5 July 2007
by Christina Zdanowicz
A remarkable plague of cicadas has been unleashed on Chicago. The insects emerge briefly once every 17 years and can reach densities of 1.5 million an acre.
Magicicada cicadas are 2.5 to 3 centimetres long. Some see their emergence as a nuisance, but they pose no threat to people.
RAKING THOUSANDS OF SQUIRMING cicadas from beneath an old ash tree in our yard, I watched my father ditch his rake for a snow shovel. He scooped hordes of the earthy-smelling insects and their vacant shells into a nearby bin, but froze when he heard me scream. My five-year-old self was terrified, because a cicada had plummeted from the tree and hit me square on the head.
Now, 17 years later, the brood of insects is back again in my home town – Glenview, near Chicago in the U.S. – but this time I'm more fascinated than frightened.
The deafening chirps of males and the clicking wings of females in the trees are choking out the sound of nature all over again. At densities of up to 150 to 200 cicadas per square metre – or 1.5 million individuals an acre – these crunchy black insects with orange wings and red eyes, provide a feeding bonanza for birds and other predators.
These countless insects burst out of their underground slumbers with one mission in mind: to mate.
Timing it just right, three species of Magicicada cicadas have been invading the greater Chicago area over the last month. They are all part of the same brood, the Northern Illinois Brood (or Brood XIII according to naming convention), and they began popping out of the soil when it reached 17.8°C in late May.
More questions than answers
While other cicadas are found worldwide from Europe to Australia, these periodical cicadas – which emerge every 17 or 13 years depending on the brood – are endemic to the east of the U.S., says biologist Gene Kritsky of the College of Mount Saint Joseph in Cincinnati, Ohio. Out of the 3,000 known cicada species, only seven engage in this unusual periodicity.
Though they resemble locusts in appearance and swarming behaviour, they are more closely related to aphids.
Periodical cicadas have intrigued scientists for hundreds of years, and we still understand little about them. "Why 17 years? Why not overlapping broods? Why don't they all come out the same year throughout the entire U.S.?" queries Phil Nixon, an entomologist at the University of Illinois in Urbana. "All of these things we don't know."
We do know from DNA evidence that these unusual cicadas have been around since at least prior to the last Ice Age, 10,000 years ago. The top theory as to why they evolved their unusual 17-years dormancy is to evade predators.
"It seems to be an evolutionary trait that allows them to come out in enormous quantities, in a big flush all at once, and overwhelm predators," says biologist and curator Doug Taron, of the Notebaert Nature Museum in Chicago. "Individual cicadas may get picked off – and in fact, are preyed upon very heavily – but by coming out in a big flush like this, the species manages to reproduce successfully and avoid having much in the way of specialized predators that prey on them exclusively."
So many cicadas have covered the streets of Chicago, it's difficult to avoid walking on them (Picture: Christina Zdanowicz).
The cicadas do end up on the menu for many animals, from squirrels to ducks, which can be spotted gorging themselves on the chirping morsels. However, the periodical cicada's lifecycle is so long that it tends to confuse its predators.
The lifecycle is "long enough that there is no predator of them that has a similar cycle, so it totally takes their predators off guard", agrees Nixon.
The insects make little attempt to move out the reach of predators or humans. "Crunch, crunch, crunch" has been a typical sound heard across Brood XIII's territory, with suburban Chicagoans smashing cicadas as they walk or jog along streets in heavily-infested areas. While these periodical cicadas are picked off and smashed by shoes, their counterpart, the annual cicada, wouldn't dare to be so easy-going around predators. Annual cicadas, also called dog-day cicadas, make a fervent effort to avoid being eating, says Nixon.
Male periodical cicadas also tend to form mass 'singing' groups in trees during the day, thus further attracting predators. "Whereas the annual cicadas sing at night when their main predators, birds, can't find them very easily," he adds.
"When you approach, they are very watchful and will walk around the twig or branch so that they are on the opposite side of you. They're much more secretive, much more furtive," says Nixon.
Birth of a brood
A platoon of entomologists has been using this most recent emergence to learn more about the screeching insects. Kritsky, who has studied periodical cicadas for over three decades, focuses on studying the evolution of broods and how one brood gives rise to another.
He says that one of his team's most exciting contributions to the field was when they successfully predicted that one brood – Brood X, or the Great Eastern Brood, which emerges across 10 states – would emerge after just 13 years in 2000, four years earlier than usual.
Kritsky already has some clues as to why a brood might appear several years early, and he used them to make his prediction.
"Right now the prevailing hypothesis is that they come out four years early to get out of cycle with their fungal disease," he says. That specialised fungus, known as Massospora, takes over the cicada's abdomen, ultimately causing it to fall off.
If enough cicadas break the cycle early, and at the same time, then they might breed and form a whole new brood with a shifted 17-year cycle. "They might be joined by additional accelerating cicadas from that lagging brood, increasing the population… after six generations, or 102 years, you could have a very large, well-established brood of cicadas created," he says.
"We're now waiting to see if Brood X switches back to 17 years, like we expect," he says. "Then, in Cincinnati we'll have the first evidence of a self-reproducing new brood of periodical cicadas."
In part, Kritsky follows the broods by mapping out trees, noting ones that had not been planted when cicadas last emerged and looking for evidence of new eggs laid in it.
When a female cicada lays her eggs, she slits open twigs and inserts creamy-coloured, spindle-shaped eggs into the wood. These eggs hatch one or two weeks later and the nymphs eventually tumble down to the ground. Here they tunnel down, look for a root, grip on and start feeding.
These cicada nymphs survive the intervening 17 years by hunkering down and feeding on sap, only switching roots a few times during that duration, said Nixon.
When their time is up, they moult and break through the surface of the soil, ready to mate and start the cycle over again. At this point they use soil temperature to gauge when to emerge. This would appear to make them a useful species to measure the biological effects of climate change.
Our reporter captures the cicadas on video (Credit: Christina Zdanowicz).
Ecologists all over the world – from those studying the fruiting time of mushrooms in the U.K. to butterfly life cycles in Switzerland – are starting to record the effects of global warming pushing back the seasons. But the effects don't seem to have hit the cicadas just yet, says Taron.
"However, I would expect cicadas to be influenced by climate change, at least eventually," he says. "You can envision a scenario where as the climate heats up the soil, it's going to hit the temperature that triggers emergence earlier and earlier in the year."
With the cicada season almost over – and little chance of the spectacular chaos returning for another 17 years – the biologists are already looking back on the last month wistfully.
"To me personally this is one of the greatest phenomena in nature… and one that we don't get to see terribly often," says Taron, while admitting he himself doesn't have to deal with battalions of cicadas trying to invade his house.
The next brood to emerge is the Great Southern Brood (Brood XIX), which will appear en masse, from the Midwest of the U.S. to the states of Maryland and Virginia, in 2011.
Christina is a former member of the Cosmos team and a freelance science writer in Glenview, Illinois in the USA.
Periodical cicadas - Wikipedia
Credit: Christina Zdanowicz