Cicada Swarm Proves a Feast for Predators

John Roach
for National Geographic News
May 24, 2004

A female thumb-size wasp known as a cicada killer might sound like the perfect predator to combat the billions of periodical cicadas swarming much of the eastern U.S. this May and June.

The wasp (Sphecius speciosus) paralyzes a cicada with her sting, carries it back to a chamber in her underground burrow, lays an egg on it, and seals the chamber. A few days later the egg hatches and the wasp larva eats the cicada alive.

A single female wasp can dispose of about a hundred cicadas in her four weeks above ground.

But the wasps' lives are in synch with cicadas of the genus Tibicen, not with cicadas of the genus Magicicada, which are currently crowding the eastern U.S. Tibicen cicadas emerge each year in July and August, according to Chuck Holliday, a biologist and cicada-killer expert at Lafayette College in Easton, Pennsylvania.

Groups, or broods, of the periodical, Magicicada cicadas emerge en masse in 13- or 17-year cycles in May and June. Brood X, which last appeared above ground in 1987 and reemerged this month, is the largest cicada brood of all.

By the time the cicada-killer wasps emerge from the ground in July and seek out Tibicen species, the Brood X Magicicada will be gone, not to be heard from again for 17 years.

"Besides, there is no point in trying to control the cicadas. They emerge in such numbers that they are effectively beyond control, because predator populations cannot increase rapidly enough to control them," Holliday said.

Biologists believe that the periodic and mass emergence of Magicicada is a survival strategy: Their sheer numbers overwhelm predators, ensuring that at least some survive. And the years-long lag between emergences mean no predator can depend on their annual availability.

But the lack of a viable predator control of the periodical cicadas doesn't mean the periodical cicadas have no predators, or no effect on their predators' lives.

"Everything—birds, rodents, small mammals, even snakes, lizards, and fish will feed heavily on cicadas when they are out," said Keith Clay, a biologist and periodical-cicada expert at Indiana University in Bloomington.

Clay and his colleagues are engaged in a long-term study of the Brood X cicadas to understand their impact on forest ecosystems. Some damage is the direct result of cicada nymphs feeding on tree roots and of adult females laying eggs in high branches. But the Brood X cicadas also have indirect effects on their environment.

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