Invasion management of the box tree moth

Box tree moths are good flyers who prefer early dusk to spread their wings (Photo credit: T Haye).

The rapidly spreading, invasive box tree moth has decimated ornamental and wild boxwoods in Europe. Its economic, environmental, and societal impact is significant, while hopes for better pest control rest on additional research and classic biological control. FloraCulture International October 2022 edition focuses on this deadly pest.

The box tree moth may kill nearly all native boxwood stands in Europe within ten to 20 years,” Marc Kenis foreshadows. He spoke at a recent global webinar organised by American Horticultural Research Institute (HRI) and Boxwood Blight Insight Group (BBIG).

Kenis is Head of Risk Analysis and Invasion Ecology at CABI (Centre for Agriculture and Bioscience International). This international, inter-governmental, not-for-profit organisation improves people’s lives worldwide by providing information and applying scientific expertise to solve agriculture and environmental problems.

What’s at stake?

Kenis notes that in Europe, the Buxus genus is either an ornamental crop with Buxus sempervirens and Buxus microphylla and a host of cultivars used as hedges, specimen plants or topiaries. Buxus sempervirens and its endeavoured sister Buxus balearica also grow wild. Buxus sempervirens occur naturally across Southern Europe but predominantly in Southern France and Northern Spain. The Balearic Islands and Andalucía host swaths of the rare species Buxus balearica.

In Europe, the invasive fungus box blight (Cylindrocladium buxicola) and box tree moth (Cydamila perspectalis) threaten to comprise the tree’s significant market position and a collapse of nature’s ecosystems.

In 2018, Martin Denny, editor of the Cyclamen Society Journal, published an article entitled Cyclamen purpurascens in Eastern France and the Swiss Jura. This report was after observing in the Jura mountains that no Cyclamen plants grew in Cydamila perspectalis-infested boxwood forests as the defoliation/death of the Buxus ground cover changed the ground-level light conditions. Cyclamen may not be the only vegetation at risk. In Georgia, where boxwood is deeply anchored in the nation’s culture and economy, this situation is even more catastrophic, with entire stands of Buxus colchica eaten away. (View online FCI’s story – Georgia’s boxwood apocalypse)

Six to seven larval stages feed on the leaves and shoots of boxwood, and severe infestations can lead to complete defoliation of plants or even death (Photo credit: T. Haye).

How widely spread is the box tree moth?

The HRI webinar focuses on the box tree moth only, a pest that is a native of China. Kenis says, “It also occurs in Japan, the Russian Far East and India. Some of these areas do not host native boxwood trees, so that the moth may be invasive there too.”

Kenis illustrates how the alien species spread between 2007 -2018 through a series of distribution maps. “The moth first arrived in the Netherlands and Germany in 2007 because of accidental importation. At the time, data indicate that these countries imported at least one million boxwood trees from China without mentioning the lively trade in boxwood trees between Belgium, China, and Italy. Currently, the moth affects nearly all of Europe and the Caucasus, wreaking havoc on ornamental crops and wild Buxus species, particularly the European boxwood or Buxus sempervirens.”

Life history and behaviour

At CABI’s research institute in Délémont, Switzerland (southwest of Basel), Kenis and his colleagues put the invasive pest under a magnifying glass.
He explained, “Where we are, Northern Switzerland, box tree moths have two generations, and that is also likely to be the case in Belgium and the Netherlands, whose climates do not hugely differ. In Northern Europe, they have a non-problematic single generation, while in Southern Europe, they have third or fourth generations causing severe damage.

“The most common pattern is two generations. Box tree moths hibernate during the winter as the third instar cocooned larvae and all larval stages are capable of diapausing. That is, a real diapause, so once the larvae enter their diapause, you can’t do anything with them. When you collect larvae between mid-September and mid-October to incubate, they will not develop. This will only happen if you break their diapause between December to January.”

Overwintering larvae start to emerge and feed from early to mid-spring until they pupate in June. The pupae stage is short, with the moth in the wing by June or July. Moths will then lay eggs that only need three days to hatch. Kenis notes, “This marks the arrival of the second generation, which is usually in flight in September. In Southern Europe, you will have multiple overlapping generations.”

Six to seven larval stages feed on the leaves and shoots of boxwood, and severe infestations can lead to complete defoliation of plants or even death. Larvae typically skeletonise the leaves consuming the entire leaf except for the midrib.

Kenis elaborates, “When the tree is completely defoliated, they also feed on the bark, limiting its chance to regrowth.”

The young caterpillars are not only voracious eaters, but they also tend to be very picky eaters. “They feed exclusively on boxwood and have no appetite for Ilex or Euonymus and thus are very genus-specific. They pupate mostly on branches between leaves, on the ground or in the event of total defoliation on other plants.”

According to Kenis, box tree moths are good flyers who prefer early dusk to spread their wings. They can quickly gather into thousands. There have been reports of pheromone traps on terraces filling up within 20 minutes.

Spread of the box tree moth in Europe (Visuals: Bras et al 2019, Journal of Pest Science).

