Coming from Asia, most likely as eggs on imported box plants, Box tree caterpillars were first found in UK gardens in 2011, although the adult moth was first seen in Britain in 2008. By the beginning of 2015, box tree moth had become established in London and the South East but is now spreading further afield. Many large, historic gardens have had years of formal box hedges and topiary wiped out by this pest and it is now becoming a real concern.
Description
The striking adult moth has semi-translucent white wings with a brown border, although they can sometimes be clear or completely brown. Although the female moth does no harm herself, she lays her eggs, between 5-30, in a sheet on the underside of the leaves. The eggs are small and pale yellow so not always easy to spot.
After 3 days, the eggs hatch into caterpillars that have greenish-yellow bodies and black heads. As the caterpillars grow, they develop thick black and thin white stripes along their body and can reach up to 4cm (1½in) in length after 2 weeks. They then spin their characteristic webbing among the leaves and branches from where they feed.
The caterpillars are most active in temperatures above 15c with the ideal temperature 18-22°C, so tend to be active between late March and early October. They can hibernate during the winter down to temperatures of -30c waiting for the weather to warm up in the spring when they start feeding again.
The caterpillars eventually develop into pupae, which are concealed in a cocoon within the webbing before emerging as adult moths who have a lifecycle of about 45 days (female).
Box tree caterpillars only feed on box (Buxus) plants and shouldn’t be confused with other caterpillars.
Symptoms
The caterpillars eat the leaves of the box plant and can defoliate a small specimen overnight leaving the telltale sign of skeleton leaves and the webbing over their feeding area. The webbing is quite thick and obvious, sticking the leaves together. This is where you will often find the caterpillars and the pupae. Don’t confuse the webbing with the webs of spiders. Once they have eaten the leaves, the caterpillars will then remove the bark and start eating the green cambium layer underneath which is most damaging to the plant.
