How to Stop Moths? A Historic Building Tries Bringing in Wasps

Empty of human visitors because of lockdowns, the darkened rooms of Britain’s historic buildings have gained thousands of unwelcome residents over the past 11 months: moths that infest closets, perch in the rafters and floorboards, and nibble away at priceless tapestries, furniture, carpets and art.

At one centuries-old estate, Blickling Hall in Norfolk, England, the charity that runs the sprawling grounds is trying to stop the moths with an even smaller set of visitors: wasps.

The National Trust, the heritage and conservation organization, hopes to fend off the moths by shipping in thousands of microscopic wasps — each one measuring only 0.5 millimeters — that hijack moth eggs.

“It’s not like our tapestries are falling off the wall and our things are being munched to bits,” said Hilary Jarvis, an assistant national conservator at the National Trust.

“It’s just that no damage is acceptable and it’s heartbreaking when you do find something,” she said. “We know they’re there and we’re not going to be complacent, and I can’t take the risk that we let those moths thrive.”

The plan is for a company that helps museums with pests, Historyonics, to place the tiny wasps, called Trichogrammaevanescens, on cards around the building. From there, they will seek out moth eggs to lay their own inside instead, halting moths’ reproduction process.

The company will also try a second tactic, spreading a female moth pheromone to confuse the male moths and reduce their chances of finding a mate.

The wasps, invisible to the human eye, pose no danger to people, said David Loughlin, the manager of Historyonics. The insects live only about two weeks and, once they die, their bodies all but vanish into house dust.

“They’re not to be confused with the wasps that sting you when you’re having your picnic,” he said. “It’s a different end of the spectrum. It’s like comparing a domestic Chihuahua with a wolf.”

Blickling Hall, one of the more than 500 historic castles, houses, parks and monuments maintained by the National Trust, has a large collection of valuable items that the charity is eager to protect.

Blickling, a 17th-century red-brick property, sits on the foundations of a manor house that is thought to have been the birthplace of Anne Boleyn, King Henry VIII’s second wife (beheaded in 1536). A star of the hall’s collection is an 18th-century tapestry of Czar Peter I of Russia, given by his eventual successor, Catherine the Great, to one of the building’s former owners.

The building’s moth problem has worsened for years, though the insects have caused only minimal damage to its treasures so far.

The anti-moth campaign will begin early next month and continue through the rest of the year. The National Trust has previously used wasps and the pheromone technique separately for short stints, to varying success, but the charity will try both together for the first time at Blickling.

Using wasps to control moth and other insect problems is a technique already used widely in agriculture, particularly in greenhouses, said James Kitson, an ecology and agriculture research fellow at Newcastle University in Britain. The measure works best in controlled environments, and could work well inside the hall, he said.

The wasps are unlikely to leave the house, as their entire and extremely short life cycle is focused on finding a mate and a new host, Dr. Kitson said. “When these things get outside into the real world, they have to compete against hundreds and hundreds of species that are also doing the same things to other native moths in the area,” he added.

If the trial significantly reduces the moth population, Ms. Jarvis said it could be used in the charity’s other properties experiencing the same problem.

A National Trust survey of its many properties found that the number of bugs had risen by 11 percent in 2020 compared with the previous year, making a thriving insect population in Britain’s stately old homes one of the many unforeseen consequences of pandemic lockdowns.

The problem extends beyond Britain: Around the world, buildings and roads suddenly quieted by stay-at-home orders gave animals a chance to venture where they normally couldn’t. Goats were spotted running through Welsh streets, coyotes roamed San Francisco and bats nestled in the spaces between walls of newly empty buildings in Austin, Texas.

“They want to take advantage of the void,” said Lee Mackenzie, a co-founder of Austin Bat Refuge, a rescue organization in the city that is often summoned to evict bats. “They simply go, ‘Oh, nobody is using this space, nobody will mind if we go here.’” (Bats, like wasps, are sometimes enlisted as pest-control cavalry to get rid of unwanted insects, like those that damage crops.)

The National Trust has also noted outbreaks of mold as a problem worsened by lockdowns, an issue that many owners of modern buildings also face in offices left unoccupied by the pandemic.

Moths, however, probably do not pose a problem in offices, Mr. Loughlin said. They mostly avoid the synthetic carpets used there.

Christine Hauser contributed reporting.