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.

"Particularly predators that might feed on other things but stop what they are doing and feed on cicadas," Clay said. "That could have any number of effects, both positive and negative."

Moles

The mole, a rodentlike insect-eater that burrows underground, is one predator in Clay's sights. Their populations are at a 17-year peak in density.

"People that care about lawns, parks, [and] golf courses are very much aware that moles are going wild," he said. "There's a superabundance of food for them to eat."

Clay's colleagues recently examined the guts of several moles and found that the rodents have been eating nothing but cicada nymphs. The nymphs have been sucking tree roots underground for the past 17 years and are now a nice shrimp-size meal.

Over the past several years Clay reckons that the abundance of maturing nymphs has provided the moles with ample sustenance to provide for more babies, leading to an overall increase in the mole population.

A few of Clay's graduate students will examine the correlation between mole and cicada density to determine if a high concentration of mole feeding is enough to reduce cicada populations.

One possibility is that, though moles are attracted to areas of high cicada concentration, they won't make a dent in the cicada population, despite voracious eating. Alternatively, if the moles really home in on the cicada in one area, they may cause a local population decline.

"We suspect it's going to be the first—that despite their abundant feeding, they are not able to significantly reduce a cicada population," Clay said.

Depending on the researchers' findings, mole studies may be one way for researchers to quantify cicada density: Where there are moles, there are abundant cicadas, or so the theory goes.

After June the only Brood X cicadas left will be tiny nymphs underground, not enough food to sustain the robust mole populations. As a result, Clay is predicting a major mole population crash by next spring.

Birds, Caterpillars, Fungus

In a related study Clay and his colleagues will examine how the cicadas influence the ecological relationship between insect-eating birds and caterpillars. During a non-cicada-emergence year in Indiana, birds mostly eat caterpillars. This in turn prevents the caterpillars from causing too much damage to trees, where they feed on leaves.

In an earlier study by Robert Marquis at the University of Missouri-St. Louis, researchers showed that oak trees covered in nets to prevent birds from eating caterpillars suffered more damage than trees without nets over them. Clay and colleagues will expand on the study by placing nets over some trees in areas of high and low cicada densities.

The researchers predict that in areas of high cicada density, the birds will be so distracted by the cicadas that, net or no net, the caterpillars will have free reign on the trees. In areas of low cicada density, however, the results of the earlier study should hold.

Clay and his colleagues are also studying a soilborne fungus called Massospora cicadina. The fungus infects cicada nymphs as they emerge and multiplies in the cicadas' bodies, eventually killing the insects.

When the cicadas die from the fungal infection their bodies spread more spores into the ground. There, the spores lie in wait for the next cicada emergence. As the spores build up over the generations, the ground becomes increasingly inhospitable for cicadas.

One theory is that the trees may benefit from the fungus as a way to kill off the cicadas, thus partly explaining why cicadas appear to prefer younger trees on forest edges.

"As trees become loaded with parasites in relation to cicadas, we have some data that suggests [cicadas] will move from older trees to younger forests within striking distance," Clay said.

Clay and his colleagues will examine tree rings from 200-year-old trees in a nature preserve at Indiana University. They want to see whether there were more cicadas at the trees during the early stages of the trees' lives. Since cicadas feed on tree sap, they impact tree ring growth in tell-tale 17 year cycles.

According to Clay, each of these experiments takes advantage of the Brood X emergence to test basic ideas in ecology.

"In the case of the pathogen [fungus], we do believe it may actually control the numbers of cicadas in particular areas. In the other cases of moles and birds, we are looking at the idea that these are opportunistic predators that just take advantage of all this food—and the possible consequences of their feeding," he said.