Billions of cicadas, dormant for the past 17 years, are waiting only for soil temperatures to reach 64 degrees before they emerge from underground and begin their noisy mating rituals.

That could happen soon in South Jersey. An online Rutgers New Jersey Agricultural Experiment Station Map indicated soil temperatures reached 61 degrees Friday.

George Hamilton, a Rutgers Extension specialist in pest management and chairman of the university’s Department of Entomology, said the cicadas will emerge later this month and will be seen through early June.

“Once it starts, they should be around for two to four weeks,” Hamilton said.

Those who think public displays of affection are annoying will not be happy when the red-eyed bugs serenade them to sleep every night for several weeks with their mating calls.

“It’s a phenomenon unique to the eastern United States,” said Jamie Cromartie, assistant professor of entomology at Richard Stockton College in Galloway Township.

This year’s invasion is one of the largest groups, or broods, of 17 total groups. It will affect states from North Carolina to Connecticut. Several experts say they don’t really have a handle on how many cicadas are lurking underground, but 30 billion in the eastern U.S. seems like a good estimate. At the Smithsonian Institution, researcher Gary Hevel thinks it may be more like 1 trillion.

But even if it’s merely 30 billion, if they were lined up head to tail, they’d reach the moon and back. Past cicada invasions have seen as many as 1.5 million bugs per acre. The last invasion in the eastern U.S. took place in 1996.

The number of years between brood sightings is 13 or 17, and each year a brood emerges somewhere in the country, Cromartie said.

The 13-year group is typically seen in the South and Midwest U.S., according to www.magicicada.org

The numbers themselves are unique, because they are prime numbers, Cromartie said, adding to the appeal for scientists. And there is no explanation as to why they emerge only every 13 or 17 years.

There are different arguments about why cicadas follow this cycle, but one thing for sure is that there is strength in numbers, similar to a school of fish, Cromartie said. Early stragglers or late-comers can become easy prey for birds.

When en masse, the only prey they cannot fend off are machines that dig up the earth — a symptom of suburbanization, Cromartie said. While the bugs don’t exactly travel, their habitats are disturbed every time new foundations and pavement are laid.

Brood II, which is the specific type of cicada that emerges in the eastern U.S. region, is one of 17 different broods identified in the country, Cromartie said.

Since 1996, this group of 1-inch bugs, in wingless nymph form, has been a few feet underground, sucking on tree roots and biding their time. After a few weeks up in the trees, they will die and their offspring will go underground, not to return until 2030.

In 2004, Gene Kritsky, an entomologist at the College of Mount St. Joseph in Cincinnati, measured cicadas’ noise at 94 decibels, saying it was so loud, “you don’t hear planes flying overhead.”

While underground, the bugs aren’t asleep. As some of the world’s longest-living insects, they go through different growth stages and molt four times before ever getting to the surface.

The cicadas feed off the sap in roots of trees — hardly a nutritious meal — so it takes a lot of effort for the bugs to develop fully, Cromartie said.

Then they go above ground, where they molt again, leaving behind a crusty brown shell, and grow a half-inch bigger.

When the massive symphony is over, the female cicadas will lay their eggs in slits at the ends of tree twigs, Cromartie said. While this isn’t harmful to the trees, it can weaken the twigs. This is especially visible in orchards, he said.

Soon after the eggs are laid, the cicadas hang out in the trees until they die. When the eggs hatch, the new generation heads back underground.

“They are not harmful. It’s just a natural phenomenon,” Cromartie said.

Contact Anjalee Khemlani:

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