How to recognise the box tree moth?

How to recognise the box tree moth?

It is essential to identify the presence of the box tree moth before it is too late. This article has all the necessary information on timely detection of this pest.

You have probably already heard that the box tree moth (Cydalima perspectalis) is a dangerous pest on box trees (Buxus). But detecting this species and distinguishing its symptoms from other diseases of the box tree can be challenging. Read below to find all the information for the protection of your beloved evergreen shrubs.

How to detect the box tree moth

In order to recognise an infestation of the box tree moth early, it is indispensable to know how to correctly control the box tree moths. In this article, you will learn when to check and how to recognise the infestation. Below you will find a few tips on fighting the disease.

Box tree moth: when to check?

You won’t find eggs, caterpillars and adults of the box tree moth all year round. The caterpillars survive the winters and are followed in spring by pupae and adult moths that lay eggs and a new generation with successive stages of development. Unfortunately, these stages occur at slightly different times each year, depending on temperature and weather conditions.

The following table summarises the occurrence of eggs, caterpillars, pupae and cocoons. However, the table depicts the normal progress in the development: any deviations from the norm in terms of weather will impact the rate at which the insects develop. Particularly warm years lead to a faster development of the insect and more generations occurring in one season.

Stage of developmentCaterpillars (overwinter)CocoonsMoths, eggsEggs, caterpillarsCocoons, mothsMoths, eggs, caterpillars
RecommendationUse trapsUse trapsUse traps
GenerationPrevious year’s generationPrevious year’s generationGeneration 1Generation 1Generation 2Generation 2

It is important to note, that the stages of development can vary depending on climate conditions. Therefore, it is important to do regular check-ups on your box trees and use pheromone traps to uncover the box tree moth presence in time.

Monitoring box trees

From April, the inner part of box trees should be checked for the overwintered box tree moth caterpillars. To be truly safe, the shrubs need to be inspected diligently from the inside and the outside at least once a month until September. Around June, the overwintered generation of caterpillars transforms into adult box tree moths and therefore can also reach your box trees from neighbouring gardens. This is the right time to use pheromone traps to notice the moths ahead of time. Finding out about the flight of the adult moths will indicate when eggs and young caterpillars will be found on their box trees. This happens about two weeks after the moths first fly.

Detecting a box tree moth infestation

The box tree moth lives pretty secretly in almost all phases of life. Groups of up to 20 eggs are deposited in the outer areas of the shrub on the undersides of the leaves. They are to be discovered only in the time directly after the moths first fly and you need to be very attentive in order to glimpse them. As soon as the small, green-yellow larvae hatch, they set off into the interior of the box tree, where they hide in the caterpillar webs and leaves. In this phase, white webs can be observed inside the shrub. If these are opened, they contain young larvae and excrement crumbs.

Older larvae move to the outside of the box tree. In this phase, the infestation is easily recognisable: the caterpillars now eat whole leaves and the bark is also destroyed – as a result, entire parts of the box tree die and turn yellow. A sure sign of a box tree moth infestation are the green-yellow caterpillars with black and white stripes and the characteristic black head capsule, which are up to 5 cm long. At this stage, even the white webs should be more visible.

During the metamorphosis, the caterpillars pupate to similarly coloured, 2 cm long pupae, that are also spun into leaves. The flight of the butterflies is not very obvious without the use of pheromone traps because the moths are active at night. If there is a fragrant butterfly bush (Buddleja davidii or Buddleja alternifolia) in the surrounding area, sometimes the box tree moth can also be found there during the day. If you have discovered adult moths in your garden, you should keep an intensive lookout for eggs and larvae in the coming days to prevent a severe infestation from occurring.

cydalima perspectalis caterpillar
Box tree caterpillars target the foliage of box trees [ PhotoStorm_22]

Summary of how to detect a box tree moth infestation:

  • Eggs can be found shortly after the moths first fly in the outer areas on the undersides of the leaves.
  • Young, yellow larvae can be found inside the box tree, hidden in white webs.
  • Older, darker coloured caterpillars can be found outside of the shrubs. The shoots of the buxus become yellow and die, the foliage is destroyed and the caterpillar webs can be seen clearly.
  • Pupae are in the webbing made of caterpillar silk and leaves.
  • The adult box tree moths can be spotted by pheromone traps or observed on butterfly bushes (Buddleja).

Box tree moth: what to do in case of an infestation?

If you discover a box tree moth infestation, it is essential to act promptly and get to the treatment right away:

  • Prune your box hedge about two weeks after you have observed the adult box tree moths. This will remove the eggs and the freshly hatched larvae.
  • Cut the webs with the young caterpillars out from the inside of the box tree.
  • Once the caterpillars have grown and move to the outside of the bush, use natural sprays. In the preceding stages, the caterpillars are protected by their webs very well. If the infestation is low, you can collect the caterpillars manually or flush them away with a strong jet of water using a gardening hose. Here you can read what you should consider when handling the poisonous box tree moth.
  • Young larvae and eggs can be disposed of alive; discard the older caterpillars and pupae after they have been killed. Read more about how the box tree moth can be disposed of here.

You can find detailed information on how to get rid of the box tree moth here.

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