Damage beyond repair

Boxwood tree moths decimate ornamental and wild boxwoods. Kenis acknowledges, “It can go very fast with one generation -usually not the first – eating your hedge away by August. The level of damage highly depends on the climate. Our city, Délémont, provides an interesting case study as temperatures in the surrounding hills with native boxwood stands are significantly lower. The species was first found in our area in 2010, although our boxwood forest remained intact until 2016. It is now destroyed due to consecutive heat waves over the past five years, allowing the pest to have a second generation. I presume this is a similar situation in the Netherlands, Belgium, and the UK,” says Kenis, who uses Climex software based on simulation and modelling techniques to define the eco-climate suitability and number of generations of boxwood moths. This research points to Southern France and Northern Italy as the moth’s most favourable places to thrive.

Do box trees survive total defoliation?

At the initial stage of infestation, most trees will regrow, ensures Kenis. “If the moth has fed on the bark, the tree will only leaf out and regrow from its base. Usually, there is some regrowth. Studies found that trees have a 30 to 60 per cent chance of surviving after one full defoliation. But survival rates decrease with every new defoliation.”

In the wild, the moth will result in plant dieback five to eight years following total defoliation and depending on the number of generations the moth has per year. We are worried that within ten to 20 years, there will be no native boxwood stands left in Europe except some mountain areas.”

In terms of ornamental boxwoods, gardeners frequently cut or uproot their infested plants or use (bio) chemicals to allow trees to regrow.

Economic and environmental impact

In calculating the economic impact of the boxwood tree moth, Kenis believes that consumers primarily carry the burden rather than horticulturists.

He elaborates, “Nursery stock growers will grow and sell alternative crops. Meanwhile, garden owners need to continue spraying. It’s challenging to keep a tree in good shape once infested. But I would not like to play down the serious impact on commercial growers affected by the pest. I know some specialist boxwood growers who have lost all their customers as people no longer buy boxwood. In my region, up to 80 per cent of ornamental boxwood has been cut.”

Kenis fears the catastrophic collapse of nature’s ecosystems even more. Boxwood forests as ecosystems will soon disappear, and Buxus balearica will be on the brink of extinction.

Kenis elaborates, “While it has now already infested this species too, the question is what will happen to the more than 20 boxwood species in Central America or the two native boxwood trees in Puerto Rico? So far, no one knows how prone they are to the moth. Ecological extinction also means a cascading impact on biodiversity. On Buxus sempervirens alone, 132 fungi, 12 chromista, 98 invertebrates, and 44 lichens develop and are at risk. And wild boxwood often protects slopes and banks from erosion.”

Societal impact

The Boxwood tree is very slow-growing, and its sturdy wood is used for cultural, religious, and practical purposes. Boxwood is, for example, well-suited for carving and turning. Some common uses for boxwood include carvings, chess pieces, musical instruments, combs, and spoons. In the Catholic religion, blessing branches of boxwood on Palm Sunday is customary. Boxwood is used in the most splendid forms of topiary art worldwide. There are many medicinal uses of boxwood in the Caucasus, while it also serves as mattress filling.

Kenis says, “Namely in the Black Sea region, the societal impact is big, and people are extremely anxious to see their boxwood forest disappear. Boxwood’s economic and societal value is more important than in Europe.”

Marc Kenis handpicks caterpillars.

Control

When infestations are tiny, handpicking caterpillars can be effective. Kenis says: “Alternatively, people can cut down and replace their boxwoods with evergreen broadleaved species such as Ilex crenata, Euonymus japonica, Lonicera nitida, or by members of the boxwood family itself such as Sarcococca or Pachysandra, both insensitive to box blight and moth. Another option is to develop more resistant Buxus species. Buxus sempervirens and Buxus microphylla varieties have been tested, but no major differences in resistance have shown up. Selecting resistant varieties is time-consuming because of boxwood’s slow-growing character.”

Regarding (bio)control, Kenis says. Bacillus thuringiensis (BT) is a biological weapon against boxwood pests. “It is efficient on young larvae. The real advantage is that 80 per cent of trees have been uprooted, so there are fewer generations able to fly and multiply. Pheromone traps can be used in gardens to monitor plantings and plan BT applications.”

Little is known about the effect of neem plant extracts (Azadirachta indica) on fighting the pest, while horticultural oils and soaps are likely to control young caterpillars.

Kenis ends by saying, “Our final hope is on classical biological control, using parasitoids of the box tree moth sourced from East Asia. The funding for the needed research is challenging. Boxwood does not fall under agriculture or forestry, so its value is often underestimated. Travelling to China to check out endemic boxwood stands is a costly exercise. Fortunately, we did carry out some literature studies and travelled to China, where we limited our research to the cities. Here, we collected larvae at 114 sites in seven provinces and were reared in the lab to obtain parasitoids. We saw a boxwood tree moth at 60 per cent of the sites, with parasitism varying between 0-50 per cent. It was higher in regions where the boxwood tree is native. A total of nine parasitoids were found, among which Dolichogenidea sp. and Chelonus sp. -an egg larval parasitoid- are the most promising. This year, we have set our hope on additional funding, which will allow us to do additional research in Japan and Korea.”