Just about every year I receive samples of fallen cottonwood leaves with a “bump” at the base and the first one for this year was from Winner. This is the cottonwood petiole gall formed by the feed activity of a very small green aphid. The insects begin feeding on the leaves in the spring and this galls forms around them. The aphids usually mature by July and the winged adults emerge from the galls and fly to another host. The damage from this insect is a nuisance – who wants to rake in August – but it does not harm the tree. The fallen leaves do not need to be burned; the insect has already left, so leaving these leaves will not increase the problem next year. An application of dormant oil can be applied on the tree to kill the aphids before they move out from their overwintering sites in the bark fissures on the tree.
