Box tree moths take wing across the Garrotxa's ancient box woods

Box tree moths are on the wing across the Garrotxa this month.¹ Sightings from late August into early September came from Olot, Riudaura, Castellfollit de la Roca, Vallfogona de Ripollès and the villages of the Vall d'en Bas.¹ These are second-generation adults. The first generation flew in early summer; this one emerges from mid-August and stays out until early October.²

The adult (Cydalima perspectalis) has a wingspan of about four centimetres. Most are white with a wide brown border around the wings and a small white crescent on the leading edge of the forewing. Roughly one in ten is mostly brown with white streaks, and that form can pass for a different species altogether. On either form, the crescent and a box bush nearby are the tell. The caterpillar is green with black stripes and a shiny black head, up to four centimetres long, and feeds inside loose white webbing laced with droppings among the box leaves. Inside a shrub the first sign is usually missing leaves and that webbing. The moth passes the winter as a small caterpillar sealed between two box leaves stitched together with silk, wakes in March, and feeds until it pupates in late April or May.

The host is what makes this matter here. Common box is native in these valleys. It grows thick under beech, oak and yew on the shallow soils of steep limestone slopes, the ground where the bigger trees themselves cannot hold, and it covers about 30 percent of the forest area in Catalonia. Charcoal from archaeological layers across La Garrotxa shows deciduous oak and box as the two most important woody plants in every period of the Holocene.² Box has been part of these woods for as long as there have been woods.

The moth is an East Asian species introduced to Europe, and its caterpillars strip box bare.³ One defoliation is survivable; box resprouts. Repeated defoliation, together with the caterpillars' habit of gnawing bark once the leaves are gone, kills the plant. A life-cycle study of this moth carried out in La Garrotxa itself describes complete defoliation of box shrubs and calls the ecological consequences unassessed.² Birds take few of the caterpillars, which store toxic alkaloids drawn from the box leaves; Great Spotted Cuckoos have been watched picking them off box branches in southern Catalonia in spring, but that cuckoo is thin on the ground and the feeding was recorded only in the spring generation.⁴

Where box dies standing, dead shrubs change the fuel in the understory, in both quantity and continuity. A model of southern Pyrenees box stands under moth, drought and fire pressure projects box mortality and burning rising as the climate warms, with the worst of it below 800 metres.⁵ Olot and Riudaura, where most of this month's moths were logged, sit below that line.¹ Fire danger around the valleys is quiet right now, with no thermal detections nearby in the past several days.