Galls of the beadlike cottonwoodgall aphid, Thecabius (=Pemphigus) populimonilis, (left) and poplar stemgall aphid, Pemphigus populicaulis.
Galls of the beadlike cottonwoodgall aphid, Thecabius (=Pemphigus) populimonilis, (left) and poplar stemgall aphid, Pemphigus populicaulis. Credit: Jack Kelly Clark