The (Very Hungry) Box Tree Caterpillar

From the title I’ve given this post I’m sort of tempted to start with ‘Once upon a time….’, but clearly there’s a significantly serious side to this tale.

Unless you’ve been locked away in your potting shed for the last few years, you’ll be hard pressed not to have noticed the significant media coverage of the gardening plague, known to us all as box blight. And yet for me, whilst being a professional gardener, out there day in, day out, I’d only seen it once in a client’s garden several years ago.

Imagine my dismay then, when, last week I saw what I thought to be the early signs of the bewildering blight on two stunning, well established box hedges in a clients’ garden.

Just one week later the box hedges were biscuit brown and almost completely defoliated. Upon closer inspection of the hedges’ bony carcasses, I was aware there was a wispy webbing wrapped around the skeletal stems of the plants. And then, crawling between the twisted twigs, tiny green and black caterpillars. Hundreds of tiny green and black caterpillars of varying sizes, some almost microscopic others about 3cm in length.

I remembered this had been covered on Gardeners’ World earlier in the year. More importantly I remembered that whilst obviously not box blight, the impact was significant and resulting in the probable death of the host plants.

Identification

The villain of the piece? Box Tree Caterpillar; or to address this diminutive devil correctly, Cydalima perspectalis. The caterpillars are the larvae of the Box Tree Moth; a highly invasive species of moth, native to Eastern Asia, which appears to have stowed away, secretively on plants imported into the UK. It was first recorded here in 2007.

The moth, generally white winged with a brown edging and an approximate 4cm wing span lays its eggs on the underside of its host plants leaves, box (Buxus sempervirens). The newly hatched caterpillars are greenish-yellow, with black heads. As they age, they can grow to 4cm in length and have an olive green/yellow body with pairs of black spots running across and down the length of the body. The caterpillars feed frenziedly on the plant’s foliage and whole plants can be stripped of all foliage in a matter of days.

Control

Early identification of Box Tree Caterpillars may prevent the certain devastation of infested plants. As gardeners regular inspections of box plants for brown patches of dieback (which will be more apparent on trimmed plants), the tell-tale silk webbing and/or an infestation of the caterpillars may put us ahead of the game. Hand picking the caterpillars from a plant isn’t an easy job, let alone practical. However, laying an old sheet around the base of the affected plant and shaking it gently will dislodge many of the pests, which must be disposed of.

  • It has been reported that birds including blue tits feed on the caterpillars. However, it is not clear if this predation will result in a reduction of box tree moth numbers. Furthermore, due to the rapid rate of defoliation, it would be unwise to leave caterpillars for birds to eat.
  • Pesticide control

    • Extensive infestations can be treated with an insecticide. Thorough spray coverage is required if control is to be achieved and the spray itself must penetrate the silk webbing if it’s to be effective.
    • Several applications of a short persistence, organic contact insecticide containing natural pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer) may be necessary to give good control. Again the webbing must be penetrated.
    • More persistent contact insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. PY Bug Killer).

    Any affected plant material must be burnt.

    The Royal Horticultural Society (RHS) are currently tracking the movement of Box Tree Caterpillar/Moth across Great Britain. Follow the link below to report a finding (http://apps.rhs.org.uk/surveys/submitrecord.asp?type=9).

    Alternative planting

  • Box Tree Caterpillars, together with Box Blight, are significant forces to be reckoned with. As such, considering alternatives plants to box are currently a more realistic, long term solution.
  • Some considerations include:
    • Ilex crenata ‘Convexa’
      Taxus baccata
      Lonicera nitida

    Conclusion

    Between 2015 and 2016 over 800 records of the Box Tree Moth were received. In 2017 this increased significantly to over 3000 and in 2018 to over 6000 topping the pest ranking for the third time.

    So will there be a happy ever after for Box Tree gang? I don’t know, but I suspect gardeners across the country will be hoping not!

    Author: Mike the Gardener

    Freelance gardening writer, consultant and designer. Mike Palmer is a passionate and professional plantsman, offering services in garden writing, consultancy and garden design. Mike is also available for garden and plant related talks and presentations. Mike has been a professional horticulturalist for over fifteen years.

    Leave a Reply

    Fill in your details below or click an icon to log in:

    WordPress.com Logo

    You are commenting using your WordPress.com account. Log Out /  Change )

    Google photo

    You are commenting using your Google account. Log Out /  Change )

    Twitter picture

    You are commenting using your Twitter account. Log Out /  Change )

    Facebook photo

    You are commenting using your Facebook account. Log Out /  Change )

    Connecting to %s