Pemphigus populivenae (Hemiptera: Aphididae)
Cottonwood Leaf Gall Aphid
Everson, Whatcom County, Washington
June 7, 2010

Leaf gall on Populus balsamifera ssp. trichocarpa (black cottonwood). Underside of the leaf, with the slit in the gall through which some of the aphids are visible.