How to identify and treat box tree moth: complete guide

Identify the symptoms of a box tree moth attack.

The first signs of a box tree moth infestation are often the silk threads around boxwoods and the numerous dark green droppings around the bushes. The caterpillars are more easily spotted in the lower parts and the centre of the infested plants.

Moth and silk threads in an infested boxwood

Boxwoods are very resilient plants and can regrow leaves, but if the moths attack the bark, it can lead to the death of the boxwoods.

Unlike other garden pests and except for rare isolated cases, the box tree moth specifically targets boxwoods (particularly Buxus sempervirens and rotundifolia), making pest prevention and care for the bushes all the more crucial.

Characteristics and life cycle of box tree moth

Buxus moth goes through several development stages, from translucent eggs to light green caterpillars and finally to night-flying moths. Understanding its life cycle is essential for integrated pest management and responsible gardening.

• Eggs

The eggs are translucent, grouped in patches, and are generally laid on the underside of leaves, making them difficult to spot. It only takes a few days for the first caterpillars to appear.

Translucent eggs of the Buxus moth

• The caterpillar (larva)

The caterpillar is identifiable by its shiny black head and light green body edged with dark green longitudinal lines, black warts, and small white hairs. It has 10 false legs and is not urticating.

Several stages of growth of Buxus moth caterpillars, from the first to the last larval stage.

• The pupa (chrysalis)

Once at ripeness, the caterpillars spin a silk cocoon to pupate. The chrysalis forms very quickly. The pupa measures about 2 cm. It is initially green and then turns brown at maturity. The moth emerges in a few weeks or even in just a few days in summer.

Pupa (chrysalis) of the Buxus moth

• The adult (moth)

These are night-flying moths. There are two forms of Buxus moths: one has pearly white wings edged with a brown band with golden and violet iridescence. The other, rarer, has entirely brown wings. Both forms are still identifiable by a small white crescent moon on their wings. Their wingspan ranges from 35 to 44 mm. A female lives about 15 days but can lay hundreds of eggs during her short life. The moths are strongly attracted to light and their population explosion is also problematic… with some homes being literally overrun.

The two forms of the Buxus moth

_{Photo credits for the different biological stages of the Buxus moth: ephytia.inra.fr}

Understanding the complete life cycle of the box tree moth

Caterpillars spend winter in well-camouflaged silk cocoons between two box leaves.

Young box tree moth caterpillar in its silk cocoon[/caption>

From February, the caterpillars emerge from their hibernation and start nibbling on the leaves. Their development is rapid. Between late March and early April, the caterpillars become pupae and between late May and early June, the first generation of butterflies can be observed.

These adults mate and then lay eggs that will become the individuals of the second generation and so on.

The adults continue to mate in summer and autumn and thus several generations of butterflies come to life. The young caterpillars of the last generation spend winter in silk cocoons and the cycle begins again.

This will allow readers to quickly grasp the key stages of the box tree moth’s life cycle and the important periods for managing this pest.

[caption id="attachment_26886" align="aligncenter" width="724"] Life cycle of the box tree moth (INRA)

Throughout the year, several generations (2 to 3 per year) can be observed with all stages of the life cycle visible at the same time: eggs, caterpillars, chrysalises, butterflies.

Effective methods to combat box tree moth

For those who prioritise sustainable gardening and ecological solutions, several methods can be effective. Bacillus thuringiensis is a natural insecticidal that can be used as part of a biological control strategy against this caterpillar. Additionally, pheromone traps can help reduce the population of box tree moths in your garden.

• Mechanical Control

As soon as the first caterpillars appear, remove them by hand. The caterpillars are not urticating.

Brush the boxwoods to remove as many silk threads as possible.

Clever gardeners spread a cloth under the plants and hit the boxwoods with a stick to knock the caterpillars onto the cloth. All that’s left is to collect and crush them. This method makes collection less tedious when dealing with large boxwoods or topiaries.

• The Basin Trap

A large basin (or even several) filled with water mixed with dish soap and illuminated at night proves effective for trapping butterflies that drown in it, but if they are very numerous, the basin fills up quickly, and it needs to be emptied daily and reset.

