I was packing for my road trip from Colorado to Alberta, Canada, when the text came in from a gentleman I’d been helping with groceries during the pandemic.

“The Canadians are actually doing damage to vehicles with United States plates on them,” he cautioned, giving me my first inkling that it wasn’t just public health officials who were serious about keeping Americans out of Canada, where the death rate from the coronavirus has been roughly half that of its southern neighbor.

As a dual citizen I was entitled to cross the border, closed to most Americans because of the pandemic. With an octogenarian father in Calgary who had been largely isolated during the stay-at-home orders, I was willing to submit to Canada’s mandatory two-week quarantine in order to visit.

But my friend’s warning proved prescient. Some concerned residents who fear that the virus will be spread to their communities have been taking matters into their own hands, spurring so many reports of intimidation that the premier of British Columbia, John Horgan, reminded angry Canadians to “Be Calm. Be Kind” at a July 27 news conference.

Addressing those Americans who are in Canada legally, he said: “With respect to those who have offshore plates and are feeling harassed, I would suggest perhaps public transit. I would suggest that they get their plates changed. I would suggest that they ride a bike.”

Before the pandemic, when Americans could choose most any country in the world to travel, Canada was their second most popular foreign destination, behind only Mexico. Lured by the proximity, advantageous exchange rate and safety of their northern neighbor, in the first six months of last year, U.S. residents made 10.5 million trips to Canada, the highest level in 12 years, according to Statistics Canada, a government agency.

But the welcome mat was rolled up on March 31, when the border between the two countries was closed to tourists.

That hasn’t kept some Americans from trying, however. Many are routinely turned away at border crossings, while others have chosen to go sightseeing instead of taking the most direct route to Alaska as required of those driving from the Lower 48 — even though violators face possible fines, jail or even being banned from Canada.

There were so many interlopers that on July 31, Canada began limiting which crossings along the border with the United States can be used by foreign nationals who are allowed to transit through the country for nondiscretionary purposes. It is also requiring them to register, and making them display a hang tag on their rear view mirror with a mandatory departure date. The crackdown comes even though the number of tickets issued was just a fraction of the number of reports coming in to Royal Canadian Mounted Police.

In the province of Alberta, for instance, there were no tickets issued to American motorists in April, May or July, and only nine tickets issued in June, all in Banff National Park, said Cpl. Tammy Keibel, a spokeswoman for the RCMP in Southern Alberta. The federal police force didn’t start recording complaints about international license plates until June 17, but there were 53 reports in the entire province between June 17 and June 29, and 121 between July 1 and July 28, she said.

The province’s most troublesome scofflaw thus far is a fellow from Alaska who was so determined to enjoy Banff with a woman from Calgary that he’d met online that he was issued two of the June tickets. His identity hasn’t yet been released, Keibel said.

His downfall, like that of many others, was precipitated by concerned citizens, not authorities. The Alaska plates on his truck were spotted June 25 during one of the regular parking lot sweeps that the Rimrock Resort Hotel in Banff conducts. Video footage was reviewed to confirm the driver’s identity, and he was questioned in his room. When he was unable to show that he had complied with quarantine laws, the police were called, said Trevor Long, the Rimrock’s general manager.

Since the border closed four months ago, only four other guests have been questioned about their plates. One was an American who had been in Canada since before the border restrictions, another was in the military and said he was an essential worker. The other two were let off with a warning.

The Alaskan, however, proved a “challenging fellow” who thought “this whole pandemic was a farce,” according to Long. He was issued an $870 ticket under the Alberta Public Health Act and instructed to leave town the following day.

Instead, the couple showed up for their massages the next morning.

“He was a little bit irritated that we said, ‘No, you’re not allowed to have your spa appointment,’” Long said.

The pair then drove to the Banff Gondola, a popular tourist destination, where the Alaska license again prompted someone to call the RCMP.

The couple was intercepted at the summit and he was charged with violating the federal Quarantine Act and faces up to six months in prison and an additional fine of up to $560,000 if convicted. A patrol car escorted them out of town, though Keibel didn’t know if the man from Alaska had left Canada, nor whether he’d be required to return for his November court date.

On a sunny day in late July, I drove around the Banff area and looked at an estimated 200 cars. Only one was from the United States, a Toyota Prius from California parked at Johnson Lake. Well, that one, and mine. And it didn’t take long for someone to notice my Colorado plates.

“Hey, how’d you get across the border?” a guy on a beater bike called to me as I was parked at the corner of Buffalo and Bear streets. When I told him I was a citizen, he retreated. “I was just wondering,” he said, cautioning that some locals get furious when they see U.S. plates.

These local efforts aren’t confined to Canada. In Hawaii, a group with more than 5,600 volunteers called the Hawai’i Quarantine Kapu Breakers works to track people breaking the mandatory two-week quarantine for visitors to the islands. The quarantine law, which will be in effect until at least Sept. 1, carries a possible $5,000 fine and up to a year in jail.

Angela Keen, who is running the sleuthing posse, said it has helped bring 45 people to the attention of the authorities, including the leader of a countercultural group called Carbon Nation and 20 of his followers. The leader, Eligio Bishop, pleaded no contest in June to breaking quarantine rules and was sentenced to 90 days in jail. His sentence was suspended and the charges against his followers were dropped when they agreed to return to the mainland.

In New York, where visitors from many states are asked to quarantine for two weeks, there have been more than 1,400 complaints to the state’s COVID-19 Enforcement Task Force regarding possible violators, according to Caitlin Girouard, a spokeswoman for Gov. Andrew M. Cuomo. That doesn’t include complaints to local health departments. Girouard did not know how many tickets had been issued under the order, which carries fines of up to $10,000 if a person causes “harm.”

The maximum penalty in Canada is far higher, with the possibility of up to three years in prison and a $750,000 fine for someone who willfully causes harm to another.

Among Canada’s citizen detectives are those spurred on by the Canada Border Services Agency.

Some who live on Vancouver Island have taken to monitoring boat traffic to see who turns off their vessel’s automatic identification transponder, which is required to be on at all times. When they notice a craft has gone dark, they assume it’s trespassing and report it to the RCMP. Two American boaters have each been fined $738 under Canada’s federal Quarantine Act. One reportedly misstated his intentions to sail to Alaska, while the other was a whale-watching vessel that had crossed the border from Washington.

The big fines and possible prison sentences are meant to be a serious deterrent. Despite the loss of revenue from American tourists, Long, who is also president of the Banff & Lake Louise Hospitality Association, said he doesn’t know a single person who wants the current restrictions to relax.

“We are very reliant on the American traveler. It makes a huge impact on the economy,” he said. “But we are ready to continue taking the hit until things get better around the world.”

Other Canadians speak of the situation with less restraint. “It makes me angry and it frightens me because Canada is obviously doing its level best, mostly successfully, to keep our country as safe as possible and our numbers low,” said Tamara B., of Calgary, who asked that her last name not be used. “You’d have to be living in a cave for the last six months to not know what the situation is down there” in the United States.

I could understand her fear of getting COVID-19. I’d been nervous about the increased risk of contracting the virus while traveling and expected that the stressful part of my trip would be the 16-hour drive from Colorado to Calgary. Or maybe the quarantine.

But my friend in Colorado with the warning about car damage was right. Besides the fear of getting sick, the real stress is parking a vehicle with American plates, and hoping that nobody notices them.