up to 120 decibels, which is approaching the pain threshold of the human ear. They compare this to the sound of an ambulance siren or a large speaker at a rock concert (think Metallica), and warn people not to hold cicadas up to their ears, which is pretty good advice, but not necessarily helpful for most people. I have never once heard of anyone actually doing such a thing. Nevertheless, audiologists must feel good because it seems that most rational people seem to be heeding their advice.

Cicada songs, that you call screaming sessions, have another purpose: mating communication. They are species-specific mating calls produced by romantic male cicadas, and can actually be heard by females for up to a mile away.

If you took a crash course in learning to speak "cicada," you might be able to decipher any of several different messages in their screams, from the general "Hey everyone, let's party." to "Hey, the party is over here." and possibly to the more specific "Hey, my parents are not home. Come on over." as an invitation to all cicadas. Then the communication becomes a bit softer and more romantic, directed at just the females, as a preface to choosing a mate from "Hey baby, let me gaze deeply into your beautiful, red, buggy eyes." to the even more intimate "Oh baby, we could make beautiful nymphs together." or "You fill in the blank". Use your imagination – whatever might be required to seduce a particularly good-looking female. But they have to do it fast. Mating season is very brief and if missed, well, there is only the one opportunity.

Now that I have attempted to answer your two main questions, let me answer a couple that you did not ask.

Q. Other than causing deafness, will cicadas hurt you?

twig damaged by cicada

twig damaged by cicada

A. Cicadas do not bite people, but they can cause some damage to trees. After mating, each female typically inserts 400 or more eggs into the twigs of any one of more than 75 species of trees. Such massive cutting into the tender bark can cause the twigs to die back and, in the case of valuable shade and fruit trees, this can become a real problem for arborists and orchardists.

Eggs then hatch and the small nymphs fall to the ground, dig themselves deep into the soil until they find a tree root to attach to and then remain there for 16.83 years, until they emerge as the next generation.

Q. How long do I have to put up with this noise? Or how long do the adults live?

A. The answer could be from a matter of seconds – if the cicada does not heed my advice of not being the first to emerge and it dies because a raccoon is sitting there waiting for it – to a bit more than a month. Adult periodical cicadas only live about 4-6 weeks before dying, dropping down to the forest floor and recycling as fertilizer for the trees. The mass emergence and the final concert is over.

This is the good news. For you, the noise is already finished for this year. The bad news is that now you will have to start anticipating the next round of periodical cicadas.

There are several different 17-year periodical cicada groups (called broods) each with a different geographic range, population size and cycle. The brood that you heard this year emerged in 2000 in several isolated areas of the East and Midwest. You just happened to buy a house near one of these areas.

Unfortunately, this brood is only a shadow, by comparison, to the "Big Brood" that we call Brood X. Brood X is the largest of the 17-year periodical cicada groups. It emerged last in 2004 and was very heavy in many states including New York, New Jersey, Pennsylvania, Delaware, Maryland, Washington D.C., Virginia, West Virginia, North Carolina, Georgia, Tennessee, Kentucky, Ohio, Illinois and Michigan, as well as in Indiana.

Plan for this big brood to emerge in wait for it wait for it 2021!

If you think you had it bad this year, you are "deaf"initely going to remember 2021. Write me again then.

Dr. Tim

(Photo credits: John Obermeyer)

 

Writer: Tim Gibb
Editor: Cindie Gosnell