Pine cone willow gall is caused by a gall midge, Rhabdophaga strobiloides. This dipteran (related to flies and mosquitoes) deposits an egg in the developing terminal leaf buds of the willow in early spring. The larva releases a chemical which interferes with the typical leaf and branch development of the willow, instead causing the formation of this cone-like structure. The adult dipteran emerges the following spring, after having spent the winter in the gall.
Check out a few more photographs of pine-cone willow gall via the BugGuide or read more about galls courtesy of the Michigan Entomological Society. Douglas Justice, the Garden’s Associate Director of Horticulture and Collections, has also identified the phenomenon in the past on the garden’s discussion forums.