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‘Surprising’ clothes moths slump of nearly 40% across National Trust houses, possibly helped by record heat and drought, charity’s annual insect pests report finds

Clothes moth numbers fell by nearly 40% across National Trust houses in 2022, driven in part by drought and record temperatures, the conservation charity’s annual insect pests report has found.

Adult Clothes Moth (Tineola bisselliella) | © ©Historyonics
Adult Clothes Moth (Tineola bisselliella) | © ©Historyonics

The report paints a picture of insect activity at historic properties across England, Wales and Northern Ireland, helping the charity protect the more than one million objects in its care.

Populations of webbing clothes moth (Tineola bisselliella) – which can cause serious damage to carpets, upholstery, taxidermy and woollen or silk objects – slumped 39% compared to 2021 figures and remained low all year.

Assistant National Conservator Hilary Jarvis, who compiles the annual pest report, said: “This was surprising news. 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?

“As ever, there are probably other factors at play, such as the doubling-down of our house teams after the alarming 18% surge in moths we saw in 2021. There could also be a natural rebalancing underway, with predators and diseases catching up with, or a shortage of food and space for, such a burgeoning moth population.

“Whatever the reason, the drop is welcome news in our properties.”

The data also show that the two main breeding seasons for insect pests appear to be merging.

Hilary continued: “Traditionally we’ve been used to two distinct spring and summer breeding periods, one often more productive than the other. But certainly with our indoor insects we’re noticing a waning difference. This seems to echo the increasingly protracted warmer weather we’re seeing these days, with earlier springs heralding a summer that often extends into a longer, mild autumn.

“Are the two periods morphing into one long breeding season for our indoor pests? If breeding was to happen anytime between late February and October, that’s potentially concerning and would certainly put more pressure on our house conservation teams.”

Overall, the report found insect pest numbers fell 3% last year, continuing their decline since the lockdown-driven insect pest boom of 2020. However, many houses recorded high numbers of ‘woolly bear’ (carpet beetle larvae which feed on silk, wool, fur and feathers) while silverfish (Lepisma saccharina) jumped 14% to the #1 slot, possibly as part of a recovery from a steady decline since 2019.

Silverfish, which feed on books, paper and cotton, continue to be more common in the east of England, possibly due to drier weather forcing them indoors in search of damp foodstuffs.

In separate research, the Trust has decided to discontinue use of a parasitoid wasp which it was trialling at Blickling Hall in Norfolk to manage persistent clothes moths there [3].

Hilary said: “The Trichogramma microwasps performed well in terms of reducing moth populations in combination with pheromones, but no better than where we’d used pheromones alone. We think that it’s too great a task for them to locate moth eggs in our lofty interiors, particularly since we can only place the dispensers where we suspect the eggs may be.”

The Trust may still use the microwasps in certain circumstances, such as to tackle moths in small collection stores or where the source of the infestation is known. The Blickling trial has now been extended to focus on exclusively testing natural moth pheromones. Delivering these for longer periods will counteract the extra-long and overlapping breeding periods and make it harder for male moths to locate females for mating. If successful, Hilary predicts this is likely to be cheaper and easier for staff to manage.

Hilary concluded: “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 – that’s exactly what our annual report helps us to do.”

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