Why do 17-year cicadas come out more often than every 17 years?

A cicada gets up close and personal with this teenage boy. Fortunately, it's harmless.
A cicada gets up close and personal with this teenage boy. Fortunately, it's harmless.
© Karen Kasmauski/Corbis

It's the kind of headline that makes entomologists stand a little taller: "Paul McCartney Plagued by Locusts."

During a May 7, 2013, concert for 47,000 fans at an open-air stadium in Brazil, McCartney's stage was swarmed by scores of insects. Many of them stayed for McCartney's three-hour performance, including one plucky bug that perched on his shoulder.

Advertisement

Advertisement

The antennae-sporting admirers probably were locusts -- a term reserved for a specific species of migratory grasshoppers exhibiting swarming behavior -- and members of the Acrididae family [sources: Eby, Vanderpool].

The same can't be said of cicadas, which are commonly and mistakenly called locusts, too. A periodical cicada of the genus Magicicada is an insect measuring 1.5 inches (3.8 centimeters) in length with a red-orange hue and protruding red eyes. In fact, this coloring is an easy way to identify periodical cicadas from other species, including the more common dog-day cicadas. Dog-day cicadas, which appear every year to mark the end of summer, have green-tinged wings [source: Penn State University].

Unlike their dog-day cousins, periodical cicadas make a rare appearance relegated only to the regions east of the United States' Great Plains -- and nowhere else in the world [source: National Geographic].

In 1893, observers began organizing periodical cicadas, also called "17-year cicadas," into broods. These Roman numeral classifications ranging from I through XVII make it easier to track the insects' infrequent arrivals. For example, in 2013, Brood II emerged for the first time since 1996 [source: Penn State University]. The previous time we saw Brood II we were still doing the Macarena.

Each brood includes different species of cicadas that emerge together in a collective bid to find suitable mates. There are 13 broods of 17-year cicadas. Along with 17-year cicadas, there are five broods of 13-year cicadas, which are more prevalent in the southeastern part of the U.S. [source: Penn State University].

Each brood operates on a different schedule, which means you don't have to wait 13 or 17 years to witness the insects' next en masse eruption. For example, in 2014, Brood III is expected to appear in Iowa, Illinois and Missouri. And in 2015, Brood IV will make a comeback in Iowa, Kansas, Missouri, Nebraska, Oklahoma and Texas [source: Cicada Mania]. In fact, there's a brood of cicadas coming out every year in a different part of the eastern U.S.

The Long (and Short) of the 17-year Cicada Lifespan

A periodical cicada (unlike your dog-day cicada) is 1.5 inches (3.8 centimeters) in length with a red-orange hue and protruding red eyes.
A periodical cicada (unlike your dog-day cicada) is 1.5 inches (3.8 centimeters) in length with a red-orange hue and protruding red eyes.
M. & C. Photography/Getty Images

The sudden appearance of 17-year cicadas is one of nature's most spectacular events, a phenomenon that perhaps led to the fanciful naming of the periodical cicadas' genus classification: Magicicada.

For centuries, periodical cicadas have fascinated scientists and casual observers. "Their incredible ability to merge by the millions ... is without parallel in the animal kingdom," wrote scientists Richard Alexander and Thomas Moore in 1962 [source: Penn State University].

Advertisement

Advertisement

Millions of cicadas may emerge in a single night after spending 17 or 13 years living 2 to 24 inches (5 to 6 centimeters) underground. It's a lengthy childhood that serves as an important distinction that enables them to have the longest life span of any insect in North America.

Seventeen-year cicadas spend the earliest months of their final year burrowing toward the soil's surface. By April, the cicadas have entered a nymph stage and rest slightly below ground level. When the soil temperature rises above 64 degrees Fahrenheit (17.78 degrees Celsius), the cicada nymphs venture out of the soil through half-inch (1.2-centimeter) diameter holes. Their population may number a million or more in a space smaller than 1 acre (0.4 hectares), which helps survival rates. Animals ranging from spiders and birds to dogs and snakes find the lumbering cicadas to be easy prey, but will reach their fill long before they've put a dent in the population [source: University of Michigan Museum of Zoology].

The then flightless young cicadas journey upward at sunset, crawling a foot or more up any nearby vertical surface -- tree trunks, weeds, woody shrubs, homes or outbuildings -- to begin the next phase of life.

Overnight, they will shed their nymphal skins in 60 minutes or less. Much like the fictional Incredible Hulk, the cicadas will split their shell-like clothing to unleash larger bodies. And, as they emerge through the tops of their shells, the cicadas will unfurl their wings for the first time.

Now the cicada is an adult -- soft, ashen and vulnerable -- waiting for its exoskeleton to harden and darken in color. It will leave its crunchy larval casing behind. Still clinging to the cicada's first aboveground resting place, the casing will be an empty and forgotten outline of the creature it once held [source: Penn State University].

Where's The Party?

Not all cicadas appear on the strict schedule. There are also some cicadas, known as stragglers that appear a year later or earlier than the rest of their brood. One-year early or one-year late is the most common time-frame for stragglers, but this isn't always the case. In 2000, for example, many cicadas appeared four years earlier than their brood X schedule. Some observers theorize unseasonably warm weather may cause some stragglers to emerge early; others suggest delayed development may cause some stragglers to arrive late.

