The Current

Send in the wasps! U.K. historical building deploys millions of parasitic wasps to tackle moth infestation

In the wake of a moth infestation, the keepers of a historical building in Britain have deployed an army of parasitic wasps to protect its artifacts. We talk to Hilary Jarvis, a National Trust conservator leading the wasp trial at Blickling Hall.

Keepers of Blickling Hall have deployed an army of wasps to protect its artifacts

The exterior of the hall at Blickling Estate, Norfolk, where millions of microscopic wasps have been set loose to stop an infestation of moths. (National Trust/Ian Ward)

Story Transcript

Conservators of a historic site in the United Kingdom have begun using millions of wasps to rid the building of moths.

"[The wasps] really are teeny tiny. They're less than half a millimetre long," Hilary Jarvis, conservator of Blickling Hall explained. "They're like a speck of dust."

Blickling Hall is thought to be the birthplace of Anne Boleyn, former queen of England. With historic artifacts at stake, the wasps are being brought in to help. This is part of a shift in the way historic buildings and museums around the world deal with pests. 

"So this is part of a slightly more holistic approach where we're trying to tackle all stages of the life cycle," Jarvis explained to The Current's host Matt Galloway.

Here's part of their conversation.

How much of a problem are these moths at Blickling Hall?

We do what we call sample monitoring across National Trust historic properties, so we never know exactly how many moths are in a building. But our samples about five years ago were showing about two hundred on our little sticky traps. Roll forward to 2020, and we've got 2,500. So there's been a huge increase.

An adult clothes moth, the type of moth infesting Blickling Hall. (Historyonics)

And what are the 2,500 moths doing in terms of damage in this historic building?

Well, I don't want to overstress. There isn't a lot of damage, and that's because we're very good at what we do. We're quite good at our preventive measures. But they do like to eat silk and wool and a lot of the things from carpets to tapestries to wall hangings to state beds.

There is some damage that we found, for example, in one of the state rooms; we've seen that they've been nibbling holes in the carpet. And, you know, for me, that's heartbreaking because, of course, we can patch it up, we can make a repair. But that's a historic thing. 

So how did you land on the idea that to deal with an infestation of insects, you would release other insects?

Actually, we've... stolen this idea, if you like, from the agriculture field, from horticulture. So we spoke to an expert who has been tending soft fruit orchards in the Mediterranean for about 20 years. 

And this is a very sensitive consumer market. And the consumers have been saying they don't want their peaches and their apricots covered in chemicals for quite some time now. So that industry has had to move quite assertively away to find a biological solution and they have been using these wasps. 

The trichogramma evanescens is a parasitic wasp that hatches its eggs within moth eggs. The newborn wasps then eat the moth egg. (Kenny Gray/National Trust)

OK, so explain how the wasps work. I was reading something about it and someone had described them as "Our winged crusaders arrived in the hall" to deal with the moths.

They are. I'm afraid… this is slightly gruesome. But how the wasps work — and I stress they're micro wasps, they're not your traditional wasps at all. 

What they do, they are primed to hatch themselves, then find a mate, and then the female is primed to lay her eggs. And what she does is lay her eggs inside the egg of the moth. So I'm afraid the baby wasp then consumes the contents of that moth egg. What hatches out then is another wasp, not a moth. And it's what we call a beneficial wasp because it will keep doing the same thing. 

So this is part of a slightly more holistic approach where we're trying to tackle all stages of the life cycle because we feel the problem is not sufficiently large, that's the best way to do this.

So how many wasps have you released into Blickling Hall?

Well, they come in a little, sort of like the size of a credit card, little cardboard packages that are really very small. But in each one there will be about 240,000 wasps. So we have released already about a million and we're going to keep releasing a million every two to three weeks across the key moth season. So they'll be more or less until August, September. 

We might take a short break, but then we'll start again in the late fall period to try and catch all the breeding seasons. Because the other thing about these wasps is that they don't live very long. Like many insects, the adult life is short. It's between three and 14 days. So we need to kind of keep replenishing the supply.

A credit card-sized wasp package containing approximately 240,000 micro wasps used to control the moth infestation at Blickling Hall. (Historyonics)

So there wouldn't be millions and millions and millions of wasps in the hall at the same time, because, as you say, they only live for a couple of weeks.

Yeah, there would probably be a good million at any one point. But just to stress, you won't notice them, you don't see them, they're that small. Our properties are currently closed at the moment due to the pandemic, but were they open and were we to have visitors, I don't think they would notice them. They wouldn't see them. 

Do you have any concerns about this? I mean, from a distance, I'm no expert, but you could imagine introducing one insect to try and deal with another one could create any host of problems. Are there any real alarm bells that you're listening for?

Well, it's a great question and, of course, we've done our research. But so this particular moth species that we're dealing with we tend to think of it as indigenous or native to the UK now, but in fact, it's not, it's from South Africa. And it first came over probably about 1880. Now the wasps predate that. I think our earliest known record is 1830.… It's also very small. The adults don't eat or excrete, so they don't change or have any impact except they lay their eggs in these moth eggs. They then die. 

Evidence of clothes moth webs and faeces on a carpet at Blickling Hall. (Kenny Gray/National Trust)

We have explored the possibility that they might escape from within the building, so to speak, into the broader Blickling estate. And that is perfectly possible. But we don't think they're going to get very far. They're tiny; they're not great fliers. There won't be any food for them to eat outside of the house because these moths don't lay their eggs in an exterior environment. 

The last thing to say is that the U.K. agriculture body, Defra [Department for Environment, Food and Rural Affairs], have approved this some time ago for use without a licence so anyone can buy them. So they've been deemed very safe from that point of view.

When will you know whether this move has worked?

It's going to take a full year and probably a little bit more, to be honest. So the moth season in the U.K. is starting about now, so we won't really know until this time next year where the numbers are significantly lower.

I should stress it's not just the wasps. We're using some disruptive pheromones that will also hopefully discourage or make it difficult for the males to find the females. So we're reducing the mating. We're approaching this holistically and trying to, kind of, clamp down on all areas and not just target the eggs.

Written by Lito Howse. Produced by Rachel Levy-McLaughlin. Q&A edited for length and clarity.

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