When the double brood of cicadas will come out - and what to expect

By Will Dunham

WASHINGTON (Reuters) -Parts of the United States are experiencing a rare natural phenomenon with the simultaneous emergence of two enormous adjacent broods of periodical cicadas. 

The noisy bugs began in April to pop out of the ground in significant numbers in the southernmost locations of their geographical distribution. The total count of cicadas is forecast to exceed a trillion by the time it is all over in their northernmost locales.

These two broods - one concentrated in U.S. Midwestern states and the other in the South and Midwest - emerge together only once every 221 years. 

Here is an explanation of this "dual emergence."

WHAT IS A CICADA?

Cicadas are relatively large insects - 1-2 inches (2.5-5 cm) long - possessing sturdy bodies, bulging compound eyes and membranous wings. There are many different kinds of cicadas.

Using needle-like mouthparts, cicadas feed on plant juices, called xylem, drawn from the roots of deciduous trees and shrubs. They spend much of their life cycle - years on end - underground as nymphs feeding on roots and drinking xylem.

After they emerge, adult males "sing" to attract females using special organs called tymbals on the first segment of the abdomen. The song pitch, tone, frequency and volume are specific to individual species. Cicadas live as adults for just a few weeks, then die after reproducing. Numerous birds and mammals eat cicadas.

HOW DO PERIODICAL AND ANNUAL CICADAS DIFFER?

With annual cicadas, some individuals emerge during any given year. They spend one to nine years underground as nymphs, varying by species, and do not have a synchronized emergence. Instead, they emerge on a staggered basis.

Periodical cicadas have more specific and longer lengths of time spent underground as nymphs - generally 13 years or 17 years - and a synchronized emergence. That means that all members of a particular brood emerge the same year. All of the periodical cicadas sharing the same life cycle that emerge together in a given year are called a brood, although any one species may be part of different broods. 

There are more than 3,000 species of cicadas worldwide, but only nine are periodical, and seven of those - of the genus Magicicada - are found in North America. In India, a periodical species of the genus Chremistica emerges every four years, while in Fiji, a periodical species of the genus Raiataena emerges every eight years.

WHAT TWO BROODS ARE INVOLVED IN THIS YEAR'S DUAL EMERGENCE?

Brood XIII, on a 17-year cycle, is restricted mostly to northern Illinois, eastern Iowa, southern Wisconsin and a few counties in extreme northwestern Indiana, according to entomologist Floyd Shockley of the Smithsonian Institution's National Museum of Natural History in Washington. Brood XIII includes three Magicicada species.

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. Brood XIX includes four Magicicada species.

These two broods together span parts of 17 states.

WHAT IS THE TIMING OF THIS DUAL EMERGENCE?

The cicadas in these broods began emerging in significant numbers in the southern parts of their geographical distribution in April. The emergence is expected to continue northward into June. Because they spend such a short time as adults, the last of the cicadas in the northernmost parts of their distribution likely will disappear by the following month. 

"Up north, it may well last until July, but down south, they'll be long gone. The adults only last a few weeks," said University of Connecticut evolutionary biologist John Cooley.

Given that most broods produce localized population numbers exceeding 1.5 million cicadas per acre (0.4 hectare) in densely populated areas of their distribution, there easily will be more than a trillion cicadas during this emergence, according to Shockley.

WHEN WAS THE LAST SUCH 'DUAL EMERGENCE'?

This will mark the first time that a 13-year brood emerges in the same year as a 17-year brood since 2015. The last time that adjacent 13-and 17-year broods emerged in the same year was 1998, according to Cooley. Brood XIX, one of the two popping out this year, emerged in 1998 at the same time as Brood IV, which spans Iowa, Kansas, Missouri, Nebraska, Oklahoma and Texas. 

The next time two 13-and 17-year broods will emerge the same year will not be until 2037 and the next time adjacent 13-and 17-year broods emerge together will not be until 2076, Cooley said.

WHAT DO CICADAS DO WHEN THEY EMERGE?

The cicadas begin emerging, 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 up any hard surfaces - tree trunks, fences, vegetation - and molt into adult winged cicadas.

After a few days, adults fly into the tree canopy, where males form loud "choruses," calling to females by vibrating their tymbals. Males have rather hollow abdomens, serving as echo chambers to amplify their calls. Cicadas are among the loudest insects. Females that are attracted to a particular male's call respond with wing flicks, which also make a sound. Pairs then mate. 

Once mated, female cicadas seek pencil-sized branches of trees and shrubs in sunny locations to lay their eggs into slits they cut in branches, according to Lill. These eggs develop for about six to seven weeks, after which hatched nymphs drop to the ground and burrow to begin the next generation of periodical cicadas.

WHEN WILL THIS BUG-TASTIC EVENT OCCUR NEXT?

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)