Moth numbers tumble in National Trust houses

Clothes moth numbers tumbled in historic houses last year, the National Trust’s annual insect pests report has found. The report collates information gathered by house staff around the Trust, helping the charity to safeguard more than 1 million collection objects, from precious books and tapestries to silk hangings on state beds.   

The 18% drop in Tineola bisselliella – the larvae of which can damage carpets, upholstery, taxidermy and woollen or silk objects – follows on from a 39% slump the previous year.  

Woman cleaning the bed in the Wisteria Bedroom at Dunster Castle, Somerset owned by the National Trust
Cleaning the bed in the Wisteria Bedroom at Dunster Castle, Somerset. National Trust Images-Sarah Allen

The Trust’s Assistant National Conservator Alexandra Radford, who compiled the report, said: “The alarming increase we saw in 2021, driven by lockdown closures, is now a distant memory. This is excellent news for all our collections. 

“The drop is likely because pest numbers are still falling from their lockdown highs, but it also reflects the decisive action and relentless efforts of house teams to manage moth numbers. We’ve put in place more training and resources to help property teams with integrated pest management, which is crucial to good collections care.” 

Overall insect counts were also down by 11%, compared to the Trust’s 2022 Integrated Pest Management data. This was likely aided by another year of turbulent weather, with record temperatures and rainfall, multiple named storms and rapid fluctuations. 

A woman cleaning a book in the Library at Ham House and Garden, Surrey
Book cleaning in the Library at Ham House and Garden, Surrey. National Trust Images-Chris Davies

Met Office data show 2023 was the second-warmest year on record for the UK, with Wales and Northern Ireland having their warmest ever years. It was also a wet year for many. 

Alexandra said: “Nature is reflecting back the impact of these extremes. Without a doubt, the ongoing unpredictability and extremes in temperatures and moisture are feeding through into insect breeding cycles and patterns.” 

Silverfish (Lepisma saccharina), which feed on books, paper and cotton, comfortably retained top spot in the pest ‘leaderboard’, with recorded numbers rising by 6%.    

Alexandra said: “This slight rise does coincide with the UK becoming wetter over the past few decades. While we aren’t necessarily getting more rain, incidents of heavy rainfall have slightly increased. These more intense periods of rainfall can lead to water run-off and flooding, and we know silverfish will seek out and thrive in damp environments.” 

Also on the rise are woolly bear (carpet beetle larvae which feed on silk, wool, fur and feathers) and booklice. Booklice and silverfish can graze on mould, with higher numbers to be expected when there is intensely wet weather and high humidity.  

Adult Clothes Moth (Tineola bisselliella). (C) Historyonics
An adult Clothes Moth (Tineola bisselliella). (C) Historyonics

Alexandra continued: “Our booklice visitors appear to be more familiar within our collection spaces over the past few years. Because mould is very appetising to booklice and silverfish, our collections and house teams continue to monitor areas that could be more vulnerable to mould outbreaks, allowing them to take a more targeted approach.”  

There was positive news from the East of England, where monitoring showed steady declines in insect pest counts in houses. These have been helped in part by work to control clothes moths and silverfish at Blickling Hall in Norfolk, where a trial of moth pheromones has been particularly effective. 

But the report also found that the traditional spring and summer breeding periods are continuing to merge into one, possibly driven by earlier springs and more protracted, mild autumns.  

Alexandra said: “In the past, we saw a more distinct spike in breeding cycles, but these are becoming blurred. Optimum breeding conditions are starting earlier and carrying on for a longer period of time to create one long reproductive season.  

She concluded: “Knowing this will help house teams prepare for the peak times when nymphs and larvae begin to emerge and take action before they start causing problems for our collections.”