Alder flea beetle

You will notice small round holes chewed through alder leaves in late spring when metallic blue-black beetles emerge from leaf litter. Adults measure 1/4 inch; larvae feed in groups beneath leaf surface, sometimes skeletonizing branches. Look for shot-hole damage on red, white, and Sitka alder; also birches, poplars, and willows throughout Puget Sound lowlands.

Most infestations cause only cosmetic damage scattered through canopy; this pest rarely kills trees. Prune heavily infested branch tips in June or July while larvae feed. Insecticidal soap or horticultural oil can suppress populations on valuable specimens, but conserve parasitoid wasps and predatory beetles that naturally control them. Avoid broad-spectrum insecticides.