Alder flea beetle (Altica ambiens)
Pest description and damage Both adults are dark, shiny blue, and about 0.25 inch long. Larvae are black and about 0.25 inch long at maturity. Adults chew holes in leaves while the larvae feed in groups and skeletonize foliage of alder, poplar, willow dogwood and other hosts. They are capable of defoliating trees but often they are in smaller numbers throughout the trees. As the insects look similar and often both hosts grow together, it is difficult for the non-professional to identify beetles by species.
For biology, life history, monitoring and management
Alder (Alnus)-Alder flea beetle
See Table 2 in: