What has caused this damage to the leaves of crack willow, Salix fragilis, found on the public footpath last week? The patches of damage are currently quite small (1-2mm in diameter), but will increase in size as the summer progresses. You will also see this on other species of willow around the site. Answer next week....
Anyone get this one? The leaf damage that you see in the picture is feeding damage caused by a beetle called Phratora vulgatissima, or the blue willow leaf beetle. It is a common and widespread member of the Family Chrysomelidae, the leaf beetles and can be found in most areas of the UK in the presence of their foodplants - various species of willows and aspen.
The imago (adult) beetles are typically around 5-6mm in length and can range in colour from blue/green to bronze/black, showing a metallic sheen on the elytra (wing cases), pronotum (thorax case) and head. Both the imago insects and the larvae feed on willow and aspen leaves, by grazing off the outer epidermal layers of the leaf. This is what causes the damage seen in the photos. The adults are present throughout most of the year and unlike many other insects, actually over-winter in the imago stage. Look for them clustered together, hibernating in sheltered areas such as on fence posts, wood piles or walls.
The larvae look like typical beetle larvae, with soft, fleshy bodies and are beige and black in colour. They can be seen from mid-summer and in numbers can cause more damage to the host plants than the imago insects. The beetle however, is a native species and is not a major threat to willow and aspen trees.