The Trust will this month begin a pest control trial at Blickling Hall in Norfolk, using biological methods in a combination never previously applied to a heritage setting as a way of mitigating the increasing damage caused by clothes moths.

Assistant national conservator Hilary Jarvis says the Trust hopes its “pioneering approach will provide a practical and sustainable method that any of our properties can use to deal with serious infestations”.

The trial will see microscopic ‘parasitoid’ wasps and specially prepared moth pheromones added to Blickling Hall’s existing pest control regime to try and reduce the impact pests are having on the site’s collections.

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A card dispenser, containing circa 2,400 parasitoid wasps, in an oak drawer © Historyonics

Measuring approximately half a millimetre, the species of wasp is barely visible to the naked eye and is said to pose no risk to humans or other animals. Supplied in small card dispensers – each containing around 2,400 wasps – the insects will be discreetly deployed in drawers and rooms throughout Blickling Hall.

The trial will also utilise pheromone ‘tabs’ as a way to disrupt adult mating and so limit moth population growth. Spreading female pheromones causes confusion in male moths and, as such, reduces their chance of finding a mate.

David Loughlin, owner of Historyonics, the company supplying the wasps and pheromones to the National Trust, believes the charity’s trial is in line with a “a global move to adopt biological techniques to manage pests” and that following examples used in food crop production is a “natural development” for the heritage sector.

A holder with pheromone 'tabs' that disrupt adult moth breeding © Historyonics

Pandemic peak

As with many heritage sites, the East Anglian estate has seen a rise in damage to its collections during lockdown. With less disturbance from visitors and staff, twinned with the effects of a mild winter and warm spring, pests have run amok.

Blickling Hall found it especially difficult to control common or ‘webbing’ clothes moths in 2020.

Coinciding with the trial, the National Trust has released the findings of its annual pest review. The latest edition charts a substantial presence of insect pests such as moths and silverfish across its portfolio of properties.

The charity’s workforce counted over 62,000 insects in 6,800 traps at 173 properties last year – record numbers for the Trust.

Top 5 pests

The five most prevalent insect pests in 2020 (and what they feed on) were:

  1. Silverfish (books, paper and cotton)
    Numbers actually dropped 8% in 2020, possibly due to warm and sunny weather at critical times drying out their water supplies
  2. Webbing clothes moth (silk, wool, fur and feathers)
    Rose 3% last year
  3. Woolly bear – various carpet beetle larvae (silk, wool, fur and feathers)
    Levels have remained stable year-on-year
  4. Australian spider beetle (dust and detritus)
    Numbers rising, though only north of the Midlands
  5. Common booklouse (paper)
    Slight rise in 2020 after a sharp increase in 2019

“There’s no doubt lockdown suited our resident bugs,” Jarvis notes. “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.”

With no results from the trial at Blickling Hall anticipated until autumn at the earliest, the Trust is urging all its sites to “maintain their vigilant checks” in perpetuity.

The trial’s findings will, if all goes to plan, be presented by the National Trust at the Pest Odyssey virtual conference in September.

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