A GERANIUM enthusiast is worried an invasive butterfly could decimate Britain’s zonal and ivy-leaf pelargoniums.

The culprit is the Geranium Bronze, an attractive-looking butterfly whose caterpillar burrows into geranium leaves and chews its way through the centre of the stalk.

AG reader Jenny Bussey has seen it before: as a former chairman of the Costa Blanca Gardeners’ Circle in the 1990s, she was at the forefront of the battle to contain the pest in Spain – a battle that was ultimately lost.

“It quickly killed off nearly all the zonal and ivy-leaf pelargoniums that used to be so colourful in window boxes, gardens and parks,” she said. “Within five years there was hardly a geranium to be seen.”

And now she fears the pest has found its way to Britain.

“I now live in north-east Lincolnshire and have been dismayed to find that the geraniums I overwintered have all the signs of attack – small holes in the flower buds, nibbled petals or flowers not developing, flower stems that go brown and wilt, and larger stems that are completely hollowed out and black.

“I haven’t seen the actual butterfly in Britain yet, but the damage to my plants is exactly the same as I saw in Spain,” she told AG.

Native to South Africa, the Geranium Bronze Butterfly was inadvertently introduced to Majorca inside cuttings in 1987, and by 1993 had got a foothold in mainland Spain.

In theory the butterfly crysalids should not be able to survive the winter, but sub-zero temperatures in Spain didn’t thwart it, and it now appears to have lived through a British winter as well.

Spotting the symptoms early means you may be able to save your geraniums, Jenny advises.

Break off any flower stems that look like they have succumbed to the caterpillar’s boring, then keep breaking off lower stems and stalks until there are no signs of any further holes.

Dispose of the suspect plant material carefully – avoid putting stems in the compost.

Spraying with a systemic insecticide such as Provado Ultimate Bug Killer should also help.