How Do Cicadas Know When to Emerge from the Ground?
By Kevin Fitzgerald
If you’ve never seen a cicada, you’ve certainly heard them, filling summer days and nights with their loud, raspy love songs. If you’ve seen any, you’re not likely to forget, since they are impressive insects. In the U.S., they grow up to 1.5 inches long, with wingspans of up to two inches.
Cicadas are insects in the order Hemiptera, suborder Auchenorrhyncha, and superfamily Cicadoidea, and they are related to leafhoppers, treehoppers, spittle bugs, and jumping plant lice. About 3,000 species of cicada have been described, with about 170 in the U.S. and Canada. They are found on all continents except Antarctica and on many islands in tropical and temperate regions.
Cicadas have bulging, faceted eyes spaced widely apart, three small eyes called ocelli between the main eyes, and short antennae. The courtship serenades of the males arise from tymbals, two drum-like membranes in the abdomen with powerful muscles to vibrate them. Each species produces its own distinctive call.
Most fascinating about cicadas are the long larval stages of some species. There are three types of cicada life cycle, the annual, periodical, and proto-periodical. After living as larvae, the annual cicadas have life cycles that vary from one to nine or more years, but they are not synchronized in emergence so some appear every year. The periodicals emerge in great hordes after 13 or 17 years, and in any given location they are synchronous. The proto-periodicals erupt in large numbers occasionally and are rare in all other years. Cicadas emerge in the U.S. in the spring and summer (earlier at southern latitudes).
How do cicadas know when to emerge?
Dr. Chris Simon is a molecular systematist at the University of Connecticut’s Department of Ecology and Evolutionary Biology who studies, among other subjects, cicadas in the U.S. and New Zealand.
“The year of cicada emergence is cued by what I and others believe to be an internal molecular clock,” she said. “The clock is most likely calibrated by environmental cues that signify the passage of a year, such as the trees leafing out, changing the composition of the xylem fluid on which they feed. The molecular clock keeps track of the passage of years. The accumulation of 13 or 17 years triggers the emergence of fifth instar nymphs. The day of emergence is triggered by accumulated ground temperature. This was demonstrated by James Heath in a study published in 1968.”
Dr. Gene Kritsky is an entomologist at Mount St. Joseph University in Cincinnati, Ohio and is the author of ten books, among them Periodical cicadas: The Plague and the Puzzle and In Ohio’s Backyard: Periodical Cicadas.
“My colleagues and I hypothesize a molecular clock in periodic cicadas that somehow keeps track of the years,” he said. “That is what we’re looking for now. We suspect it is tied to yearly cycles of the trees they are feeding upon.”
In 2007 in Cincinnati, it was warm in January but there was a hard freeze in February, and then a normal spring, which caused maple trees to produce two leaf sets that year. Hundreds of cicadas feeding on those trees emerged a year early, after the trees produced 17 leaf sets in 16 years.
During years when they are set to emerge, the time of year is determined by soil temperature.
“Emergence is temperature driven,” said Dr. Kritsky. “During the last emergence in our area, I planted temperature probes in the ground all over campus to track the emergence. The cicadas emerged over a period of two weeks after the soil temperature reached 65 °F (18 °C).”
Why study cicadas?
“I became intrigued by historical research in graduate school, and applied it to cicadas,” Dr. Kritsky said. “I found the earliest record of periodical cicadas at the Plymouth pilgrim colony in 1633. I was intrigued by the notion that historical records could be helpful in tracking the evolution of periodical cicada broods. I was following in the footsteps of my professors. Dr. Frank Young at Indiana University and Dr. Lewis Stannard at the University of Illinois were the cicada specialists for their respective states. Their enthusiasm for these ‘bugs of history’ inspired me to continue their work.”
Dr. Simon said, “I started by studying periodical cicadas. I chose periodical cicadas because as an undergraduate student I became interested in understanding 1) why there are so many species on earth, 2) what a species is, and 3) how species form. When I went to graduate school at Stony Brook, NY, I decided that given that I wanted to study speciation, I needed a study organism that was in the process of speciating. My literature search turned up two good candidate groups: crickets in the genus Gryllus, with spring versus fall mating, and cicadas of the genus Magicicada, with reproductively isolated year classes that are essentially incipient species. I have been studying Magicicada ever since.”
“The fact that I was familiar with cicadas and molecular techniques and had an interest in island evolution led me to New Zealand, where I studied speciation and radiation of more than 50 endemic cicada species,” she continued. “My idea was to study the complete history of a species radiation. Twenty-five years later we are nearly finished with that project. In addition, we traced the origins of New Zealand cicadas to Australia and New Caledonia. That led to a global study of the origin, evolution, and spread of the tribe of cicadas (Cicadettini) to which New Zealand cicadas belong, followed by a study of the evolution of all cicada tribes worldwide. Other projects have investigated the impact of hybridization and gene flow among cicada species and the co-evolution of the obligate bacterial endosymbionts of cicadas in collaboration with the McCutcheon Lab at the University of Montana. These endosymbionts manufacture the essential amino acids necessary for survival.”
Cicadas nymphs remain underground, molting through five instars, and emerge from the ground in the fifth instar. Then they molt for the last time, assuming the adult form. The adults live for four to six weeks, feeding on tree sap with their long, beak-like mouthparts. The males sing in trees. Females hear and respond, and mating takes place. Following mating, the female cuts V-shaped slits into the bark of twigs with her saw-toothed ovipositor and lays about twenty eggs to a nest. In six to ten weeks, the larvae hatch, then drop to the ground and burrow into the soil by the tree and make their way to a tree root, from which they suck sap for 13 or 17 years.
The mass eruption of 13- and 17-year cicadas is a ploy to flood the land with more prey than all of the predators can possibly consume, and there are many, including foxes, dogs, squirrels, birds, praying mantises, bats, wasps, spiders, robber flies, and humans. Populations of periodical cicadas can reach 1.5 million per acre. The seven species making up the periodical cicadas are of the genus Magicicada. They are found only in the eastern half of the U.S., the westernmost reach occurring in Kansas.
Periodical cicada species have limited ranges, usually spanning several states, and the periodical cicadas within a range are called a brood. Broods are numbered sequentially, using Roman numerals, which was proposed by entomologist Charles Lester Marlatt, who established the numbering system in 1898. For the most part, the brood ranges fit together like a jigsaw puzzle, although the larger broods sometimes overlap in distribution.
More than one species typically occupies a brood. For instance, 13-year Brood XXIII emerged in the lower Mississippi River Valley in 2015 with four species: Magicicada neotredecim, M. tredecim, M. tredecassini, and M. tredecula. That same year, 17-year Brood IV (also known as the Kansan Brood) emerged in the western edge of the general periodical cicada range with three species: Magicicada septendecim, M. cassini, and M. septendecula. So all seven species of Magicicada emerged that year.
Dr. Simon and her colleague Erin Dwyer have devised a school exercise called “Experimental Studies of the Biology of 13- and 17-year Periodical Cicadas: A Laboratory Exercise for University and AP Biology Laboratory Classes.” The exercise teaches students to examine the soil and pick out Magicicada nymphs, put them in vials labeled with appropriate data, and bring them back to class to study in depth.
“We would like more students to try our cicada life cycle exercises,” Dr. Simon said.
So let’s drum up some respect for these hardy little creatures, especially next summer, when their grating serenades fill the forests of eastern Ohio, Western Pennsylvania, and West Virginia. Be sure to report Magicicada sightings at www.Magicicada.org!
Kevin Fitzgerald is a freelance science writer living in Connecticut. He has published in newspapers, encyclopedias, and online.