Back in 1996, Bill Clinton was still in his first term of office, Charles and Diana agreed to divorce, the Unabomber was apprehended, and Ella Fitzgerald died. That year I hadn’t even dreamed of moving permanently to North America: I was living in Yorkshire where mad cow disease was seizing the headlines in England. While so much history has flowed through newspaper presses since then, Brood II cicadas were all the while secretly sucking at tree roots waiting for the calendar to flip over to 2013.
I encountered the brood a couple of weeks ago at a rest area on the I-64 west of Richmond. The first thing travelers noticed after stepping out of their cars was a chirping racket coming from every direction. I had heard the sound only once before when a 13-year brood of cicadas was emerging near home in 2011. This year it is the turn of the 17-year ensemble to sing.
For a few weeks every 17 years local residents have sleepless nights. The noise can be as loud as a passing truck, and at 90 decibels reaches a level at which the Occupational and Safety Administration warns we should not be exposed to for more than 8 hours a day to avoid hearing damage. Not being respecters of regulations, male cicadas chirp round the clock.
They are among the great wonders of the insect world, but not only because they have one of the longest life cycles. The question that intrigues me more is how they manage to coordinate mass emergence. When ground temperatures rise above 64 °F (18 °C) in the Year of the Brood, the fossorial cicada nymphs start to burrow upwards, breaking the surface first at the southern edge of their range (North Carolina) and progressively towards northern limits (Connecticut). Soon there are incalculable billions above ground, more than a million per acre, but during intervening years you are unlikely to see any at all. So precise is their timing that local residents can plan when to be on vacation in 2030 during the next big pulse.
They look scary and are the biggest of their kind. They are an inch-and-a-half long with bulging red eyes, orange wing veins and leg stripes, and through their transparent wings you can see a black cigar-shaped body. They look like bugs in zoot suits and were given the marvelous scientific name, Magicicada septendecim.
I was a member of a naturalist group heading for a wildlife center in the Blue Ridge, but the cicadas in the car park – not you would think the most auspicious place to watch wildlife – were the most memorable sights of the day. They were thought to be a bad omen by superstitious early colonists, who assumed (wrongly) that they were the same as locusts in the Bible which warned in the Book of Revelation: “Then from the smoke came locusts on the earth …” I guess that every seventeen (or thirteen years) there were particularly fiery sermons from the pulpit about the seven last plagues of Armageddon.
For all their fearsome appearance, they are harmless insects. They don’t bite or sting, nor do they eat vegetation as true locusts do. They can cause minor tree damage from “flagging” (browning) during nest-building, but are generally rather beneficial. Soon after mating and egg-laying the adults die, providing food for critters and fertilizing the soil. The only remaining cicadas are immature “instar larvae” hatched from eggs. These nymphs fall from the tree canopy to the ground where those that avoid predation burrow underground to find juicy roots to suck on … and on … and on until the calendar turns.
For the most part, the distribution of 17-year broods and 13-year broods don’t overlap, although they evolved from a common stock a few million years ago. Today there are 15 broods in North America, twelve with 17-year cycles and three with 13-year, and you will not find them anywhere else. Of the 3,000 species of cicada worldwide, only seven have periodical behavior.
We watched hordes of them lumbering up tree trunks and walls; we saw many flying unsteadily like old flying boats, often crashing into branches or to the ground where robins were waiting to pounce. For a small bird, a cicada is hamburger-sized (and probably just as nutritious), so the eaters were soon sated. Everywhere across the hard-trodden ground there were holes, about ten to the square yard and a little larger than earthworms make. These were the burrows from which cicadas had recently emerged, and scattered close by were the transparent brown shells (exuvia) which they had worn for so many years underground. They gave us a spectacle I may never see or hear again.
The greatest enigmas are how they know the time, and why coordinate their emergence instead of being independent like most insects. We know of genes that regulate daily cycles in animals, but I can’t understand how molecules make a 17-year timer. And if cicada nymphs were synchronizing eruption by communicating with each other underground, it stumps my imagination. Perhaps it serves to overwhelm the appetites of predators so that at least some survive to breed, although that theory is controversial. A similar explanation has been offered for mast seeding of long-lived plants. Last year was a mast year for oaks, with acorns lying so thick in our yard that every step sounded like walking on cornflakes. But mast years are unpredictable – they can be consecutive or after long gaps – whereas cicada years can be written into almanacs for years, even centuries, ahead.
There are few pat answers in nature and the life sciences. That was frustrating when I was a student preparing for exams, but now I think the mysteries are far more wonderful than the facts.
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