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  • Sue

Box Clever

....Or what to do about box moths and their very hungry caterpillars.

Damage caused by box moth caterpillars - and a dead box moth caterpillar which was caught in the act


We are being bedevilled by box moths and their offspring this year. They have been a problem in France for well over a decade, but this year, due to the mild winter, they have become a plague. The box moth is yet another invasive species which is causing chaos here because there are no natural predators. This little character came from Asia into Europe early in the 21st century and because of the comparatively mild weather we have here in France, four generations of the pest can be produced each year. They have also now spread to the UK where they are slightly less prolific, but are nevertheless starting to cause terrible damage to box plants everywhere.


A happy and healthy box moth caterpillar


The box moth is an anonymous, small, brown moth which can travel up to 10 kilometers, so if you spot some near your home be aware that they could be moving into your garden soon. The female moth lays eggs on the undersides of the leaves of a box tree where they hatch into caterpillars. These attractive little monsters then gorge themselves on the young leaves of the host plant. This will not of itself destroy a box shrub, but the removal of the leaves will reduce the plant's ability to photosynthesis and so feed and grow. It will become severely weakened. However, if you leave the plant untreated the caterpillars can then start to eat the plant's bark and cambium. As this bit of the plant includes the its phloem and xylem (the pathways by which any plant transports food and water throughout its structure), eating the bark in this way can result in death. Meanwhile the caterpillar, having eaten all it wants, forms a chrysalis which, not long afterwards, hatches into a new moth and the whole process begins again.


So much for the biology lesson - and what can a gardener do to halt or slow down the process?


When dealing with an invasive species of this type it can be hard to find a solution which does not also destroy native species - which can severely upset the ecostructure of your garden. It is important to try to be as selective as possible and to this end I have experiments with several ways of getting rid of box moths. The easiest, and most pleasing, was to watch our hens pick the caterpillars off the plants. However, this has a very limited success as the hens can't reach high enough to clean up the entire plant and, even if they had a step ladder, they cannot possible find all of the caterpillars.


I then resorted to standard insecticides which you can buy in a supermarket or garden centre. These are marked as being effective against "pyrale du buis", which is the French name for the box moth caterpillar. The problem here is, as outlined above, that these products are too aggressive and will kill other species including those which are beneficial to the garden.


I next spoke to Stephane Jay, the owner of a nursery not far from us and he recommended a biological control which contains a micro-organism called Bacillus thuringiensis. This occurs naturally in the soil and works by attaching itself to the walls of a caterpillar's gut which stops it from digesting food. The larva quickly dies of starvation. Different strains of the bacterium affect specific insect groups, so it is important to choose the one which is effective against box moths. I tried this product and it has proved to be very effective, but it does need to be applied four times a year because it is not residual - so will have no effect on caterpillars which hatch in the future.



The treatment I used successfully



The magic ingredient - Bacillus Thuringiensis sub species kurstaki.


Another even less invasive treatment is to use pheromone traps. These are little containers containing a pheromone which attracts and traps the male box moth. They are then unable to find and fertilise the female box moth, so she cannot lay eggs. The problem is resolved before it has had the opportunity to hatch, as it were, but has the drawback of never being able to trap all of the males....and as you know you only need one.... You can buy these traps on line (Amazon France supplies them). The phrase to use in the search engine is "piège à pheromone pour capturer le papillon de la pyrale du buis"


Box is extremely resilient and after treatment, providing the bark has not been stripped, it will re-grow, but it is clear that the problem of the box moth is here to stay and we can only minimise the damage the moths cause. My strategy for the future is, therefore, to plant no more box, but to find a suitable alternative for any topiary or hedging which I want to create in years to come. Fortunately, there are quite a few alternatives - here are a few to consider:


Podocarpus nivalis

Taxus (yew)

Berberis

Pittosporum tenuifolium

Lonicera nitida (shrubby honeysuckle)

Cotoneaster conspicuus

Phillyrea angustifolia and Phillyrea latifolia

Euonymous fortunii species

Euonymous japonicus species


Ilex crenulata has long been recommended as an alternative to box, but it does not thrive in hot dry climates so, despite the wet winter and spring we have just experienced, it would probably not be a safe bet in most of France.




All is not lost - if you can control the moth your box will regrow.





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