Alder Leaf Beetle

Alder Leaf Beetle (Agelastica alni). The shiny blue-black beetles are 6 to 7 mm long and belong to the family of leaf beetles (Chrysomelidae). The Alder Leaf Beetle feed on the leaves of alder (Alnus) and are also spotted on poplars (Populus), willows (Salix) and hazels (Corylus avellana). Sometimes they are also found on fruit trees.
The beetles hibernate in the ground under fallen leaves and plant debris. The Alder Leaf Beetle is active from April to June: it then eat elongated holes in the leaves. Birds (great tits!) are the natural enemies of the Alder Leaf Beetle.
The female Alder Leaf Beetle lays hundreds of orange eggs on the underside of leaves. After one to two weeks, the eggs hatch and the larvae appear. The larvae eat the mesophyll between the veins of the leaf (skeletonizing). Initially, these are light green before turning to black. The larvae pupate on the ground among plant debris.