Box tree moth

Identification

Bright green caterpillars with a black head, black dots and dark longitudinal stripes along the body, growing to 4–5 cm. They feed inside a webbing of dense, white threads, eating the leaves and gnawing the bark of the shoots, so that they can strip the shrub completely bare. An infested box leaves behind characteristic webbing, greenish droppings and dry leaf remains. The adult moth has white wings with a broad brown border and a wingspan of about 4 cm. As an invasive species it produces 2–3 generations per season, so feeding can continue from spring until autumn. The female lays pale yellow eggs in flat, overlapping (roof-tile-like) clusters on the underside of the leaves, usually deep inside the shrub. The youngest caterpillars scrape away only the lower skin and mesophyll of the leaf (leaving transparent 'windows' and a skeleton of veins), older ones eat whole leaf blades, and when leaves run out they gnaw the green bark of the shoots — such ringing leads to the dieback of entire twigs. The pupae sit in white webbing between the leaves and twigs. Young caterpillars overwinter hidden in cocoons (so-called hibernaria) in the dense interior of the shrub and become active as early as early spring, before any symptoms are visible from the outside. Alongside the typical form there is a rarer dark (melanistic) form of the moth, with almost entirely brown wings bearing a light spot on the forewing. From a distance an infested box looks parched and faded — it turns brown, and between the shoots a white webbing with trapped greenish droppings is visible. The species originates from East Asia and has spread across Europe since about 2007, recorded in Poland since about 2012; it has no natural enemies here able to limit its numbers, hence the sudden, invasive nature of the damage.

Treatment

Regular inspection of the shrubs and hand-picking of the caterpillars together with their webbing, especially inside the shrub where feeding begins unnoticed. Treatment with a Bacillus thuringiensis product (effective against young caterpillars) and, under heavy pressure, thorough spraying of the shrub's interior as well. Pheromone traps allow the moths' flight to be monitored and the treatment timed, while a strong jet of water washes off some of the caterpillars. The key is early detection: inspection of the shrub's interior should begin as early as early spring, when the overwintering caterpillars resume feeding, and be repeated every 1–2 weeks throughout the season (2–3 generations). Bacillus thuringiensis subspecies kurstaki acts through the gut and is most effective against the youngest stages (the caterpillars must eat it while feeding), so the treatment is carried out in warm weather, thoroughly wetting the centre of the shrub as well, and repeated after 7–10 days; on large caterpillars it loses effectiveness. Pheromone traps and sticky boards serve for monitoring, not for control — they mark the peak of flight, after which (about 1–2 weeks later, when the caterpillars hatch) the spray is planned. Under heavy, multi-generation pressure insecticides are used: those selective for moth caterpillars (spinosyns or insect growth regulators) spare beneficial insects — spinosyns cover a broader spectrum of stages, while growth regulators are most effective on young larvae, like Bt — whereas contact products of the pyrethroid group act quickly and on all stages but are non-selective and harmful to pollinators and predators, so use them spot-wise and with care. Heavily stripped shrubs usually rebuild their foliage once the pest is removed, because box regenerates well from the shoots; cut, webbed fragments are worth destroying to reduce the number of overwintering caterpillars.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.