by By Will Dunham
WASHINGTON (Reuters) – Parts of the United States are about to experience a rare natural phenomenon with the simultaneous emergence of two giant broods of periodical cicadas in close proximity.
More than a trillion of these noisy bugs are about to come out of the ground starting around April.
The two broods—one concentrated in the US Midwestern states and the other in the South and Midwest, with a small area of overlap in Illinois—emerge together only once every 221 years.
Here’s an explanation of what is expected to happen during the “dual advance.”
WHAT IS A CICADA?
Cicadas are relatively large insects – 1-2 inches (2.5-5 cm) long – with firm bodies, bulging compound eyes and membranous wings. There are many different types of cicadas.
Using needle-like mouths, cicadas feed on plant juices, called xylem, drawn from the roots of deciduous trees and shrubs. They spend much of their life cycle – years later – underground as nymphs feeding on roots and drinking xylem.
After emerging, adult males “sing” to attract females using special organs called tymbals on the first part of the abdomen. The pitch, tone, frequency and volume of song are specific to individual species. Cicadas live as adults for a few weeks, then die after reproducing. Many birds and mammals eat cicadas.
HOW DO SEASONAL AND ANNUAL CICADAS DIFFER?
With annual cicadas, several individuals emerge during any given year. They spend one to nine years underground as nymphs, varying by species, and do not have synchronized emergence. Instead, they emerge on a staggered basis.
Periodic cicadas are more specific and have a longer time spent underground as nymphs – generally 13 or 17 years – and synchronized emergence. This means that each individual brood member hatches in the same year, from late April to June, depending on their location. Each of the periodical cicadas that share the same life cycle and emerge together in a given year is called a brood, although a single species may be part of several broods.
There are more than 3,000 species of cicadas in the world, but only nine of them are periodic, seven of which – of the genus Magicicada – are found in North America. In India, a periodic species of the genus Chremistica occurs every four years, and in Fiji, a periodic species of the genus Raiataena occurs every eight years.
WHY ARE TWO EVENTS RELATED TO DOUBLE GRADUATION THIS YEAR?
Brood XIII, on a 17-year cycle, is mainly confined to northern Illinois, eastern Iowa, southern Wisconsin and a few counties in far northern Indiana, according to entomologist Floyd Shockley of the Smithsonian Institution’s National Museum of Natural History in Washington. Brood XIII includes three species of Magicicada.
Brood XIX, on a 13-year cycle, is widely distributed from Alabama, Arkansas, Georgia, Indiana, Illinois, Kentucky, Louisiana, Maryland, Missouri, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee and Virginia – a total of 15 states, according to Shockley. The two broods together comprise parts of 17 states. Brood XIX contains four Magicicada species.
These two broods overlap only in a small area in central Illinois. They are close enough that there may be some interbreeding between broods.
WHEN DOES THIS SECOND STAGE OCCUR?
Period cicadas are expected to begin emerging in the southern parts of their geographic distribution in mid-April. The emergence continues northwards in June. Since most hatcheries produce localized populations of over 1.5 million cicadas per acre (0.4 ha) in densely populated areas of their distribution, there can easily be over a trillion cicadas during emergence this, according to Shockley.
WHAT DO CICADES DO WHEN THEY COME BACK?
The cicadas begin to emerge, mainly at night, once the soil warms to about 64 degrees Fahrenheit (17.8 degrees C), according to George Washington University entomologist John Lill. These nymphs crawl over any hard surfaces – tree trunks, fences, vegetation – and molt into adult winged cicadas.
After a few days, the adults fly into the tree canopy, where males create a loud “chorus” calling to the females by vibrating their tympals. Males have relatively empty bellies, acting as echo chambers to amplify their calls. Cicadas are among the tallest insects. Females attracted to a particular male’s call respond with wing flicks, which also make a sound. Couples will live there.
Once mated, female cicadas seek out pencil-sized branches of trees and shrubs in sunny areas to lay their eggs in slits they cut into branches, according to Lill. These eggs develop for about six to seven weeks, after which hungry nymphs drop to the ground and burrow to start the next generation of periodical chickadees.
WHEN DID THIS WONDERFUL EVENT HAPPEN?
These two broods last emerged in the same year in 1803. The next time is set for 2245.
(Reporting by Will Dunham in Washington, editing by Rosalba O’Brien)
