National Trust identifies the five insects most commonly threatening collections in its historic properties

Press release
Furniture beetle damage to a book
Published : 16 Jan 2020

For the first time the National Trust has revealed some of the findings from its annual assessment of the key insect pests which can cause irreparable damage to its historic collections.

The charity found silverfish and webbing clothes moth were the two most prolific insect pests monitored in its properties in 2019 and revealed a north-south divide for several key species. Its findings also suggested warmer winters will support pest populations, making diligent housekeeping across its houses even more crucial. 

Throughout the year, house staff around the Trust monitor insect activity indoors to safeguard more than 1 million collection objects. The data, which has been collected centrally since 2012, is used to produce a national picture of insect activity at historic properties across England, Wales and Northern Ireland. 

During 2019, 164 historic properties reported on 15 different insect pest species. 

Of these, the top five most prevalent (and what they feed on) were: 

1/ Silverfish (Lepisma saccharina) – books, paper and cotton
2/ Webbing clothes moth (Tineola bisselliella) – silk, wool, fur and feathers
3/ Woolly bear (a generic term for various carpet beetle larvae) – silk, wool, fur and feathers
4/ Australian spider beetle (Ptinus tectus) – dust and detritus
5/ Common booklouse (Liposcelis bostrychophila) - paper

The Trust expanded its insect pest monitoring and analysis after a big leap in insect numbers during 2015. These insect numbers rose again in 2016, fell slightly in 2017 and have remained broadly static since then. 

Hilary Jarvis, Assistant Preventive Conservator, who collates the data for the annual review, says: “We are finding each year that the same pest culprits consistently appear in the top five, although the rankings can differ: we don’t always know why. For example, in 2019, silverfish returned to the number one slot, having been displaced by webbing clothes moth in 2017 and 2018.”

As part of its annual review, the Trust tracks weather patterns, drawing on data from the Met Office to see whether any particularly hot, cold, wet or dry periods may have impacted on insect numbers in properties. 

Hilary says: “The Met Office is describing 2019 as a year of extremes, with some chilly spells and windy weather but record-breaking rain and heat. 

“There is rarely one single driver of pest activity or relative species prevalence, but it is likely that warmer winters and hotter summers lead to more pest cycles. Although pest numbers at our places have actually remained relatively static in recent years, our housekeeping teams need to remain diligent in their efforts to keep insect pests at bay.”

Nigel Blades, the Trust’s Preventive Conservation Adviser responsible for interior environmental conditions, says: “Insect damage to objects is not new, we know for instance that textiles and food stuffs as far back as ancient Egypt suffered from insect infestations. 

“Fortunately, only a tiny proportion of insect species in the UK attack or eat historic material.

However, a small percentage of these have the potential to become serious pests and can cause irreversible damage to collections in a short period of time. 

“Our staff monitor insect activity using small removeable sticky traps, tucked into the corners of rooms and collection stores, onto which any insects that are wandering around can get stuck. We may sometimes use lures like pheromone traps, for example, to attract the webbing clothes moth, to help us locate the source of an infestation. Four times a year we collect the traps and look at what we’ve caught to identify trends and spot a growing problem before it gets too big.”

The conservation charity doesn’t apply treatment to collection items for every insect pest that it finds. Some pests can be kept away by regulating the environment around the collection. 

Nigel continues: “Pests, such as silverfish and furniture beetle, thrive in more humid conditions, so we aim to keep the environment at a moderate humidity and temperature using a ‘conservation heating’ approach. Not only does this help prevent insect infestation, it also helps maintain the correct atmosphere to protect the fabric of the houses and their contents.” 

The Trust’s review also tracks any regional variations and showed a north-south divide for several key species. Silverfish remain a common problem across most of the UK, with webbing clothes moth skewed mostly to the southern half of the country and Australian spider beetle more prevalent from the Midlands northwards. 

Hilary adds: “Regional variations aside, the most interesting stories are often at individual properties: staff at Castle Drogo in Devon have worked hard to get on top of a webbing clothes moth infestation that flared up in inaccessible areas during a major building conservation project. 

“Meanwhile, the ever-alert house team at Hardwick Hall in Derbyshire managed to stem an increase in the number of silverfish by scaling back the dampening of the rush matting in the Mary Queen of Scots room, which is done to keep the rushes from drying out and becoming damaged.”

Nigel continues: “Our staff use a number of housekeeping routines to deter and manage insect damage, and they are always happy to talk to visitors about these, and what people can do in their own homes. Prevention is better than cure – for example, regularly checking in dark corners, under furniture, and in folds of textiles where insects like to hide undisturbed, along with meticulous vacuuming to remove dirt and dust. Chimneys can be vulnerable, too, so it is important to have them swept.

“As a conservation charity we do all we can to deter insects in our historic properties. If we do discover an infestation, we use a water-based, odourless and biodegradable insecticide that is harmless both to humans and the collection. For moveable textiles like rugs, we might also freeze the item (below -29°C (-20°F) for two weeks) to kill larvae and eggs. Humidity-controlled heat treatment is also an option for certain objects.

“Through diligence, good housekeeping and regular monitoring of insect activity, our aim is to slow down deterioration of our collections, which will allow them to be enjoyed by future generations of visitors.”