Cicada invasion overwhelms predators
5 July 2007
by Christina Zdanowicz
Cosmos Online


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.

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."