National Trust launches pioneering new trial to protect precious collections from a rise in insect pests
This month, the National Trust is set to begin a groundbreaking new pest-control trial, using natural methods in a combination not previously used in a heritage setting to tackle clothes moths at the estate believed to be the birthplace of Anne Boleyn.
Despite vigilant housekeeping and other preventive measures, common or ‘webbing’ clothes moths (Tineola bisselliella) have proved hard to control at Blickling Hall in Norfolk.
Now, a multi-pronged trial will use a microscopic ‘parasitoid’ wasp, Trichogramma evanescens, together with specially prepared moth pheromones and the Trust’s existing regime, to target the whole lifecycle of the moth, which can cause serious damage to carpets, furniture, clothing and other wool and silk objects.
While wasps and pheromones have been used separately against clothes moths, and pheromones have been used to manage moth pests in agricultural settings, the Trust believes the combination hasn’t been deployed in a heritage setting until now.
David Loughlin, owner of Historyonics, the company supplying the wasps and pheromones to the Trust, says: “There has been a global move to adopt biological techniques to manage pests of food crops, so this is a natural development to adopt similar tools to manage the pests threatening our valuable national heritage.”
Trichogramma evanescens is a natural enemy of the clothes moth, searching out moth eggs and laying its own eggs inside, so that a new beneficial wasp hatches, rather than a moth larva. Once the wasp eggs are laid, the wasps die naturally and disappear inconspicuously into house dust.
Measuring about 0.5mm, the wasp is barely visible and is not harmful to humans or other animals. The wasps are supplied in small card dispensers (each containing around 2,400 wasps) which can be discreetly hung or placed in drawers or open rooms.
The trial will also use pheromone ‘tabs’ to disrupt adult mating. These work continuously to spread female pheromones (chemicals released to attract males of the same species) which confuse male moths, reducing their chance of finding a female mate. The tabs use electrostatic technology to physically transfer the pheromone onto the bodies of male moths, turning them into portable female pheromone dispensers.
Used alongside existing measures, it is hoped the two biological control methods will vastly reduce the moth population at Blickling, safeguarding one of the Trust’s most significant collections. Among its most treasured items are the ‘Peter the Great’ tapestry, gifted by Catherine the Great to Blickling’s then owner in the 1760s, and a State Bed whose ambassadorial canopy and headcloth are the most complete 18th-century examples of their kind. Recent research has shown that the bed’s counterpane is likely to be one of only two surviving pieces of Queen Anne’s throne canopy.
Assistant National Conservator Hilary Jarvis says: “We are really hoping this pioneering approach will provide a practical and sustainable method that any of our properties can use to deal with serious infestations.
“Although these are rare, they can sometimes prove immune to our usual, more gentle approaches, with potentially serious results.”
The National Trust hopes to present the trial at the Pest Odyssey virtual conference in September, which advocates for sustainable pest management in cultural heritage institutions.
The trial comes as the Trust releases results of its annual pest review, which found that insect pests such as moths and silverfish thrived during lockdown, partly due to less disturbance from house staff and visitors, both inside and outdoors.
Hilary continues: “There's no doubt lockdown suited our resident bugs. The relative quiet, darkness and absence of disruption from visitors and staff provided perfect conditions for larvae and adults alike from March onwards.
“When we closed all of our houses, we knew insects would likely thrive, so pest monitoring was high on our list of essential tasks in 2020. Staff did monthly checks, which meant we could take swift action before outbreaks could take hold.”
Despite widespread staff furloughing, it was a record-breaking year for the annual pest review, with at times mere handfuls of individuals at a record 173 properties counting more than 62,000 insects in 6,800 traps; also records for the Trust.
Hilary says: “The dedication of our house staff throughout 2020 is remarkable, given the impact of Covid-19 on all our lives, let alone working practices, in a truly challenging year.”
Analysis showed that normalised insect numbers rose 11% in 2020 compared to 2019, with many houses also reporting mould outbreaks, due to a lack of activity to drive airflow.
As well as lockdowns, the mild winter and early – and in many cases very warm – spring also helped push pest numbers to record levels. The absence of harsh frosts appeared to particularly favour cluster flies, with many houses inundated throughout the summer. These are a good food source for other insects so can compound insect pest problems if not cleared up promptly.
The review also shows a continued north-south divide for several key species, probably due to temperature and rainfall patterns. Hilary says: “Clothes moths remain more common in the south of England, and don’t look to be creeping northwards, though we are alert to that prospect, given the ever-warming climate.”
The top five most prevalent insect pests in 2020 (and what they feed on) were:
1. Silverfish (Lepisma saccharina; books, paper and cotton): dropped 8% in 2020 (to just above webbing clothes moth), possibly due to warm and sunny weather at critical times drying out their water supplies.
2. Webbing clothes moth (Tineola bisselliella; silk, wool, fur and feathers): rose 3%.
3. Woolly bear (a generic term for various carpet beetle larvae; silk, wool, fur and feathers): appears stable.
4. Australian spider beetle (Ptinus tectus; dust and detritus): numbers rising, though only north of the Midlands.
5. Common booklouse (Liposcelis bostrychophila; paper): slight rise in 2020, after a sharp increase in 2019.
Hilary says: “The results of our trial at Blickling won’t be clear until at least autumn, after the main breeding season, and we may find we need to continue our multi-pronged approach into next spring and summer too.
“In the meantime, staff will maintain their vigilant checks and are looking forward to being able to welcome visitors back as soon as it’s safe to do so.”