Dealing with Box Tree Caterpillar

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The most recent unwelcome visitor causing problems in our box hedges is the Box tree caterpillar Cydalima perspectalis. First reported on our shores in its adult moth stage in 2008, the devastating caterpillars were not discovered in private gardens until 2011 but they have since become more widespread in and around London.

The adults lay their eggs on the underside of leaves on Box hedges, the striped green and black caterpillars that hatch defoliate the host before covering it in the same silky webbing used to form its cocoon. Three or four generations of this voracious new pest are possible in a long warm summer. Affected Box plants are often severely defoliated, the leaves are devoured almost in their entirety leaving little more than a network of green stems and leaf ‘skeletons’ behind. The remaining leaf parts will die and an untreated outbreak will weaken a Box hedge making it more susceptible to other pests and diseases.

The adult moth usually has white wings with a slightly iridescent brown border although the wings can be completely brown, or clear. The pest appears to overwinter as small caterpillars hidden in between box leaves that have been woven together with its distinctive silky webbing, the caterpillars complete their development the following spring completing a new generation.

Practical ways to deal with Box Tree Caterpillar

These stripy imposters are never likely to be welcome in any keen gardener’s plot but they are thankfully somewhat easier to deal with than box blight – which is a good thing as they are becoming more common with each passing year according to the responses to the RHS box tree moth survey. (On a side note – At home I am a great believer in letting our brood of (very free range) chickens have full garden access on special occasions, they do a sterling job with slugs, snails and vine weevils leaving no fallen leaf unturned I am confident they would seek out deal with Box caterpillars too)

• Remove the caterpillars by hand, with a little patience (and manual dexterity!)
• Use a biological control, although this approach may take time as the predatory parasites will need to multiply and will only be effective in warmer temperatures. The nature of a biological control will need the pests to remain, albeit at acceptably low numbers to sustain the predators.
• Pheremone traps – these can be very effective and a specific impregnated lure for the box tree moth is available from Agralan ltd.
• Chemical control – by spraying with a suitable insecticide such as Bug Clear (pyrethrum) or Bayer Provado Ultimate Fruit & Vegetable Bug Killer (deltamethrin). Because of the persistent nature of the silky webbing surrounding the caterpillars as they feed, attention is needed to achieve effective spray penetration. (Special care should be exercised when applying insecticides to avoid plants with open flowers so that bees and other pollinating insects are not harmed).

Once an outbreak is brought under control monitoring is advised to determine if the caterpillars have returned and if so, further measures should be undertaken.