Bloodsucking, green-headed monsters.

That’s how Hempstead Town officials are describing a breed of biting horsefly that is swarming the salt marshes off the South Shore.

Town workers are adding an extra 25 traps this season for the greenhead flies, which seek out blood from humans and animals. The boxes will be installed from Seaford to the New York City line in Inwood, capturing much of the town’s 73 square miles of waterways.

“The cool, wet spring followed by hot, sultry summer days has resulted in a bumper crop of these flies along Long Island’s South Shore,” Hempstead Supervisor Anthony Santino said Thursday in Lido Beach. “Anyone in the water can tell you the nuisance of these flies.”

Santino said the town is going “low-tech” against the greenheads by using fly traps, a non-pesticide method to catch the flies. The traps are nothing more than a bottomless brown-painted box with a screen on top, placed on stilts in the marshes and along the water off Reynolds Channel.

Officials said the flies are drawn to the warmth and darkness of the boxes, and they fly in through the open bottom and become trapped, then dehydrate and die.

Santino, grabbing a handful of the dead bugs during a Thursday news conference, said the traps need to be emptied about once a week by town workers.

The traps can catch up to 2,500 greenheads per day. The 60 boxes across the region could gather more than 150,000 flies per day. Each box costs about $100 to make.

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The flies populate coastal areas along the East Coast from late June to August, peaking in mid-July. They grow to about a half-inch, and the female greenhead, which seeks out blood for nourishment, can lay 200 eggs at a time. The flies are not known to carry disease.

The flies can travel up to 2 miles, and have been a nuisance, not only on the beach, but across the outdoors, with “scissor-like jaws,” Santino said.

The flies can live for about four weeks and are largely resistant to pesticides, which only affect the weakest in the population and end up creating a breed of “super greenheads,” Santino said

Hempstead Councilman Anthony D’Esposito said the flies can be a painful presence in waterfront communities like Island Park.

“I know what a green fly can do to a beautiful day on a boat or even at a Little League game,” D’Esposito said. “These traps help reduce the number of biting flies, so we can enjoy the summer with no pesticides affecting the environment.”