Macrohaltica ambiens and others
Pest description and crop damage Adults are dark shiny blue and about 0.25 inch long. Larvae are dull brown to black and about 0.25 inch long at maturity. They feed on and skeletonize foliage of alder, poplar, and willow. Both adults and larvae feed on the foliage.
Biology and life history Adults become active in spring. Larvae usually are present in June and July. Populations of alder flea beetles vary year-to-year. In outbreak years, these beetles can cause substantial defoliation but rarely enough to effect long-term health of the trees. Occasionally applications are necessary in nurseries.
For more information
Johnson, W.T. and H.H. Lyon (1991), Insects That Feed on Trees and Shrubs, 2nd ed., Cornell University Press (p. 228).
Pacific Northwest Nursery IPM (http://oregonstate.edu/dept/nurspest/alder%20flea%20beetle.htm)