Your garden, terrace or balcony with its elegantly shaped box shrubs may be looking its best, but beware. The smart look of your home may be in jeopardy.
Your decorative box shrubs may become the unwitting host and victim of the box tree moth (Cydalima perspectalis), an unwelcome Asian invader with no natural predators. Originally a native of East Asia, the box tree moth was first seen around Basel in 2007 but is now ravaging box (Buxus) bushes in the cantons of Geneva and Vaud.
The caterpillars become active in the spring (around April, depending on the weather) and begin devouring the leaves. They are yellowy green, with a black head; older caterpillars have thin white and thick black stripes the length of the body and can grow up to 5 cm long. After four weeks, they spin a cocoon of among the leaves and twigs and pupate for about 10 days.
The adult moth usually has white wings with a faintly iridescent brown border, although they can be completely brown or clear. The wingspan is around 4 cm. Once hatched, the moths find a nearby bush and lay their yellowish eggs in clusters of 15–20 on the underside of the leaves. The moths die after about eight days, but in the meantime the new caterpillars have been born. The species can produce up to four generations in a year.
A tell-tale sign of a box moth infestation is the white webbing covering the bush, reminiscent of a spider’s web, which the caterpillars produce over their feeding area. Other indications are brown leaves – often rather lacy when only the veins and stalks remain – and tiny greenish brown droppings. The caterpillars first feed on the leaves, then on the green bark of the branches, and the “bite marks” are very evident. Their voracious appetite can decimate whole groups of box almost before your eyes.
Since the caterpillars begin their attack from the interior of the bush, it can take a while before any damage is visible – by which time it might be too late. It is important to check box plants regularly and thoroughly from the beginning of April to the end of September. If there is a weak infestation, the caterpillars can be picked off by hand and thrown away with the household waste.
For heavier infestations, the use of a pesticide is inevitable. Delfin (Bacillus thuringiensis) is an organic pesticide that works against small caterpillars. It has the added advantage of being bee-friendly. Other products, such as Karate Zeon, Lambda-Cyhalothrin 100 CS, Alanto Garden, Alanto Spray and Gesal Calypso, are effective against larger caterpillars but are more toxic, so necessary measures to protect bees and other wildlife need to be taken. You can also try pheromone traps, filled with poison, that attract the moths as they hatch. More drastic measures are called for if the plants are totally destroyed – rip them up and dispose of them.
Many garden centres are open on Sundays and public holidays in the spring so you can maximize gardening time. Check out individual websites for details.