Sandbar willow can also be laden with aphids, which are typically protected by ants that harvest excess sugars passed by aphids.
Amy Savage and Merrill Peterson, both based at Western Washington University, examined relative abundances of ants, aphids and midges, which can all occupy the same willow. They found that aphid populations grew faster when they were tended by ants and in the company of midges. Galls were more common on plants that also had ants and aphids. Finally, numbers of ants, aphids and midges were all positively correlated.