Box caterpillars feed within webbing and can completely defoliate box plants. Relatively new to Britain, it is a pest native to East Asia. Whilst the adult moth was first reported in Britain in 2007, caterpillars were not found in private gardens until 2011, it has since become widely distributed across England.
Symptoms to look for:
Gardeners are likely to become aware of box tree caterpillar when they find webbing and caterpillars on box plants, however it is often the case that they are only noticed when the damage has been done.
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
Caterpillars are also capable of stripping bark which can result in girdling of affected sections
Due to its short life cycle (45 days) there are often multiple waves of infestation, up to as frequently as once a month. It generally occurs between March and October and is best treated quickly as the pesticides are more effective the sooner they are used.
There are a few ways to deal with box moth, although the most effective treatment (Bacillus bacteria) not legal for use in the UK, it is hoped in a couple of years it will become more available. In the meantime there are two insecticidal sprays that can be used to mitigate and reduce damage, while having limited effect on wildlife.
Contact pesticide (Provanto or Py) can be used to quickly kill the caterpillars, this works much faster on the larger caterpillars than systemic killers. However it can fail to penetrate the web-cocoons they use to protect themselves and provides no long lasting protection. It is best used as an emergency fix when the plant is in dire straits.
Systemic pesticides (Bug Clear Ultra) are not ideal against larger caterpillars but they provide 4 weeks protection against younger caterpillars. By spraying every 4 weeks, you can maintain a constant protection that will give a decent amount of control for emerging caterpillars. As box plants rarely flower then there is little risk of chemicals building up in the pollen and harming bees.
Physical Control not recommended, removal by hand is slow and likely to leave many still on the plant, whilst water blasting the box may remove more but also provides perfect conditions for box blight, another highly damaging problem.
If the box is struggling to regrow, spraying it with a seaweed solution (Maxicrop plant growth stimulant) can help awaken dormant buds, this is not often a problem on young plants but older larger plants can be slow to recover from damage.