People ask: why do periodical cicadas stay underground for 17 or 13 years?
There are three parts to this puzzle that people are interested in:
How cicadas count the years as they go by.
Why prime numbers? 13 and 17 are prime.
Why is their life cycle so long? They are one of the longest living insects.
Cicadas likely don’t count like people do (“1,2,3,4…”) and you won’t find scratch marks inside the cell (where they live underground) of a Magicicada, marking off the years as they go by. However, there is a kind of counting going on, and a good paper to read on that topic is How 17-year cicadas keep track of time by Richard Karban, Carrie A. Black, and Steven A. Weinbaum. (Ecology Letters, (2000) Q : 253-256). By altering the seasonal cycles of trees they were able to make Magicicada emerge early, proving that cicadas “count” seasonal cycles, perhaps by monitoring the flow and quality of xylem sap, and not the passage of real time.
Why prime numbers, and why is the life cycle so long? This topic fascinates people. The general consensus is that the long, prime-numbered life-cycle makes it difficult for an above-ground animal predator to evolve to specifically predate them. ReadEmergence of Prime Numbers as the Result of Evolutionary Strategy by Paulo R. A. Campos, Viviane M. de Oliveira, Ronaldo Giro, and Douglas S. Galva Ìƒo (PhysRevLett.93.098107) for more on this topic. An argument against that theory is that a fungus, Massospora cicadina, has evolved to attack periodical cicadas regardless of their life cycle. Of course, a fungus is not an animal. Maths are easy for fungi.
There are also questions about why there are 13 and 17 year life cycles, why a 4 year acceleration of a brood might occur1 and why Magicicada straggle.
1 This is a good place to start: Genetic Evidence For Assortative Mating Between 13-Year Cicadas And Sympatric”17-Year Cicadas With 13-Year Life Cycles” Provides Support For Allochronic Speciation by Chris Simon, et al, Evolution, 54(4), 2000, pp. 1326—1336.
One of the most frequently asked questions we receive is: “what do cicadas do“? This question is similar to the question “what is the purpose of cicadas” — the answers to both questions help people understand why these fascinating, unusual creatures exist at all.
The simplest reduction of their life cycle is:
1) They hatch from an egg.
2) They burrow underground where they will drink from plant roots for most of their lives.
3) They leave the underground and become adults.
4) The males make sounds that attract females.
5) Males & females court & mate.
6) Females lay fertilized eggs in the branches of plants, and the cycle continues.
7) They die.
The specifics of a cicada’s life cycle varies from species to species, but here is a more detailed view of what cicadas do:
From egg to 1st instar nymph:
1) Cicada nymphs hatch from eggs.
2) Nymphs feed on plant fluids which they access thanks to the egg-nest groove made by their mothers.
3) They leave the groove, and drift to the ground. Their descent to the ground doesn’t hurt them because they weigh so little.
4) Once on the ground, they dig into the soil until they find small rootlets, from which they will feed.
5) Underground, they will tunnel/dig…
6) and establish a cell…
7) from where they’ll comfortably feed. Cicadas feed on the xylem sap of plants. With the help of bacteriathey transform the water, minerals and amino acids found in tree fluids to the tissues of their own bodies.
8) They pee, in fact they seem to use excess plant fluid to moisten soil to help mold the walls of their cells.
9) Throughout their life underground they will move from root to root… as plant root systems change with the seasons, when roots die off, or perhaps to avoid predators.
23a) If you are a Male cicada, you are going to sing… unless you belong to a species that cannot sing, in which case, you’ll move your wings in a way that will produce a sounds.
There are many types of songs: a) distress calls, b) calls to establish territory, c) calls to attract females, d) including choruses of many cicadas and e) courting calls
23b) Female cicadas, and some male cicadas, move their wings to make sounds, also in an effort to attract and engage a mate.
24) Most cicadas (aside from Magicicada during the early days of their adult lives) will try to avoid being eaten by predators.
25) They’ll fly, of course.
26) Cicadas, like Magicicada, will establish chorusing centers, which are places where the male cicadas sing together and females come to meet them.
27) Male and female cicadas will court…
28) and mate…
29) the female cicada will lay her eggs in grooves (ovipositing) she etches into a suitable plant stem, and we’re back to step 1.
30) The last thing cicadas do, of course, is die, and return the nutrients found in their bodies to the soil, where they will be broken down and absorbed by the plants they fed upon.
Some things cicada do not do:
Here are some things cicadas do not do:
1) They don’t seek shelter during the fall months (i.e. they don’t try to live inside your house), unlike Ladybugs or Stinkbugs.
2) They don’t sting or otherwise pass venom onto people.
3) They don’t chew plant leaves, like caterpillars or grasshoppers.
4) They don’t dump garbage in the ocean.
Once they become adults, cicadas live on and around plants similar to their host plants, often the very same tree where they were born. Depending on the species of cicada, this could be a tree, or perhaps a grass (sugar cane, which some cicadas use as hosts, are giant grasses).
