Box tree caterpillar

Box tree caterpillars are the larvae of a moth that feeds on box (Buxus) plants. It is native to East Asia and it became established in Europe in 2007. Although adult moths were first found in the UK in light traps in 2008, it was not until 2011 that larvae were reported in private gardens in the home counties. By the end of 2014 the moth had become established in parts of London and surrounding counties; in many cases the caterpillars had caused severe defoliation indicating that the moth is likely to become a serious problem.

Seen box tree moth? We would like to know.

As part of our research the RHS would like to know where box tree moth has been seen.

Please submit your records via our box tree moth survey (expected time to complete survey = two minutes). The RHS confirms that your information will be stored in compliance with the Data Protection Act and personal data will not be shared with third parties. If you prefer not to have your information used in this way, please indicate when submitting your record.

Between 2015 and 2016 over 800 records of the moth were received, this jumped to over 3000 in 2017,  indicating that the moth is a widespread problem in London and surrounding areas.

Thank you to everyone who has submitted records so far – read a blog about the surveys 

Watch an animated map of the results from the box tree caterpillar survey (links to YouTube)

Gardeners are likely to become aware of box tree caterpillar when they find webbing and caterpillars on box plants.

• The pale yellow flattish eggs are laid sheet-like, overlapping each other on the underside of box leaves
• Newly hatched caterpillars are greenish-yellow, with black heads. Older caterpillars reach up to 4cm (1¼in) in length and have a greenish/yellow body with thick black and thin white stripes along the length of the body
• The pupae are concealed in a cocoon of white webbing spun among leaves and twigs
• The adult moth usually has white wings with a faintly iridescent brown border, although the wings can be completely brown or clear. The moth has a wingspan of around 4cm (1¼in)
• The caterpillars eat box leaves and produce webbing over their feeding area. Plants may also show patches of dieback which may be especially apparent on trimmed plants. This is not to be confused with dieback caused by the disease known as box blight

Non chemical control

• Where practical, caterpillars should be removed by hand
• A pheromone trap which can help monitor adult moth activity is available from several suppliers including Agralan
• The mixed nematode biological control sold as Fruit and Vegetable Protection may have some effect on the larvae

Chemical control

• Extensive infestations can be treated with an insecticide. Thorough spray coverage is required if control is to be achieved
• Forceful spraying is needed to penetrate silk webbing
• The contact pyrethroid insecticides pyrethrum (considered organic e.g. Bug Clear Gun for Fruit and Veg or Defenders Bug Killer,), deltamethrin (e.g. Provado Ultimate Fruit & Vegetable Bug Killer, Sprayday Greenfly Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) may have some effect.
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) can also be used
• Plants in flower should not be sprayed due to the danger to pollinating insects
• Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Choose alternatives to box

Where it has become impractical to control box tree caterpillar an alternative hedge or topiary plant can be used. Ongoing trials of a variety of compact, evergreen shrubs at RHS Garden, Wisley are showing promise as alternatives. The following all have small leaves and can be clipped into formal hedging styles:

Berberis darwinii 'Compacta'
Berberis thunbergii ‘Atropurpurea Nana' AGM
Elaeagnus × submacrophylla 'Compacta'
Euonymus fortunei (various cultivars)
Ilex crenata
Lonicera nitida 'Maigrün'
L. nitida 'Baggesen's Gold' AGM
Luma apiculata 'Glanleam Gold'
Osmanthus delavayi AGM
Pittosporum 'Arundel Green' AGM
P. 'Collaig Silver'
P. 'Oliver Twist'
P. tenuifolium 'Golf Ball' (PBR)
Podocarpus 'Chocolate Box'
P. 'Young Rusty'
Rhododendron Bloombux ('Microhirs3'PBR)
Taxus baccata 'Repandens' AGM
Ugni molinae,U. molinae 'Butterball'

Downloads

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biological control suppliers (Adobe Acrobat pdf document)

The biology of the box tree caterpillar in the UK is not yet fully known, as it is a recent discovery here, but it may have two or three generations per year. It overwinters as small caterpillars, hidden between box leaves that have been spun together with silk in autumn, and completes its development in spring. The adult moth is capable of flight, but it is not known how far it can travel.

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