Gardeners' World: Expert on removing box tree caterpillars
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Earlier this year, Gardeners’ World presenter Monty Don shared photos on his Instagram account showing how his box hedges had been “ravaged” by box blight. Box blight is a devastating fungus which causes leaves to turn brown and the growing tips to shrivel. To ensure the disease had been completely eradicated, Monty said he “grubbed and burnt” the hedges.
In the sixth episode of the show’s 2021 series, Monty began planting yews in place of his beloved box hedges.
Monty said: “Now, it’s worth remembering that I’m planting yews in the first place because our box hedges, which I loved when they were in their prime, proved to be hopelessly riddled with box blight.
“We grubbed them all out earlier this year and burnt them.
“However, in London and the South East, box tree caterpillar has arguably become a bigger problem than blight.”
The programme then visited gardening expert Nick Bailey at Ham House near the River Tames in Richmond, London where they were trying to get rid of box tree caterpillars.
Nick said: “Commonly known as box, buxus is a true stalwart in British gardens.
“It’s a versatile plant which can work in spaces both big and small from large practical hedges through to 17th-century-inspired gardens like this one.”
Nick said the box tree is “seriously in the wars” due to box blight and the box tree caterpillar.
The gardening expert showed viewers the small caterpillar which he said can cause “absolute devastation” in a garden.
Box tree caterpillars are active from March to October.
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They appear because female box moths lay their eggs on the underside of box leaves.
Once these hatch, the caterpillars rapidly feed on leaves which can cause “almost complete defoliation” of the plant.
Nick branded the caterpillar “one of the top garden pests in the UK”.
The horticultural pro spoke to fellow gardener Rosie Fyles who has been battling the pest.
The expert explained what the main signs are for box tree caterpillar.
She said: “What you see here is leaves that have been eaten and then some webbing. Where there is webbing, the caterpillar tends to be hiding in leaves.”
Rosie said you can pick off the caterpillars individually but you have to spot them first.
Box caterpillars usually appear in May, but according to Rosie, it can depend on the weather.
Instead of controlling the problem, Rosie is planning to replace the box hedging in Ham House’s kitchen garden.
In other areas, once the caterpillars are spotted, they are targeted using a biological spray.
Chris Poole, the chairman of the European Boxwood and Topiary Society and his team have been testing out unique, new ways to tackle the problem.
They were contacted by a German company who have a deterrent which could stop the female moths laying their eggs.
In gas permeable pouches, Chris and his team put caterpillar excrement into the hedge.
Another, more simple solution is using the herb thyme.
Chris said: “What they found was that female moths were repelled by the smell of thyme.”
The substance in the pouch was “thymol” which is the active compound in the herb.
Thymol is found in the oil of thyme and is a naturally occurring mixture of compounds in the herb.
Thymol is also an active ingredient in pesticide products registered for use as animal repellents, fungicides/fungistats, medical disinfectants, tuberculocides, and virucides.
However, Chris warned that growing thyme next to a box hedge is not a “strong enough concentration” of it.
Gardeners’ World airs on BBC Two at 9pm on May 14. You can catch up with the latest episode on BBC iPlayer
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