As with most aphids it is the eggs that over-winter, deposited by females on the bark of cottonwood trees. These hatch in the spring to nymphs that feed on the petioles (the stem) of new leaves, causing the stem to swell and create a gall that completely encloses the aphids. The next generation will include winged females that fly to the alternate host plants in the Mustard family, and several successive generations will feed on the roots of these plants. This may include various crop plants as well, allowing this aphid to be an economic pest. In the fall new winged females return to cottonwood trees to deposit their eggs. The gall is the most noticeable feature for this aphid, but it appears to cause little harm to the infested tree.
Difficult to impossible to control once the gall encloses the aphids, but since infested cottonwoods suffer no damage from the gall control is not necessary at this stage. Control should focus on dormant oil applications in the winter, applied to the trunks and branches of the trees to smother the over-wintering eggs.