An alarming increase in the population of clothes moths in National Trust (NT) properties appears to be in retreat, with a dramatic plunge in numbers of the pests in 2022.
The trust, which monitors the presence of insect pests each year, said it was surprised by the findings of its 2022 survey. Overall insect counts fell by 3% compared with 2021, but the presence of Tineola bisselliella slumped by 39%.
The clothes moths can cause serious damage to carpets, upholstery, taxidermy and woollen or silk objects. The larvae feed on protein in animal-based fabrics such as wool, fur, velvet and silk before they become winged insects.
The NT monitors 6,500 sticky traps placed in 180 properties each year. The traps are checked four times a year, with staff and volunteers counting and classifying tens of thousands of insects.
Hilary Jarvis, the trust’s assistant national conservator, who compiles the annual pest report, said the fall in the number of clothes moths was “surprising news” after an 18% rise in 2021.
“We had thought climate change would be a boon for this moth, which originates from South Africa and is no stranger to heat.
“But changing weather patterns are challenging our thinking. Was it simply too hot – and perhaps more importantly too dry – for these particular moths to thrive in the warmest year on record?”
Other factors could be a natural correction and increased efforts by house teams to control the infestation, including the use of pheromones.
During the Covid lockdowns, when NT properties were closed, insect populations increased, said Jarvis. “Everything was shut up, there were no visitors, no dusting. The numbers really rose. The key thing is not to let them get too comfortable.”
Despite the overall drop in insect pest numbers, silverfish (Lepisma saccharina) jumped 14% to the No 1 position. Silverfish feed on books, paper and cotton.
Many NT houses also reported high numbers of “woolly bear” – carpet beetle larvae, which feed on silk, wool, fur and feathers. Booklice and the Australian spider beetle were also among the five most prevalent insect pests last year.
The trust adopts a preventive approach to insect pests, including removing dust and maintaining environments that are neither too warm or too damp. It uses insecticide if necessary.
It has decided to discontinue a trial at Blickling Hall in Norfolk using Trichogramma parasitic microwasps that lay their own eggs inside moth eggs.
“The microwasps performed well in terms of reducing moth populations in combination with pheromones, but no better than where we’d used pheromones alone,” said Jarvis.
“With so many potential drivers of insect activity, it’s not easy to predict what might happen next with clothes moths or any other insect pest. Our main job is to keep robust records of the typical insect profile in our houses, so we can be alert to changes and potential risks, and be ready for action.”
