Right (and background cicada) from original
photo courtesy ©2004 Vergil
Want to hear cicadas?
1. The calling song of Tibicen
72 degrees F.
93 degrees F.
Got a song to sing... This cicada nymph (young) was
unearthed from the Indiana University Research and Teaching Preserve
about six weeks before its scheduled emergence, late May 2004. Copyright
©David Bricker, Indiana University Media Relations
Two cicadas mate during their brief above-ground
reproductive period. Copyright Chris
Simon, University of Connecticut.
are on the wing in Washington, D.C. Voracious. Unstoppable. A force of
Yep, cicadas have lots in common with legislators. But while our elected representatives belly up to the public trough in an annual feeding frenzy, the 17-year cicadas now taking wing across D.C. and states like Indiana and Ohio fast only one year in 17.
Oddly, that's the only year when the periodic cicadas are above ground.
Cicadas are parasites. For 16 years, larvae of 17-year cicadas suck root juice from host trees. In the fall of the 16th year, the cicada nymphs burrow upward. The following spring, in the 17th year, they emerge, mature, and mate, but do little or no feeding.
(Various cicada species have different cycles. Some appear annually, others on a 13- or 17-year schedule. BTW, cicadas are technically not locusts, which are related to grasshoppers.)
It's their mating chorus that makes cicadas so obvious during the mating year.
Males sing, females give a little click in response, and they mate. Afterwards,
the females lay eggs in tree bark, cutting an incision that kills some leaves.
Cicada larvae emerge the same fall, drop to the ground and burrow into the soil, and start sucking root juices in their subterranean vampire act. Seventeen years later, they stage a re-run: call it the return of the fly.
When tree leaves that have been slathered with cicadas die, it's natural to pity the trees. But like a late-night pork-barrel appropriation that gets slipped into unrelated legislation, the parasite's real impact is concealed.
Most of the damage occurs during the 16-year underground period, says James Speer, an assistant professor of geography at Indiana State University, who collaborated with Keith Clay, at Indiana University, to relate red maple growth in Indiana to a 1989 cicada outbreak. The trees actually thrived, he found, when the cicadas were flying.
In 1989, the trees grew twice as fast as in 1988, when the larvae were maturing underground, but the flush of growth was not due to good weather. "With red maples, it is visually obvious that the trees are being suppressed" immediately before and after the mating year, Speer says.
And while he has only studied one cicada outbreak in one tree species, Speer says the counterintuitive result makes sense. "Everybody expects a high effect in the emergence years, because that's when we notice the cicadas. But 99 percent of its life cycle is underground... most of its life is spent as a root parasite."
With help from a new (competitively awarded) grant from the National Science Foundation,
Speer and colleagues plan to expand their tree-ring research to
other cicada species, which mate in other years. Using dendrochronology,
the study of tree rings, he says, "You can sample any past outbreak,
match narrow and wide rings, by taking a core from any living tree."
Just as removing a core does not harm trees,
cicadas themselves don't do much damage to established trees, says
Cliff Sadow, an entomologist at Purdue University. "If the trees
are not new, I tell a homeowner not to bother [with protection],
because they only lay eggs in twigs that are 5 to 10 millimeters
in diameter." Cicadas, he stresses, are a natural part of the landscape
that trees routinely survive.
Still, cicadas may play an underappreciated role in changing forests, says Speer. By looking at data on such factors as climate, insect outbreaks and fire, he hopes to get a better broad-scale view of how forests change. Cicadas, by feeding on certain species, could affect the ever-changing mix of species in a forest. "We can see how multiple factors affect the way the forest interacts; it's a new direction, based on looking at multiple records."
Have you been bugged by the odd number? Why do periodic cicadas stage their mating flights every 13 or 17 years, and not, say, every 10, 12 or 15 years? Glenn Webb, a professor of mathematics at Vanderbilt University, became intrigued by the question during a couple of massive outbreaks in Nashville.
Thirteen and seventeen are both prime numbers, divisible only by themselves and one. Having heard a suggestion that prime-number cycles offered an evolutionary benefit, Webb did computer simulations, and found support, but not proof, for the hypothesis.
The seeming evolutionary advantage stems from the way the cicada cycle may accelerate or dampen the population cycles of a predator. Cicadas are slow-moving, easily-captured beasts (remind anyone of the American taxpayer?), simple prey for mammals and birds. Instead of evolving poison or camouflage, cicadas survive through sheer numbers. With up to 1.5 million cicadas living on a single acre, some will inevitably survive to lay eggs, no matter how many mammals and birds saunter up to the cafeteria.
The population cycles of predators,
Webb explains, is not an all-or-nothing cycle like periodic cicadas, but their
numbers do vary over certain periods. Cicada photo: National
So what would happen if cicadas appeared on a non-prime schedule, say 15 years, and the predator's population tended to peak every five years? Each time the predator feasted on cicadas, its population would get a boost, which would be echoed 5, 10 and 15 years later. Then, when the cicadas were out mating 15 years later, the predator would get another boost.
"If the cicada population is synchronized [with the predator], then the predator would have its cycle reinforced, because it could rely on a feast," says Webb. "That would be bad for the cicadas, because they would suffer more predation."
The prime-number cycles found among at least
five cicada species, he adds, prevents cicadas from giving aid and
comfort to the enemy.
The Prime Number Periodical Cicada Problem, Glenn Webb, Discrete And Continuous Dynamical Systems-Series B, Volume 1, Number 3, August 2001 Pp. 387-399.