When they are nymphs, which they are during the first stages or instars of their life, they live underground amongst the root systems of the plants they derive nourishment from. While they are there, they dig tunnels and build cells (their living quarters) where they can feed from the fluid of rootlets of plants in comfort.
It is important to note that when we talk about cicada broods, we are talking about the 17 & 13-year periodical Magicicada cicadas. We are not talking about Tibicen or other species.
There are 12 groups of Magicicadas with 17-year life cycles and 3 groups of Magicicadas with 13-year life cycles. Each of these groups emerge in a specific series of years, rarely overlapping (17 & 13-year groups co-emerge every 221 years, for example). Each of these groups emerge in the same geographic area their parents emerged. These groups, each assigned a specific Roman numeral, are called broods.
Gene Kritsky’s book, Periodical Cicadas: The Plague and the Puzzle, documents the history of the recognition and naming of the broods. The first person to document that different groups of periodical cicadas emerged in different years was Nathaniel Potter in 1839. Benjamin D. Walsh and Charles V. Riley devised the system for numbering the different broods in 1868, and then C. L. Marlatt sorted the 17 year broods out from the 13-year broods, giving us the system we have today.
Visit our Broods page which features a grid of the Brood names, their lifespan, when & where they’ll emerge next and links to maps.
You might ask, “why were there once cicadas in my area, but now there are none?” There are a number of reasons why cicadas might die off in a particular area, or go totally extinct.
1) Destruction of host trees by a blight or destructive insect infestation. Tibicen bermudiana went extinct in the 1950s in Bermuda because of a cedar tree blight. Emerald Ash Borer insects are currently devastating Ash trees in North America. Ash trees are a favorite tree of Magicicada cassini, in particular.
2) Destruction of host trees by humans. Consider that every time a forest is removed to make room for another neighborhood, factory, strip mall, or highway, the cicadas that inhabited those areas died. Each time the human race expands, the cicadas must decline. The paper The Distribution of Brood Ten of the Periodical Cicadas in New Jersey in 1970 by John B. Schmitt documents the reduction of cicada populations in New Jersey, the nation’s most populous state. Also, the entire Brood XI went extinct in Connecticut as of 1954.
3) Extreme weather such as tornadoes, hurricanes, and flooding can destroy cicada habitat. While there are cases where cicadas were able to survive some pretty horrific weather, if trees are destroyed, or grasses that are hosts to young cicada nymphs are destroyed, or if floodwater sits too long, the cicadas are doomed.
4) Pesticides. It should be obvious that pesticides will kill cicadas.
Can pets, including dogs and cats, or other animals sense cicadas below ground? Yes they can. Animals have a better sense of hearing than humans, and they are able to sense the subtle sounds of cicadas tunneling underground.
You might discover animals, including your pet dog, digging up your lawn in advance of a periodical cicada emergence. That is because they can sense the cicadas preparing for the day they will emerge.
Are cicadas attracted to the sound of lawnmowers and other machinery? Yes! Cicadas, particularly Magicicada periodical cicadas, are attracted to lawnmowers, weed-whackers, leaf blowers, hedge trimmers, power drills, etc. If it is loud and vibrates, cicadas will be attracted to it. Why? Most likely because they think your tool is a particularly impressive periodical cicada chorusing center, so males want to join in with the chorus and females want to mate with the particularly impressive males.
So, next time you’ve got the old angle grinder out, don’t be surprised if a cicada lands on your shoulder.
Are Cicadas Locusts? The short answer is NO. However, in the U.S.A. we’ve been calling cicadas “locusts” for hundreds of years.
People have seen referring to cicadas, particularly Periodical cicadas, as both flies and “locusts” since the 1600’s, when colonists first documented them.
Gene Kritsky’s book Periodical Cicadas: The Plague and the Puzzle provides a chronology and historical texts of people referring to cicadas as “locusts”. Consider this quote from Pehr Kalm from 1756:
By the Englishmen here they are called Locusts and by the Swedes living here, they have gotten the name Grasshoppers. In Latin, they could be called Cicada.
It makes some sense that Englishmen would call cicadas Locusts, and Swedes would call them Grasshoppers because there was only one species of cicada in both England and Sweden. This cicada, Cicadetta montana montana, call is so high-pitched you need electronic assistance to hear it, so most people were not aware of its existence. So, when Englishmen and women encountered cicadas they likely thought “there are a lot of them, they’re big, I’m afraid they’re going to eat my carrots — these must be LOCUSTS”!
Cicadas are indeed not Locusts, Grasshoppers or Flies.
Take a look that the illustration of a true locust below. You’ll notice the true locusts have HUGE rear legs for hopping, long antennae, and relatively long bodies. True locusts chew the plants they consume, while Magicicadas suck fluids from trees.
Giant hind legs for jumping
Hind legs about the same size as other legs; great for climbing and perching.
What they eat
Everything green they can find to eat
They’re in your town
All the plants in your town have been stripped bare