The alder leaf beetle, Agelastica alni, is a species of leaf beetle (Chrysomelidae) in the genus Agelastica. Previously rare, it now appears to be on the increase in the Northwest. The beetle and the beetle's larvae host on the grey or speckled alder (Alnus incana), but will be found on hazel, birch and/or hornbeam if there is a shortage of food. Causing large holes and gaps from the end of April onwards, which are produced by the larvae in the first two stages, but the main damage is only produced by larvae in the third stage.
The alder leaf beetle is a relatively small beetle, around 6–7 mm, black or metallic blue in colour. The larvae are normally black.
The skeletonized remains of a black alder