What is ‘Brood X’? A massive swarm of cicadas that emerges once every 17 years, It will appear in 12 states ranging from Illinois to Georgia to Pennsylvania. Once the weather warms up, trillions of cicadas will emerge from the ground, living for just a few weeks to mate, with their larva returning to the ground to hibernate for another 17 years. It will be the first appearance from Brood X since 2004. While the cicadas are harmless, the buzzing will be loud and can be heard from miles away.