Aphids - many species of aphids occur on plants and some are capable of producing galls. The most common galls are the stem and petiole galls on poplar or cottonwood. An example is poplar petiole gall (Fig. 4). Adelgids - these aphid-like insects are most prevalent on conifers with the eastern spruce gall (Fig. 7) and Cooley spruce gall (Figs.