Jamming cicadas: after years underground, these insects emerge on cue and sing in concert

Natural History, June, 2002 by Peter J. Marchand

Folklore has it that cicadas singing on a summer morning portend a hot day. If true, then this particular June day had the makings of a scorcher.

When I first stepped out of my house in Arizona's White Mountains, only a single cicada was singing. Within two hours the air was ringing with the metallic buzz of hundreds of males. The chorus steadily crescendoed as more and more insects joined in, singing and flying, singing and flying. By noon it seemed that every branch among the junipers and pinon pines surrounding my home was occupied by an adult cicada. The ground beneath the trees looked like a battlefield, pockmarked with half-inch-diameter emergence holes and littered with molted skins, the last vestige of the cicadas' subterranean existence. For an insect that spends better than 95 percent of its life below ground, this would be the final hurrah: a single, frantic mating effort that would last no more than a few days.

I was witnessing the emergence of annual cicadas, which, despite their name, spend up to seven years underground before coming to the surface to mate, lay eggs, and die. As impressive as the numbers of annual cicadas can be, however, their emergence pales in comparison with that of periodical cicadas (genus Magicicada) found in the eastern United States. Populations of the latter, after remaining below ground for a full thirteen or seventeen years, appear synchronously in almost unimaginable numbers--up to 150 per square yard, or nearly three-quarters of a million per acre--and produce a mating chorus that is nothing short of deafening.

Periodical cicadas are among the longest-lived insects presently known. The seven species of Magicicada are distributed among fifteen discrete populations, or "broods," all found east of the Great Plains. Four of these species require thirteen years to complete their life cycles; the remaining three species need seventeen. In most years, there is an emergence event of science-fiction proportions somewhere in the eastern states, usually involving more than one species of periodical cicada. This year, thirteen-year cicadas will appear in Louisiana, Mississippi, Arkansas, Tennessee, Kentucky, Missouri, and Illinois, and seventeen-year cicadas will show up in Ohio, Pennsylvania, and West Virginia.

The clockwork appearance of periodical cicadas in late May to mid-June has long fascinated, and perplexed, entomologists. Cicadas live all but their last days feeding on the sap of tree roots located a few inches to a couple of feet below ground. There the nymphs are buffered from the environmental cues, such as changes in temperature and hours of daylight, that provide most plants and animals with a means of keeping a biological calendar. But researchers Richard Karban, Carrie Black, and Steven Weinbaum, of the University of California, Davis, believe they have discovered how periodical cicadas time their emergence. They suspected that these cicadas might receive their cues indirectly by monitoring the annual flowering cycles of their host trees. (Flowering temporarily alters the quantity or quality of nutrients available to the feeding nymphs.) So Karban and his colleagues reared seventeen-year cicadas on the roots of a variety of peach that they induced to flower twice during the nymphs' fifteenth year. The cicadas emerged the following summer, one year ahead of schedule.

But why are these insects bound by such a rigid calendar in the first place? The answer probably has much to do with predator avoidance. During a mass emergence of periodical cicadas, almost any animal, from raccoons to raptors, will prey on them. But with the cicadas' appearance only once every thirteen or seventeen years in different parts of the country, predators are unable to predict just when or where they will next turn up. And any predators that do happen upon the superabundance of emerging cicadas can satiate their appetites and still leave the vast majority of the insects alive and free to reproduce.

The simultaneous emergence of more than one species of periodical cicada creates an interesting mating challenge. Females must be able to recognize their own kind, by sound, amid hordes of individuals--including willing males of the wrong species--and a drone so loud that even cicadas have trouble hearing. Chris Simon and her research associates David Marshall and John Cooley at the University of Connecticut have recently deciphered some of the subtleties of mate recognition by recording sonograms of different species that had been identified in the laboratory by their DNA. Similar though their mating signals may sound to us, each species vibrates its sound-producing tymbals at a slightly different frequency. Where one species, recently discovered by Marshall and Cooley, overlaps in territory with another having a similar call note, the frequency of the first species' song is adjusted upward by about 0.3 kilohertz. The female, demonstrating remarkable discrimination of mating signals, can then pick the males of her species out of the crowd.