Cicadas are back and buzzing in many states this year. But for most Washington area residents, it is — fortunately — a distant buzz.

Inch-long, winged cicadas, which are often mistakenly called locusts, last enveloped the area in 2004. They aren’t expected back until 2021, part of a brood that spends most of its 17-year life cycle underground before coming to the surface to mate.

There is, however, another large brood stirring this year. Brood XIX, which has a 13-year life cycle, mostly appears throughout the South, but there are confirmed reports that they are surfacing as close to Washington as Richmond and Williamsburg in Virginia and St. Mary’s County in Maryland.

“I think if it were [to get any closer], it would be here by now, and we surely would have seen it if they were going to be here in any numbers,” said Mike Raupp, an entomologist at the University of Maryland, who said he has “been chasing cicadas for decades.”

Fifteen different cicada broods inhabit the eastern part of the country. Each year, at most one of the 13-year and one of the 17-year broods rises from the earth. Brood X has the largest geographic reach of the 17-year cicadas, appearing in the District and 15 East Coast states. This year’s Brood XIX is the largest of the 13-year cicadas, and it is now emerging from the soils of Oklahoma, Arkansas, Mississippi, Alabama, Georgia, Tennessee, the Carolinas, Kentucky and Illinois, in addition to Virginia and Southern Maryland.

It’s not just by luck that this year’s cicadas aren’t quite reaching Washington. As Christine Simon, an ecology professor of the University of Connecticut, described it, broods “fit together like an intricate jigsaw puzzle.” So where Brood XIX ends in Southern Maryland, for instance, Brood II begins and then meets up with Brood X.

“There’s lots of competition underground among the nymphs [earlier-stage cicadas] — that’s one of the things we believe stops them from overlapping,” Simon said. “There are millions and millions of nymphs, so if another [brood] comes along, there’s not going to be enough room for them.”

Competition is also a suspected reason for occasional early emergence of cicadas. Yes, Brood X cicadas, which carpeted area lawns, trees and sidewalks in 2004, could be back before 2021, according to Simon. Scientists can’t fully explain it, but small populations have been known to emerge before they’re due; four years early is the most common deviation from their regular schedule. In 2017, Simon said, the Washington area could see thousands, maybe tens of thousands, of Brood X cicadas, though that is a small number compared with the hundreds of millions of the actual brood year.

“They’ll be obvious, but there won’t be that many,” Simon said of any Brood X cicadas that emerge in 2017.

As for those who think they’ve already seen Brood X reappearing, it’s highly unlikely that the cicadas would surface 10 years early, experts said., a Web site run by Simon and her research colleague John Cooley, has unconfirmed reports from the public about cicadas in Manassas and Rockville. Cooley and Simon said those sightings are either errors or early arrivals of Brood II, a smaller invasion expected in parts of Maryland and Virginia in 2013.