How do Cicadas Know When to Emerge?

Smite thine enemy with cicada shells (Courtesy Jason Sterner)

When I was a weird kid growing up in cotton country, nothing could beat the exceptional gross-out factor of cicadas. The summers when various broods emerged in my town, I coveted the crispy, alien exoskeletons left behind by metamorphosed cicadas. They clung to tree trunks and bushes, and as my friends and I learned young, they clung just as well to our clothes. I collected and wore them, turning my t-shirts into demented, bejeweled regalia. The shells and dead cicadas—which positively reek after a few days—were also great weapons in turf wars with the boys down the street. Pro-tip: Nothing sends your enemies running like rotting, bulgy-eyed insects the size of your palm.

The buzz surrounding the emergence of periodical seventeen-year cicadas (Magicicada septemdecim) reached a fever pitch this year,1 which got me thinking fondly about the insects again. I made my radio debut this week talking about a great project at North Carolina State University, where they’re asking folks to mail in dead cicadas to study how urbanization is impacting the insects. Reporting on that got me wondering—how do cicadas know when to emerge in unison?

It’s a fascinating question about a phenomenon that’s been called “one of the great biological wonders of North America.”2 In a given brood of periodical cicadas, millions of nymphs remain buried underground for seventeen years with only a tree root as their lifeline. Their specialized mouthparts allow them to survive anchored to roots, where they suck from the xylem of trees (which, you may recall from high school biology, is the tissue that carries water and stem-building nutrients upward). After the long wait, which makes these insects the longest-living in the world, the nymphs synchronize their emergence to within days of one another.

Seventeen-year periodical cicada (Courtesy Chris Root)

Seventeen-year periodical cicada (Courtesy Chris Root)

The closest science has come to explaining this spectacle involved a study in the early 2000s by a guy self-described as having “dreamed about tricking [cicadas] into emerging early for most of his adult life.”2 Entomologist Greg Karban and his team at UC-Davis wanted to know if seventeen-year cicadas synchronize their emergence by marking the passage of real time, or by marking the passage of some seasonal cycle. They collected hundreds of nymphs that were already in the ground and transferred them to the roots of potted peach trees, a painstaking process at which they’d already failed three times.

The team then placed the peach trees in growth chambers that mimicked the normal temperature and length of daylight in Davis, California—but progressed at twice the normal rate. The scientists effectively sped up the life cycle of the trees, exposing them to eight seasons in a year. Karban’s dream came true—this tricked the cicadas into emerging early, supporting the theory that cicada nymphs synchronize according to some kind of seasonal cue. We still don’t know what that cue might be, but Karban believed it might be changes in the quality of xylem fluid. After blossoming in spring, xylem fluid tends to be richer in nutrients. The circadian rhythms of other insects, including aphids and treehoppers, have also been shown to depend upon seasonal changes in their host plants’ fluid.

The synchronized emergence of a cicada brood is a feat of nature, but that’s not to say every cicada always gets it right. Every year, there are lonely stragglers who didn’t time their appearance right. Sometimes there are enough stragglers that they can find one another and mate; sometimes, they just missed the party train.

The reason for the season (Courtesy Jonathan Szczepanski)

The reason for the season (Courtesy Jonathan Szczepanski)

1) There were maps of where the red-eyed adults appeared; a fantastic Radiolab episode about deciphering between different species’ mating calls; and countless articles assuring people no, they won’t hurt you, but yes, I suppose you can eat them.

2) Karban, Richard, Carrie Black, and Steven Weinbaum. 2000. “How 17-year cicadas keep track of time.” Ecology Letters, 3: 253-256.

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