Attacks by box tree caterpillars are growing in France, with the invasive species particularly damaging to garden hedges.
The insects - called les chenilles de la pyrale du buis in French - can cause long-term damage to box vegetation (Buxus sempervirens), before gardeners may have even noticed their presence.
Reports of the attacks in France have spread in recent weeks.
“You might not see them, unlike certain caterpillars, on the outside of the plant,” said Olivier Bézuel, garden manager of the Jardins d’Avremesnil, near Dieppe (Seine-Maritime, Normandy), to France 3.
“They'll start on the inside. So when you do see them, it means that a good part of the plant has already been attacked. In two or three days, they can completely strip the leaves off a hedge.”
Most of the damage occurs between March and October.
Climate change spread
Originally from Asia, the insect - properly called the caterpillar of the box tree moth; Cydalima perspectalis in their Latin name - was reportedly accidentally introduced into France in the Alsace region in 2008. It has gradually spread across the country.
Rising temperatures as a result of climate change have also contributed to the spread, and it is now also being seen in higher-altitude mountain areas that would normally manage to escape it.
“In recent years, with no more sub-zero temperatures, the caterpillars have managed to remain dormant over the winter. As long as there's no big frost, they don't disappear,” said Mr Bézuel.
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The species now appears on the alert list from the European and Mediterranean Plant Protection Organization (EPPO).
No danger to humans or pets…but to hedges
It has a light green body and black head, with stripes. It is not dangerous for humans or pets, but it appears to only eat the type of vegetation seen on bushes and hedges, making it particularly damaging to gardens.
Each insect can lay up to 200-300 eggs at a time, meaning they can spread very quickly.
As a moth, the insect has white wings with brown borders, sometimes with purple reflections, and can measure as large as 44mm across. They are, like most moths, attracted to light.
How to get rid of the caterpillars
Mr Bézuel recommends a several-pronged approach, including:
Pheromone traps
Attracting tit birds (called chickadees in the US)
Bacillus thuringiensis bacterial powder
“Don't hesitate to use pheromone traps, which will attract the moths,” said Mr Bézuel. “The traps won't destroy them, but they will let you know if you have box moths in the garden. That way, you know whether you can start the first treatments.”
Tits can be attracted by installing nest boxes all over the garden.
“Keep them high enough so that cats don't come and chase the birds away,” advises Mr Bézuel. “They are very important as they are a natural predator in gardens, which is great.”
Bacillus thuringiensis bacterial powder can be sprayed onto the caterpillars as a last resort. This may not be a “miracle solution”, Mr Bézuel said, as it can be washed away by rain, and can be difficult to use. However, it is non-toxic to humans, and can help to save your hedges.
Other methods, such as using washing up liquid or throwing boiling water on the caterpillars, may get rid of them in the short-term, but could also damage the plant, and are not advised.
Mr Bézuel has said that the spread of the insect, as well as climate change, may require gardeners to start thinking differently when it comes to hedges and plants, and move away from the slow-growing box vegetation that is particularly popular with these caterpillars.
“It is difficult to break a horticultural tradition in one go,” he said. “But this is a critical year; gardeners are going to have to evolve their thinking.”
Some gardeners are already considering alternatives to the box hedge; the Royal Horticultural Society (RHS) in the UK is among the groups to be trialling other evergreen shrubs that could be used in areas where it “has become impractical to control the box tree caterpillar”, it states on its website.
