Published March 29, 2013
Periodical cicadas (Magicicada septendecim),
the cousins of katydids and crickets, have a unique breeding schedule,
and after 17 years of living underground, a large group of them are
preparing to fill the skies along the U.S. East Coast, from North
Carolina up to Connecticut.
Normally, periodic
cicadas spend their lives in complete darkness underground, sucking the
fluid out of the roots of trees and shrubs. At the end of their life,
they emerge, breed, and almost instantly die, completing a lifecycle
that humans have studied for centuries.
In the process,
however, they annoy millions of people with their constant chirping
and, of course, the piles of dead cicada bodies on the ground. While
some areas may see no cicadas at all, others in the past have seen
millions of cicadas in a single acre. (Listen to the cicada’s love song.)
"It
can be like raking leaves in the fall, except instead of leaves, it’s
dead cicada bodies," said Dan Mozgai, a cicada researcher who keeps a
clearinghouse of cicada information and breeding schedules at cicadamania.com.
Consistency
Cicadas
are easy to anticipate because of their extremely consistent mating
behavior. Every 13 or 17 years, depending on the population, species of
periodic cicadas will emerge as part of a specific brood in order to
look for a mate.
The group expected this spring, known
as Brood 2, are the offspring of cicadas last seen in 1996. If they
follow the same tracks as their parents, they'll emerge in Connecticut,
Maryland, North Carolina, New Jersey, New York, Pennsylvania, Virginia,
and Washington, D.C.
The genetic mechanism that prompts
periodical cicadas to emerge kicks in every 17 years (or every 13 years
for other broods) when the ground warms up to 64°F (18°C).
Some
researchers think the timing of a brood’s emergence is a defensive
mechanism—appearing at infrequent intervals means that it’s harder for
would-be predators like birds and squirrels to anticipate when the
insects will be available to eat.
Others suggest that the 13- and 17-year cycles, prime numbers in mathematics, help cicadas avoid parasites. A 2004 study
from the University of Campinas in Brazil suggested that a cicada with a
17-year cycle and a parasite with a two-year cycle, for example, would
meet only twice each century.
But not all cicadas breed
on this multiyear cycle. Some, like the tibicen cicadas, work on an
annual rotation, leaving them more susceptible to predators like the
cicada killer wasp (Sphecius speciosus).
The
wasps know exactly when to expect the annual cicadas in late summer or
early fall. The wasp lays its eggs on the cicadas, and the larvae slowly
kill the cicada and feed off its carcass.
Anticipation
For
the periodical cicadas, their temperature thresholds for emergence are
getting harder to anticipate. During mild winters, like in 2012, cicadas
from Brood 1 appeared in mid-April. Scientists think that this year,
Brood 2 cicadas could wait until early May or even later.
John Cooley,
a cicada researcher at the University of Connecticut, has led efforts
to map where cicadas live, breed, and die. With funding from a grant
from the National Geographic Society, Cooley has attempted to trace
their evolutionary history since the last ice age, trying to understand
how the species evolved as the world's climate changed.
"We
know that they've had to change in the past because of postglacial
forests that exist now," Cooley said. If scientists understand how
cicadas once responded to climate change, then they can imagine how
another episode of global warming might impact both cicadas and other
insects.
In the meantime, the offspring of the insects
that last appeared when Bill Clinton was President will soon be making
their presence very well known.
The cicada brood scheduled to emerge from underground this year could contain millions of individuals.
