Sunday, June 2, 2024

Periodical Cicadas--Cicada Tourism

I flew to St. Louis to see the periodical cicadas, brood XIX, 13-year periodical cicadas. Why? Because they are a wonder of the world. No other organism spends 12 or 16 years underground to emerge, mate and lay eggs in the 13th or 17th year, with all of the periodical cicadas in an area synchronized at these very long intervals. Nothing in the world comes even close to it. How could this evolve? They are found only in the eastern U.S. from just west the Missouri River east almost to the Atlantic Ocean, from Wisconsin and New York south to Louisiana, Alabama, northern Georgia, South Carolina and North Carolina, and nowhere else in the world. I somehow was never in the right place at the right time to see them emerge. 

periodical cicada, St. Louis, MO
periodical cicada, St. Louis, Missouri

I have some "eek a bug!" reaction, but periodical cicadas, and cicadas generally, are nonaggressive. The periodical cicadas sat still for a close-up photo and will let you handle them. And that is part of the marvel. Cicadas have virtually no defenses against predators--they don't jump like grasshoppers or sting like bees or..., or anything else--so when they emerge, everything that can eat them feasts, which is pretty much everything, insects, frogs, reptiles, birds, mammals. They survive as a species because despite incredible losses to predators, there are so many that thousands live to reproduce. Predator satiation, it is called. 

Predator satiation is tricky because large numbers are required, especially if you are as big and tasty as a periodical cicada. It really only works if the animals emerge infrequently. The birds that gorged on cicadas will have a better than usual year and more of their fledglings will survive, so bird populations will rise. If the cicadas emerged again next year in the same place, and the year after, presently the predator populations would catch up, and the predators would eat so many cicadas that that brood would die out. A long or unpredictable interval between emergences lets predators eat until they are sick, but not build numbers over time, because food drops down to normal levels the next year. The periodical cicadas do that the best of any species: 13 and 17 are prime numbers, not a multiple of any smaller number. Consequently predators cannot live twice as long, or cycle their numbers so that big families produce big families three years later, to be ready when the next emergence happens. In fact, periodical cicadas have only one specialized predator: the fungus Massospora cicadina, which affects only periodical cicadas. It produces spores that lie inactive in the soil for 13 or 17 years until an emerging cicada encounters one on its way out of the soil. 

periodical cicadas, St. Louis, Missouri
tree trunk with cicadas

They are really very handsome, with black bodies, yellow wings, orange outlining, red eyes.

Going to St. Louis to see them was an adventure. First, I needed to guess the right week to arrive. The emergence started at the southern edge of this brood of cicadas and as the season progressed, moved north. When would it reach St. Louis? Once the first adults emerge, it takes about six weeks before all adults have laid eggs and died. I did pretty well; cicadas were numerous in St. Louis the last few days of May.

Second, I had to actually find cicadas. I got to my hotel and quickly went outside expecting to hear cicadas. Nothing. And no big insects on the trees. That had been a worry of mine, that human landuse had destroyed the cicadas. Cicadas were common in some parts of St. Louis, virtually absent in others. I guess that near my hotel, and in a lot of other neighborhoods, at some time in the last 13 years they dug up the soil, removed the host trees, or spread insecticide and so destroyed the cicada larvae in the soil. Cicada conservation is going to be difficult: for years we don't know they are there. The good news is that the nymphs are pretty deep in the soil, usually 12 to 40", but they do feed on tree roots: remove the tree for a lawn or a building and what are they going to eat?

cicada emergence holes
Under a tree, holes left by cicadas when they emerged from the ground

Everyone had a cicada story. Cicadas bouncing on car windows, or being so loud you couldn't hear another person speaking, or intimidating the cat with their numbers. I was warned to watch for cottonmouth snakes, which liked eating cicadas and might climb several feet up a tree trunk to reach them. I heard of places where the empty shells of the nymphs made a carpet on the ground and homeowners complained of fishing cicadas out of the swimming pool. 

cicada exoskeleton
exoskeleton left on a tree trunk
by cicada which molted into an adult

If you live in the eastern United States, many places will have a periodical cicada emergence, once in a childhood. A memorable event! They are predicted in Kentucky in 2025, eastern Louisiana in 2027, southern Missouri to Louisiana in 2028. For the rest of us around the world, there is nothing like it. Cicada tourism, to see a marvelous insect I had first read about about 1970, was really great. 

