The Elm Balloon-gall Aphids (Eriosoma lanigerum) are tiny, soft-bodied insects that feed on elm trees. Their feeding causes distinctive, balloon-like galls to form on leaves. These galls provide shelter and nourishment for the aphids. They are usually green or red and can distort leaf shape. Though unsightly, these aphids rarely cause serious harm to healthy trees. Natural predators often keep their populations in check.