ITHACA, N.Y. -- Any day now, Brood X, the largest, most prolific brood of 17-year cicadas, will emerge from the ground and cut a swath across the Eastern seaboard. But many won't even make it to the surface: While the cicada nymphs have been developing into adults underground, their habitats have been paved over by parking lots, enormous shopping malls and large tracts of homes.
"The Eastern U.S. corridor is so developed that cicada habitats have been destroyed," says Cole Gilbert, Cornell University associate professor of entomology. "They need gigantic numbers to swamp their predators and survive."
An indicator of the devastation that urban development has caused to periodical cicada breeding areas can be seen by what has happened to a tiny brood that won't be emerging this year, the Finger Lakes brood. Periodical cicadas emerge in specific locations across North America once every 13 or 17 years, and the males begin courting females with a high-pitched trill, made by vibrating membranes on the sides of the first abdominal segment.
The Finger Lakes brood last emerged in 2001, and it won't be seen again until 2018. But already urbanization has limited the insect's habitat to just one area in New York state, Onondaga County. Yet 200 years ago, the brood thrived in 10 upstate New York counties.
Using data going back to John Adams' presidency in 1797, Cornell entomologists have tracked the Finger Lakes cicada group (Brood VII, Magicicada septemdecim) and have found their numbers have plunged. (Cicadas often are mistakenly called locusts, a term that refers to migratory grasshoppers.)
Urbanization is not the only enemy of the slow-flying periodical cicadas. "As anyone knows who has been in an emergence area, specimens are relatively easy to catch by hand," Gilbert says. Consequently many omnivorous animals, such as birds, red foxes and raccoons, feed voraciously on the cicadas during the two or three weeks of their above-ground existence.
Gilbert says the cicadas' only defense is a great offense: their large numbers. "Emergence densities are so vast that all predators in the region become satiated for a short time, and enough cicadas survive to breed and sustain the population," Gilbert says.
The Finger Lakes brood has the most restricted geographical range of any cicada group distributed throughout the eastern United States. During the past 200 years, this brood has been found only in Wyoming, Livingston, Monroe, Ontario, Yates, Cayuga, Tompkins, Oswego, Onondaga and Madison counties in New York.
Cicadas need large forest tracts to reproduce, says Gilbert, but during the 20th century, much of their breeding habitat was converted to agriculture. During the 1967 Finger Lakes brood emergence, the late Cornell entomologist LaVerne "Verne" Pechuman was unable to find any periodical cicadas in any counties other than Livingston, Cayuga and Onondaga.
Pechuman returned to the same sites in 1984 and found that in Livingston and Cayuga counties the populations of cicadas were very small and had been devastated by birds, primarily grackles and red-winged blackbirds. Pechuman looked for signs of egg laying, such as a series of small slits in twigs that prevent leaves from receiving nutrients so that they turn prematurely brown. A wooded hillside in a periodical cicada emergence area typically is cloaked with brown "flags" on branch tips by the end of July, well before the leaves of unaffected trees have begun to show their fall colors, Gilbert explains. Pechuman found signs of egg laying only in Onondaga County and predicted that the Finger Lakes brood would be restricted to that county during the next emergence in 2001.
Pechuman died in 1991, but Carolyn Klass, Cornell extension researcher in entomology, and Gilbert checked the 2001 emergence prediction. "Unfortunately, Verne's prediction was right, and the Finger Lakes brood now appears to be restricted to Onondaga County," says Gilbert.
There is a very healthy population of M. septemdecim on the Onondaga Indian Reservation and other areas closer to Syracuse, says Gilbert. Human development south of Syracuse, however, could soon limit the brood to the reservation.
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