A cicada "invasion" is imminent in the US, with millions of the large, cricket-like insects poised to emerge from the earth after 17 years lying in wait. The first of the bugs that are expected to blanket the US east coast have been spotted in North Carolina and New Jersey. "The Brood II emergence has begun!" cheered website cicadamania.org upon which a subculture of insect fans record their sightings. But the onslaught will truly begin later this month, once the average ground temperature hits 17 degrees Celsius. At its peak, there could be swarms with 1,000-2,000 chirping insects per square metre. Although cicadas are common around the world, a cyclical phenomenon happens only in the United States. Every 17 years, "periodical cicadas" mature, mate, lay eggs and die in a deafening concerto. Their offspring - which won't be seen again for another 17 years - burrow 20cm into the ground, where they will feed on the sap from roots until their day in the sun arrives. The broods - there are 15 of them - are classified by Roman numeral. Most are on a 17-year cycle, although three reproduce every 13 years and the cycles are staggered, meaning that at least one of the broods hatches each year. But not all broods are created equal and "Brood II" is a big one. Over the next few weeks, the insects will emerge and begin a reproductive orgy when the larvae split their skins and mature into adult form, said University of Maryland entomologist Michael Raupp. "Then they'll scramble in a flight to the treetops. The males will begin to court the females. They will mate," he said. "[The females will] lay their eggs, these eggs will hatch and will tumble down to the earth, and feed again for another 17 years."