Poplar Vagabond Aphid Gall

From a Forest Health Protection publication: "Poplar vagabond aphids have a life cycle similar to other gall-making aphid species in that it overwinters as an egg and alternates between winter and summer host plants. Newly hatched aphids feed on developing leaves of aspen and certain cottonwoods, which produce an enlarged, twisted clump of growth at the leaf base. Several generations of aphids occur within the folds of such galls, which may contain as many as 1600 individuals each."