Also to discover is the vacuum technique…

• Alternative Treatments and Effectiveness Against Box Tree Moth

Although black soap and white vinegar are often mentioned as organic treatments, their effectiveness has not been scientifically proven. For more reliable garden protection, we recommend following proven methods and promoting garden biodiversity to create an ecological balance.

Biocontrol and natural treatments against box tree moth

Biocontrol is defined as the set of protection methods that use only living organisms or natural substances such as predators, the Thuringian bacillus, or pheromones. Several research projects are underway at INRA to find solutions to curb the phenomenon. The latest study concerns the insufficient effectiveness of trichogrammes.

• Predators

In its region of origin, the box tree moth is regulated by natural predators. Here, these predators are absent. Predation by great tits and house sparrows has nevertheless been observed, as well as by the Asian hornet (Vespa velutina).

The installation of nesting boxes facilitates the nesting of birds that will help regulate box tree moths. Count on 10 to 20 nesting boxes per hectare. It is imperative to comb the boxwoods to remove the silk and facilitate predation.

INRA is conducting studies using micro-parasitic wasps that are egg-laying: trichogrammes whose females lay their eggs inside the eggs of the box tree moths, leading to the death of the latter.

• Treatment with Thuringian Bacillus

Repeated treatments of Thuringian bacillus (Bacillus thuringiensisssp kurstaki or BTK or more simply BT) are effective but only on young caterpillars.

The older ones resist the treatment. The Thuringian bacillus is a bacterium naturally present in nature. This bacterium attacks the intestinal wall of caterpillars. They stop feeding and die quickly, within 1 to 5 days, from septicemia.

The treatment is applied to the entire foliage with a sprayer. Its duration of action is a maximum of 3 to 7 days. It is essential to treat when caterpillars are present and to couple the treatment with trapping butterflies.

To treat your boxwoods:

Start by removing the silk threads that often surround the attacked branches, then treat as soon as possible, on the young caterpillars with BT, preferably in the evening or early in the morning. Repeat the treatment two to three times, at 3-week intervals, to destroy the different generations of young caterpillars. BT is only active on young caterpillars that feed a lot. Older caterpillars, at the end of their cycle, are little affected, as they stop feeding. Intervene again in summer and autumn if new generations of caterpillars appear.

After a first attack and treatment, water the boxwoods well to stimulate the regrowth of new leaves.

→ Also read Bacillus thuringiensis: a natural insecticide

• Pheromone Trap or Buxatrap

Pheromones are sexual substances produced by female insects to attract males. INRA, as part of the “SaveBuxus” programme, has developed and patented a pheromone trap called BUXatrap. The pheromones attract male moths into the traps. This is not a miracle solution, but it allows, when combined with other control methods, to limit the invasion.

Each species emits a specific pheromone. It is therefore necessary to install one trap per type of pest, in this case, for the box tree moth. These traps are used as a warning system to determine the start and extent of the presence of these pests. Each capsule must be changed every 4 weeks.

By increasing the number of traps, it is possible to slightly reduce the number of males, fertilised females, eggs laid, and contaminated boxwoods. However, the cost of these interventions then becomes very expensive.

One of the pheromone traps installed in the Harmas of Jean Henri Fabre in Sérignan-du-Comtat.

To know the periods of pest presence in the garden, it is advisable to regularly read the plant health bulletins (BSV). Supplied by a network of professional and volunteer observers, the BSV allows one to know the importance of problems (diseases, pests) in each region and for each type of crop. It is a valuable aid for observing problems in one’s garden and triggering, if necessary, appropriate preventive and curative interventions.

Tip: to consult the BSV, simply type “Plant health bulletin” followed by the name of your region into your search engine.

Finally, discover our advice sheet dedicated to pheromone traps.

Prevention strategies against box tree moth attacks

Prevention revolves around three axes:

• From March onwards, remove and burn the caterpillars and nymphs present on the bushes to limit the appearance of the first caterpillars;
• In case of death of the bushes, cut them down and burn the aerial parts;
• Avoid planting bushes and do not replace them with new ones. Choosing resistant plants and diversifying trimmed hedges can also contribute to ecological gardening.

Note that an alternative would be to replace the bushes with other plant species that are ignored by the box tree moth. Discover our selection of potential candidates in our advice sheet: “Box, what to replace it with?“.

Also watch the video by Olivier on: alternatives to box.

To find out more

• Box tree moth, a stowaway from the East – INRAE