They scarf potato chips and whole bags of marshmallows late in the night, leaving behind trashed campsites and ruined tents. They break into stranger’s coolers and make off with watermelons. They carelessly turn on water spigots and leave them running.

Rangers are dealing with a problem that has all the hallmarks of a classic beach-week bender, but the culprits aren’t rowdy teens. They’re Assateague Island’s famous wild horses.

Park officials said the very thing that makes the horses a national treasure and draws millions of visitors to this 48,000-acre seashore south of Ocean City each year is under threat: Their wildness.

The horses have been mooching food for years, but their brazenness has grown worse in the past few, said Trish Kicklighter, the park’s superintendent. Some of Assateague’s 113 horses have become particularly fond of junk food and interact with people who bring it into the park. The horses beg. They pester. They even run a hustle that wouldn’t be out of place on a D.C. street corner.

“I didn’t believe it until I saw it,” Kicklighter said. “Two horses put their youngest, cutest pony in front of a car, and then the older horses went around to the windows to panhandle for food.”

The fact that horses have learned how to open coolers and turn on spigots draws chuckles from visitors, but park officials warn too much contact with humans puts the horses and people at risk.

In June, biologists took the unusual step of moving a stallion from the park to a horse rescue center after it head-butted a woman, leaving a gash on her head. The horse had harassed other visitors for food, park officials said.

For the horses, a diet rich in human food could make them sick. Debris such as tinfoil has been found in some horses’ excrement.

Park officials are quick to place the blame for the horses’ behavior on those with two legs, not four. This year, they have rolled out measures to reestablish the boundaries between horses and humans, inspired by national parks out West that have managed bears. Officials have hired a full-time equine chaperone for its Pony Patrol team and instituted a $100 citation for “willfully” getting within 10 feet of a horse. Previously, there was only a fine for petting or feeding the horses.

Park officials said the horses’ behaviors undermine the majesty of the animals, which inspired the novel “Misty of Chincoteague.” Tourists expect postcard images of horses galloping on beaches and manes aflutter on dunes, not a horse with its head stuck in a bag of Cool Ranch Doritos.

“A horse that is raiding your campsite and is getting into your cooler — is that a wild horse anymore?” asked Carl S. Zimmerman, a park spokesman. “That’s a shame, because that wildness is what makes them so special.”

Phil Gregoli is one of the best hopes for keeping Assateague’s wild horses wild. With a neon-colored safety vest and walkie-talkie, the retired printer is a member of the park’s mostly volunteer Pony Patrol.

The patrol spends its days shooing horses off roads, camping areas and more crowded beaches, but the job is as much about herding humans as horses. Gregoli’s walkie-talkie crackled often on a recent July morning.

“Horses up on six-one-one,” a ranger radios, referring to the main road onto Assateague Island.

Gregoli hopped in a green golf cart to head to the scene but only made it halfway before running into a “horse jam” — that’s park lingo for a backup caused by horses lingering on a road and visitors slowing down to rubberneck.

A driver with rolled-down windows pulled up next to a chestnut mare with a blond mane. The horse hurdled a guardrail and attempted to stick its head inside the car. The driver pumped the gas and bolted away.

Park officials said many horses have come to expect food from passing motorists, so they linger on roadways. It’s one of the behaviors officials would most like to change, because it can be deadly for the horses: The park averages about one car-related death a year, they said.

Gregoli jumped out of the golf cart and shooed the horses off the road. He said many of the problems with horses boil down to some visitors’ perceptions of the horses as not truly wild, like a bear or moose.

Gregoli said he once came upon a man hugging a horse and feeding it potato chips. Park officials said they have seen parents stick a child on a horse’s back to get a snapshot.

To combat such behavior, a new sign states: “Horses, Bite, Kick & Charge/KEEP AT LEAST 10 FEET AWAY.” To help keep the animals away from campsites, park officials installed wooden blocks to prevent them from nudging open water spigots.

Many longtime Assateague Island campers said an encounter with the horses is just part of a trip.

Sally Cohen of Tampa, Fla., who has been going to Assateague for more than 25 years, said she once returned to her campsite to find five horses munching on hot-dog buns she had left out. Another time, she said, a horse rooting around in a tent got spooked and charged off — with the tent around its neck.

“This island belongs to the ponies,” Cohen said. “They are mischievous little buggers. You almost expect to lose some food or camping gear each time you come out.”