Online reports – Ecology – When the box tree moth munches on the box tree

© Photos by Florine Leuthardt and OnlineReports.ch

“Do not compost under any circumstances”: bare boxwood hedge*

Biologist Florine Leuthardt advocates that the neglected privacy screen should be cared for like a rose in the future

From Peter Knechtli

Whatever box tree leaves are still available are currently being eaten bare by box tree moth caterpillars. But there is no reason to be hopeless, says Basel biologist Florine Leuthardt – all that is needed is a different attitude towards the undemanding native privacy screen plant.

Former banker Tim Schweizer (name changed) drove his camper van on holiday to the Mediterranean. When he sold it three weeks later, he found his box tree, which had slowly grown to a sizeable size over the years, in such a state that he was shocked: a skeleton. Box tree moths had eaten the leaves of the magnificent evergreen bush bare in a very short space of time. The chance that this garden ornament will recover and sprout again is zero.

300 caterpillars on a tree

The phenomenon is new. Until a few years ago, the box tree, introduced by the Romans from the Middle East, was one of the most popular and culturally and historically valuable native ornamental, dividing and privacy shrubs in Switzerland. For centuries, private gardeners, park gardeners and cemetery gardeners have valued Its unpretentious art, its dense and solid foliage, its strong dark green and its ability to withstand even the harshest winters.

But what seemed to be a timeless certainty began to waver when a pest from Asia found its way to Central Europe a few years ago and took an even greater liking to the boxwood than its rightful owners. A pearly white butterfly with dark brown-edged wings (pictured top left) lays its eggs on the underside of the leaves as small droplets that are barely visible to the naked eye. After just one week, well-camouflaged green caterpillars hatch and immediately develop a huge appetite. A caterpillar, which can reach a length of up to five centimeters, eats leaves the size of a palm. The extent of the damage is easy to imagine given that up to 300 caterpillars can attack a tree.

In August 2010, the wild boxwood forest in Grenzach suffered a fatal blow: the pests devoured it down to the roots. The extensive stands in the “Hörnli” cemetery in Riehen and in Basel's Kannenfeldpark are also so acutely threatened that the area had to be closed to the public at short notice for pest control. “We are constantly working,” said Basel's city gardener Emanuel Trüb to OnlineReports.

No professional enemies.

The pest first became known regionally in 2007 in Weil am Rhein. From the fact that it was also found in Cologne and Kehl, scientists conclude that “there must be some connection with the port” and that the box tree moth must have been introduced from Japan, South Korea or China. Basel biologist Florine Leuthardt (picture), who is currently working on a dissertation on the box tree moth, believes that the moth could have been introduced as early as 2006 without being noticed. From 2008 onwards, it was already found in the Zurich and Lake Zurich area. Recently, it has even attacked the Bernese Seeland, which had previously been spared, where the centre of the Swiss Boxwood cultivation. Infected plants can also be found in garden centers.

The problem: According to Florine Leuthardt, the box tree moth “most likely has no natural enemies”. The reason: the caterpillar produces the poisonous water-soluble alkaloids contained in the box tree leaves. If a bird eats a caterpillar, it immediately vomits it up again. There are also no known parasites that could put an end to the unwanted leaf-eater. “In a few years,” said a well-known gardener from the region to OnlineReports, “there will be no more box trees in Switzerland.”

“Treat boxwood like a rose”

27-year-old Florine Leuthardt does not want to go that far. “Yes, I would still buy a box tree today,” she says, but immediately adds: “Without care, the box tree will die out.” This is why the pygmy moth expert advocates a completely new understanding of the box tree: “It should no longer be a plant that you can plant and then forget about. Instead, you should look after it like a rose.” The more people look after the well-being of the box tree, the better the pest population regulates itself and the less “the perceived damage” becomes.

Are the first pearly white butterflies fluttering through the garden or is there an unpleasant smell of seichele “it is time to start fighting it.” The biologist mentions two different treatment methods – physical and chemical. Individual trees or small hedges can be sprayed with a high-pressure cleaner (“but not with full pressure!”). This usually kills the moths. Caterpillars can also be picked off by hand, knocked off with a stick or vacuumed up.

Be careful when injecting poison

Florine Leuthardt warns against simply leaving caterpillars in a garbage bag: firstly, they can easily eat through plastic bags, and secondly, they smell terrible. The best thing to do is to burn the animals or quickly dispose of them in the garbage. “Under no circumstances should infected leaves or branches be composted. The caterpillars will continue to develop there and will soon be back on the tree.”

If chemical control does not cause you any pangs of conscience, you can choose between two spray methods: contact poison and feeding poison. Contact poison has one serious disadvantage: it also kills beneficial insects such as spiders or butterflies. Florine Leuthardt recommends feeding poison with the active ingredient Bacillus thuringiensis, which attacks the pest from the inside after it has eaten the leaves. It is important to seek advice from the gardener and “spray precisely”. The Basel municipal gardening department also achieves “excellent results” with this feeding method (according to Emanuel Trüb).

Nevertheless, the head of the Basel municipal gardening department is not sure whether the boxwood will survive in Switzerland in the long term. “There is no equivalent native replacement,” Florine Leuthardt is convinced. Trueb, on the other hand, is of the opinion that boxwood hedges can at least be partially replaced, for example by squat privet, yew, evergreen viburnum, barberry, small-leaved lonicera or false cypress.

* in the Hörnli cemetery in Riehen

21 August 2012

“It doesn’t necessarily have to be a gift”

You don't necessarily have to use poison. (Sure, the stuff is expensive and will be sold!) But there is also an efficient trap (with pheromone attractant) for the moths! We have used it successfully. If you are interested, you can find x providers on the Internet.

Bruno SiegenthalerLiestal

“Wasps eat caterpillars”

Wasps eat caterpillars. I watched wasps flying into the box tree. I saw one eating a pygmy moth caterpillar.

Ruth DucreyArni AG

“There is an enemy: ducks”

I read the above article with interest and would like to point out that there is indeed a natural enemy for the box tree moth.

We have been keeping Indian runner ducks in our garden for over 25 years. The nursery is fenced off with a low fence, but otherwise they can move freely around the entire area (approx. 1300 m2). They also avoid the biotope; that's what they were taught!

Recently I looked into the garden from my office on the first floor and called out to my wife: “Look down in the garden, our runner ducks can climb.” My wife immediately made the right assumption – as is always the case with the female sex – and replied that our boxwood probably has caterpillars too. When I checked, I found another specimen, and since then the boxwood has been “cared for” by our ducks and is enjoying a healthy existence. However, the boxwood plants are no taller than about 1.20 meters.

Perhaps my observation can shed some light on the bleak prospects for the boxwood.

Franz BorerRöschenz

“The best and most recent article”

The best and most up-to-date article I have read about this pest on the internet so far!

Heinz BielsteinWiesbaden