I didn't begin to see everything I might have. For example, there are three species of periodical 17-year cicadas (Magicacada septendecim, M. cassini, and M. septendecula), which are synchronized to emerge with each other (same year's brood) but have very different calls and avoid mating with each other. 13-year cicadas form four species, closely related to the 17s, with likewise coordinated emergences. What is unusual this year, two related broods, one 13  year and one 17 year,  are emerging in the same year and with a little bit of geographic overlap (not in St. Louis tho). Because of their long life cycle of prime numbers, it has been 221 years since these two broods cooccurred. It has also been 26 years since two geographically adjacent broods emerged in the same year and 9 years since any two broods emerged the same year. People who study periodical cicadas have to be patient!

The explanation of their evolution is odd and technical, as you might expect for something that seems to have happened only in one place in our world. It is believed to be a result of the interplay of climate, glaciation in particular, and predator pressure. Periodic cicadas, very edible, are eliminated if they do not appear in large numbers. If large numbers are regularly present, predator numbers will rise to endanger the species. So irregular, synchronized emergences are needed. Reserchers postulate that as the glaciers advanced, and the growing season got shorter and shorter, the cicadas get less and less food from slower growing trees, grew more slowly, and took longer to mature into adults. Nymphs from the same mother emerged over two or three or five years, as each grew large enough to mature. But if only a few emerged in a particular year, they all got eaten and left no descendants. So poor growth conditions plus predators selected for changes that synchronized them, for example to mature after a certain time, not at a certain size. A mathematical model suggests that this will work. Thus newly-synchronized cicada populations survived and grow to spread. Non sychronized, non periodic populations died out. The long development times that resulted from poor growing conditions turned into a benefit since they allowed so much time to elapse that no predators specialized on these cicadas.

Note that predation keeps these broods synchronized: those cicadas that emerge a year early or a year late are eliminated by predators. That is true in general, but genetic studies have show a very complex relationship between broods;  early- and late-emerging cicadas as migrants and founders have to have caused that. Researchers have also worked out the relations between the broods, finding them to have, over the last 100,000 years or so, developed from each other by waiting an extra one or four (four is a base number in cicada development) to mature, or by maturing a year or four sooner. 

I found them marvelous. Dramatic and mysterious big insects, endemic to the eastern United States. Most jaw-dropping natural history stories are from far away tropical places. I simply got on a plane and flew into the region of Brood XIX emergence, picking St. Louis. I thank aquatic biologist Steve Schwartz for taking me just west of the city, where we found lots of cicadas in Laumeier Sculpture Park. Most were high in the trees, hard to see, but everywhere they were buzzing. Not so loud we couldn't talk, but surely I heard 10,000 cicadas. Just by walking the Laumeier loop trail.

And yet, I saw none in the six blocks around my hotel (within sight of Forest Park). I had expected them at the Missouri Botanical Garden, since it is full of plants, but was disappointed. I saw a dead one on the path and sometimes heard them buzz. But the birds in the park were very active and appeared to be hunting cicadas in the trees and chasing each other when one caught a cicada. Periodical cicadas will not easily survive there. Large areas of St. Louis are already cicada-free. Despite the maps and descriptions of millions of cicadas, cicada conservation is going to be important in the next decades. It will be 13 years until St. Louis notices cicadas again, a lot of time for landuse changes to accidentally kill the nymphs in the ground. 

Really numerous animals are often at more extinction risk than we realized and both because people thought they couldn't harm them and because the species failed quickly when their numbers dropped. Examples would be passenger pigeons but also the only North American insect that went extinct before 2000, the Rocky Mountain locust, the one that 150 years ago darkened the sky on the plains in its great migrations. 

periodical cicadas on branches and tree trunk
periodical cicadas all over this tree

I'm very glad I went. The periodical cicadas really are a wonder of the world, and one confined to the eastern United States. (Helpful map: link)


Periodical Cicada Information Page. University of Connecticut. Accessed 6/1624. Lots of information!

Cane, J. 2009. The Migratory Locust in North America; a post mortem. Wild About Utah. link (Accessed 6/1/24).

Ito, H., S. Kakishima, T. Uehara, S. Morita, T. Koyama, T. Sota, J. R. Cooley and J. Yoshimura. 2015. Evolution of periodicity in periodical cicadas. Scientific Reports. 5: 14094 pp. 1-10. DOI 10.1038/srep14094.

Simon, C., J.R. Cooley, R. Karban and T. Sota. 2022. Advances in the evolution and ecology of 13- and 17- year periodical cicadas. Annual Review of Entomology. 67: 457-82. Available from the Periodical Cicada Information Page, all kinds of neat information. 

Williams, K.S. and C. Simon. 1994. The ecology, behavior, and evolution of periodical cicadas. Annual Review of Entomology. 40: 269-295.

 Kathy Keeler
A Wandering Botanist

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