The cicadas are coming! Millions of them. The country is buzzing this year for an epic, entomological event: the emergence of two different broods of periodical cicadas. One of the broods appears every 13 years; another group — known as Brood XIII — appears every 17 years. The last time the two broods emerged simultaneously was 1803, long before Wisconsin was even a state.
Southern Wisconsin is home to Brood XIII, which will soon carpet the ground with exoskeletons and fill the air with raucous buzzing. These differ from so-called "dog day" cicadas, which also call Wisconsin home and appear each summer. To learn more about the periodical cicadas, I spoke with PJ Liesch, an entomologist with the University of Wisconsin-Madison extension.
This interview has been edited for length and clarity.
What can we expect this summer?
In just a few weeks, for about a month or so, things are going to get pretty noisy in some parts of Wisconsin. And that's because of the emergence of our Brood XIII periodical cicadas, which have a 17-year lifecycle. They were last out in 2007. And prior to that, back in 1990, and you can keep counting backwards all the way into the 1800s.
These insects are really fascinating. Because they emerge at these long, extended time intervals of 17 years — it's a little mind-boggling to think of an insect living that long. They come in at these long time intervals and also in massive numbers.
That might be part of a survival strategy. These insects are fairly clumsy, so vertebrates like birds, mammals and fish are going to gorge themselves on these things. Plenty of cicadas will be eaten, but there's going to be lots more that can still survive and reproduce. That makes these insects pretty unique. Some parts of the state will have them coming out in huge numbers — hundreds of thousands, if not millions, in relatively small areas.
That is so cool. What kinds of conditions are the cicadas waiting for?
These insects have been feeding below ground for essentially 17 years. They feed on the roots of trees. And in that 17th year, they're waiting for the soil conditions to be just right. A key factor for emergence is the temperature of the soil at a depth of eight inches. That has to get above 64 degrees Fahrenheit. I've been checking weather stations around the state. At the moment, soil temperatures in southern Wisconsin are in the mid- to high-50s. We're getting close. A couple more weeks, and we're going to be in that temperature range that they need.
That won't necessarily trigger it immediately. It primes the system, so to speak. But around that same time, if we get a decent rainfall and that rain percolates down into these channels, that also seems to spur activity.
The cool thing about this, I think, is that it almost lets you time travel back to where you were the last time this happened. Where were you 17 years ago? Do you have cicada memories?
Unfortunately, I do not. I actually turn 40 years old next year and I have never seen these before with my own eyes.
Aw, man. And you’re a Wisconsin native?
Yes, I grew up in southeastern Wisconsin in Racine County. Back in 2007, when they were last out, I missed them because I moved from Racine County to come to Madison to start graduate school. So I'm kicking myself, I missed them that year, I missed them in 1990. I'm going to make sure I see them this year and make sure my kids get to see them too.
It's a little bit like that solar eclipse. Folks drove across the country to experience something like that. Well, these cicadas in Wisconsin, because they only come out every 17 years, you might only have a handful of opportunities in your entire life to witness these insects coming out in these tremendous numbers in the Badger State. That's really unique and really cool and really special.
It is really special. So they spend most of their lives underground. What does the best summer ever look like to a periodical cicada? How does emergence fit into their life cycle?
Only adult insects can reproduce. So they are juveniles below ground. They are going to come out of the ground, crawl out, and then the juveniles are going to climb onto vertical surfaces — that could be the trunk of a tree or a fence post. They are going to molt and leave that old exoskeleton behind. You'll see those around; those will be evidence of molting. Once they molt and they transform to the adult stage, it takes a little while for them to darken and harden. Those adults will only be out for about a month. Really, 98% or so of their entire life is spent below ground in the dark, feeding on tree roots.
They come out for about a month as adults. The main goal that they have is to find a mate. And the way to go about doing that is to make a lot of noise. Specifically, the males have these membranes on the side of their bodies called tymbals. It's like the membrane on a drum. They can use muscles to cause vibrations, and that produces the sound. So for this month or so while they are out, it's the males singing — or screaming — to attract the attention of females. The females can't sing back. But at a close range, the females can snap their wings. It's going to sound just like someone snapping their fingers.
