The alder leaf beetle (Agelastica alni) is a strikingly dark, iridescent member of the leaf beetle family (Chrysomelidae), widely distributed across Europe, the Caucasus, Siberia, and northeastern Kazakhstan, and introduced to the United States in the 19th century. Both adults and their larvae are notorious for their voracious appetite for alder trees (Alnus). While adult beetles chew random, unsightly holes throughout the canopy, it is the developing summer larvae that cause the most severe damage. Feeding in dense, communal groups, they skeletonize foliage and can completely defoliate an entire tree. Fortunately, despite the dramatic aesthetic damage, healthy alders usually recover well. While walking in a Swedish forest close to my home during late summer or autumn, I occasionally came across large, dramatic aggregations of these beetles huddled together as they prepare to overwinter.