Scientists are not exactly sure how cicadas know 17 years or 13 years has passed in the first place. They think it might have something to do with some property in xylem, the fluid they suck out of tree roots for nourishment when they are underground, that reflects the yearly cycle of the tree. Perhaps if the xylem signal is weaker for insects under one type of tree versus another, that will exacerbate the off-schedule appearance. Interestingly, the prime number appearances are thought to help with survival; since the cicadas are not on the same two-year or five-year cycle as their predators, the predators don't become too dependent on them for food (though they do enjoy eating them to a point when they are around) [source: Hayes]. Unfortunately, stragglers who miss the big cicada party more likely to be eaten off as their quantities are smaller.

Advertisement

Advertisement

Even if scientists don't know why stragglers exist, most agree on one thing: Stragglers make it difficult to create accurate emergence maps. Historical accounts of stragglers, because they can range from a single periodical cicada to just a few dozen -- instead of thousands -- are nearly non-existent. To date, scientists and other observers have not been able to pinpoint geographic patterns of stragglers [sources: University of Michigan Museum of Zoology, Magicicada, Cicada Mania].

Feeding, Mating and Other Habits of 17-year Cicadas

Periodical cicadas, whether they are a 17-year or 13-year species, will live for just three or four weeks aboveground -- just long enough to mate and, in the case of female cicadas, deposit eggs. (Sad as this may seem, this mate-then-die cycle is quite common in the insect world.)

While cicadas don't excel at longevity as adults, they are quite good at getting noticed. Male cicadas attempt to attract females with a loud buzzing that seems to undulate up and down between two distinct notes.

Advertisement

Advertisement

Although each species of periodical cicada has its own song, the response of female cicadas varies little. Exactly one-third of one second after the male's song ceases, a female responds with a click of her wings -- but only if she wishes him to make further advances. The male counters with the same call, hoping to hear another wing click. If he does, he makes two particular sounds in quick succession, waits for a third wing click and then vibrates his forelegs as he gets busy with his new partner [source: Bug Music].

Within about 10 days, the female will deposit some 400 eggs in small twigs and branches of nearby trees and shrubs, choosing from among the 80 different species they prefer to use as a nursery. The females have protruding, needle-like devices at the end of their abdomens, which are used to create an egg cradle in the wood. Female cicadas may repeat the egg-laying process dozens of times [source: Ohio State University].

In about six weeks, white nymphs the size of ants will emerge from the eggs, drop to the ground and work their way into the soil. Like their parents, they'll spend the next 17 or 13 years feeding on fluid found in plant roots.

Although cicadas' root-feeding is considered harmless, damage caused by female cicadas burrowing into trees and shrubs to deposit eggs can be significant. Twigs may break off, which can create production issues for fruit and nut orchards. Some caretakers cover small trees with mesh cloth to prevent access to bark; others apply insecticides before egg-laying occurs or simply opt to wait out the cicadas' limited life span [source: Penn State University]. Not only are these insects generally harmless to plants, they are harmless to people, so no need to panic if one lands on you.

Author's Note: Why do 17-year cicadas come out more often than every 17 years?

Biblical plagues come to mind. Native Americans thought they were evil. Early colonists believed a plague had been visited upon them. And in 2013, parts of the East Coast spent much of spring under a sky of buzzing insects. Welcome to the 17th year in the lifespan of periodical cicadas. Emerging by the millions, these large, clumsily flying insects have made their mark upon humans. Including me. The trill of the male cicada, whether the periodical version of the more common annual cicada, is the siren song of summer -- at least in my part of the country.

Related Articles

Sources

  • Bug Music. "Magic Cicadas." (May 16, 2013) http://www.bugmusicbook.com/#!magic-cicadas/cki0
  • Cicada Mania. ""Are There Periodical Cicadas Coming to Your Town?" (May 16, 2013) http://www.cicadamania.com/where.html
  • Cicada Mania. "2012 Brood I Wrap-Up." Sept. 24, 2012. (May 23, 2013) http://www.cicadamania.com/cicadas/category/broods/brood-i/
  • Eby, Margaret. "Paul McCartney Attacked by Grasshoppers During Concert in Brazil." New York Daily News. May 8, 2013. http://www.nydailynews.com/entertainment/music-arts/paul-mccartney-attacked-grasshoppers-brazil-concert-article-1.1338663
  • Hayes, Brian. "Bugs that Count" American Scientist (May 23, 2013). http://www.americanscientist.org/issues/issue.aspx?id=3437&y=0&no=&content=true&page=2&css=print
  • Magicicada. "Why Do Periodical Cicadas Appear in Unexpected Years?" (May 23, 2013) http://www.magicicada.org/magicicada_straggler.php
  • National Geographic. "Cicada Facts: Understanding the Invasion." May 21, 2007. (May 16, 2013) http://news.nationalgeographic.com/news/2007/05/070521-cicada-facts.html
  • Ohio State University. "Periodical Cicadas: Life Cycles and Behavior." (May 17, 2013) http://www.bugs.osu.edu/~bugdoc/PerioCicada/PeriCicadaBehav.htm
  • Penn State University. "Periodical Cicada." (May 16, 2013) http://ento.psu.edu/extension/factsheets/periodical-cicada
  • Staten Island Museum. "They're Baaack! Return of the 17-year Cicadas." (May 16, 2013) http://www.statenislandmuseum.org/exhibitions/current-exhibitions/
  • University of Michigan Museum of Zoology. "Periodical Cicada Page." (May 16, 2013) http://insects.ummz.lsa.umich.edu/fauna/Michigan_Cicadas/Periodical/Index.html
  • Vanderpool, Scott. "Paul McCartney Plagued by Locusts." KZOK. May 8, 2013. http://kzok.cbslocal.com/2013/05/08/paul-mccartney-plagued-by-locusts/