What is a box tree borer and how do you fight it?

The box tree borer is a small butterfly. The up to 5-cm-long caterpillars with their yellow-to-dark green color, the black head capsule and the black-and white-stripes are pretty to look at. But their pretty appearance is deceiving because they are voracious pests that eat box trees and shrubs and destroy them.

 

Where does the box tree borer come from?

The box tree borer (lat. Cydalima perspectalis) was probably introduced to Europe through international nursery plants trade with East Asia. Since 2012, it has spread rapidly throughout Switzerland and destroyed countless box tree and shrub plants.

What does the box tree borer look like?

Its wings are silky white with a wide brown border on the edges of its wings. The borer is striking with its crescent-shaped white spot on both wings.

How does it multiply?

The box tree borer is located on the underside of leaves of plants and only lives about 9 days. The female lays its eggs only in the box tree and shrub. The caterpillars hatch within a few days or weeks. The voracious little creatures first start with the leaves on the inside the box tree and eat their way outward, which means that damage is not immediately visible. They pupate several times and go through five to seven larval stages until they hatch in July as a butterfly. By this time, the massive extent of their damage from eating the box tree is now easy to see. Two to three generations of box tree borers are born per year. In September, the last generation of the year hatches and it lays eggs again. These caterpillars overwinter in cocoons between the leaves. Even low temperatures in winter do not bother them. They become active again in spring and the eating frenzy starts all over again.

Caution: This frenzied “borer eating orgy” can also cause “box tree dieback,” a fungal disease that weakens the box tree even more. This fungal disease alone causes the box tree to lose its leaves and look bald.

 

How can you identify the fact that the box tree borer has been at work?

If you notice dried and gray leaves on your otherwise always-green box tree around July, and you also notice damage to leaves from the borer eating them, then the box tree borer caterpillars have been at work. In addition, this pest has whitish webs, which look like spider webs, are all over the box tree or shrub and has bright bits of fecal matter. The yellowish clutch of eggs lies on the underside of the leaf and stands out because of its black dots in the middle. During the cold season you can see box tree borer pupae, which overwinter in the web.

 

What should you do if the box tree is already infected?

  • Natural enemies: Unfortunately, so far, only the wasp has been observed as this caterpillar’s natural enemy.
  • Mechanical treatment: Caterpillars can be picked off by hand or vacuumed with a vacuum cleaner. Catch the falling caterpillars with a cloth or plastic tarpaulin under the tree.
  • Spray: Biological sprays or insecticides are recommended for heavy infestation or large box trees. Biological sprays should be harmless to  beneficial organisms and therefore not harm bees. Be careful with insecticides. Strictly follow the instructions for use and safety instructions to protect people and animals. It is best to ask the specialist retailer for the appropriate means for applying insecticide to your box tree.

How can a person prevent the box tree borer from attacking?

Start checking regularly in March and cut back  the shrub way back in spring. This will help destroy any clutches of eggs. Water the box tree or shrub early in the morning only and do not water the leaves directly. Fertilize it regularly, especially if it is growing in pots. Healthy plants are less susceptible to pests and diseases.