So you get this back-and-forth calling between males and females. Eventually, they mate. The females will lay eggs. And a few weeks later, they're done and gone for 17 more years.
And the young cicadas will head back underground for the next 17 years?
Not immediately. [The females] are really fond of twigs on trees that are about the diameter of a pencil, give or take. The female has this egg-laying structure that, under the microscope, it looks like a spear or a pocket knife. She'll literally cut slits into these twigs, and then insert batches of eggs and those eggs will sit there for many weeks, maybe six or so. Then these very small juveniles come out of those eggs, and they drop down to the ground. They rain down to the soil, but they're so small, it's probably going unnoticed. They burrow below ground where they're going to hang out for 17 more years.
When they're underground are they in a dormant state?
It would depend on the time of the year. They are actively feeding and growing and developing over the course of those 17 years below ground. These insects, both as juveniles and adults, have tubular mouthparts, somewhat like a mosquito. Except they use these to poke into and pierce tree roots, and they drink sap from those trees. So the juveniles are actively feeding on tree roots. When the temperatures decline in the fall and winter months, it's too cold and their activity is essentially going to seize. But then during the warmer months, they're going to be actively feeding and growing and developing.
What is this lifecycle an evolutionary response to? Why do we think that they do this?
That's a great question. We don't fully understand. We can speculate and make some educated guesses, but there's some unknowns about their biology. One thought, though, is that if you are an insect that emerges in tremendous numbers on a very regular basis — say every year or every other year — animals that feed on you are going to catch on. If raccoons and skunks and birds know they can rely on you on an annual basis, their populations are probably going to increase. That's probably going to decrease your populations, because there's more of them to feed on us cicadas.
But if you come out at these very long timescales, most other forms of wildlife — vertebrates, birds and mammals — probably aren't living quite that long, so they can't catch on to this pattern. It’s thought that evolutionarily, it might help with their survival. Again, it goes hand in hand [with the idea that] you emerge at these long time intervals, but you do so in such massive numbers that you overwhelm, and essentially satiate, anything that feeds on you. They're going to have full bellies, and there's still plenty of cicadas around to mate and successfully reproduce.
What kinds of threats do cicadas face? I'm thinking about all the development that could happen in a 17-year period or how climate change might throw them off.
My impression is that on shorter timescales, in our lifetime, probably the biggest immediate threat to them would be land use changes. Because the cicadas feed on tree roots, those trees need to be there for very long periods of time. If those trees are cut down for an agricultural field or a parking lot or housing or whatever, the cicadas that were feeding on those roots will die out. Over time, the range of these insects is probably shrinking due to land use changes.
Here in Wisconsin, for example, we historically have had records in counties like Waukesha and Milwaukee County, but I have not seen any confirmed reports from counties like that in decades. We had some documented here in the Madison area, but the last record of those I could find was 1956. They haven't been spotted in 60 to 70 years at this point. So those land use changes can be a really important threat to cicadas in a shorter, immediate time-scale.
When you think about climate change, we don't fully know what that is going to mean for cicadas. One thing that is almost certainly going to happen is if we have a changing climate — earlier springs, as an example — that's probably going to shift their emergence time. So they'll pop out and emerge earlier in the year. What will the impact of that be? It's really hard to tell at this point.
One other potential complication, though, is if with climate change, we get more extreme weather swings. If you had years where you maybe had an early warm-up in say, February, and then it got brutally cold again in March — these weird weather swings, that might have some potential impacts on these insects. In the long run, there's been some hints at that in the literature. If that has an impact on the host trees and some changes in the phytochemicals within the plant, that might have some subtle impacts on the cicadas too. But for the most part, we don't fully understand what these changes will mean for these insects in the long run.
So you've been poring through all of those historical records, trying to understand where they'll occur. Where can folks in southern Wisconsin go to experience this themselves?
There's a number of spots tucked around. What I did this last winter, I dug through about 150 years of old newspaper articles, government reports and other documents to try and find our best idea of where these occur. One really important thing to understand about these insects — and this is often overlooked — is they can be really geographically isolated. I've been telling folks, their distribution, it's like pins on a map. Some of the maps will shade in entire counties, giving the impression that they're widely distributed everywhere, which isn't the case. They’re restricted to individual isolated geographic spots.
In southern Wisconsin, perhaps the single best spot to see them would be the Lake Geneva area. I have seen records from that part of the state going back well into the 1800s. We also know there's some decent populations near Beloit, near Janesville. There's one particular park just northwest of Janesville, it used to be a Boy Scout camp. Now it is Rock River Heritage County Park. There was a strong emergence noted there in 2007. We've got some activity in Spring Green and a lot of tucked-away spots in Iowa County as well.
If you want to know more about the distribution, earlier this year, I released a periodical cicada website for Wisconsin. I have a page dedicated to when and where they occur. It includes a map with this research and also a column going county by county, talking about some of the important locations.
Are there things that we don't know about cicadas that entomologists like yourself hope to use this summer to explore?
That's a great question. My main interest at the moment is to get a better feel for where these insects really occur. In Wisconsin, again, we talked about some of the threats that might be impacting where they occur in the state. Maybe they were in spots historically, and no longer occur there. We know that's the case in Madison.
So on that cicada website that I created, I developed a community science project. Folks can go on there and submit sightings. I'd really love to hear about any sightings to help fine-tune our map and better understand where these occur.
But there are some other really cool research questions out there. One thing that I've heard discussed recently, that we just don't fully understand, is how do cicadas know to all emerge at the same time? We talked about soil temperatures and rainfall [which] can influence them. But how do they really know to emerge at the same time? What if some emerged early? Those would probably get picked off by birds or other wildlife. That is a mystery we don't fully understand at this point. I'd love to see someone try and figure that out.
That is really exciting.
One thing — and this is a common question that I've gotten from the public — is are these cicadas harmful or dangerous in any way? Wildlife is going to gorge themselves on these things. They are pretty palatable. Humans can even eat them. They're really quite harmless to people. They don't bite, they can't sting you or anything like that.
They technically can cause a little bit of damage to plants and that is when the female cicadas lay their eggs. Again, she uses that spear-like egg-laying structure called an ovipositor. She cuts these slits in small twigs, and that can cause those twigs to die and break off, so those leaves will turn brownish. We call it flagging. If this occurs on a large tree, it's going to be noticeable, but it really doesn't have an impact on the tree's health in the long run, so you don't need to worry about it.
Perhaps the only situation I would be concerned about is if I lived in a known cicada hotspot, and I had recently planted trees. If cicadas end up harming the outer, maybe six to 12 inches of those twigs on a big tree, that's a drop in the bucket. But on a very small, young tree, that might end up being a pretty high percentage of the overall tree's canopy. So if you have a recently planted tree, I suggest covering it with fine mesh netting and tying it along the trunk of the tree. That will physically keep the insects off. If the cicadas can't get to the tree, they can't harm it.
One other thing is if you live in a known cicada hotspot, and you're planning on planting a tree this year, I would probably wait until July when the cicadas are going to be done for the year.
Good to know. What are you personally looking forward to?
The thing I'm most excited about is simply to experience this phenomenon. In Wisconsin, you can live here your entire life and you might only have a handful of opportunities to see this insect in the state. I turn 40 next year. I could have seen them twice already, but I've missed it both times. So I'm really excited to see them, to experience the sheer numbers of them and to experience their singing. It’s this otherworldly, science fiction-esque sound. I'm really excited to see and hear them with my own eyes and ears.
For more information on where to experience the Brood XIII cicadas, view Liesch's county-by-county rundown here. Then, participate in a community science project by sharing your sightings here.
