The second gall Leonard and I found on willows along the Lower Hat Creek Trail (Shasta County CA) is the willow stem gall. I discussed the willow rosette galls that infect leaf buds in my last post (“Willow Rosette Gall” 02-07-18).
There are over six species of sawflies in the genus Euura that induce integral stem galls in willows in the West. Sawflies are small wasps with saw-like ovipositors. In most galls, eggs are laid in the host and gall formation is initiated once the larvae hatch. Adult Euura sawflies apparently program the stem to produce galls by fluids injected when the eggs are laid. The gall, instead of being initiated by the larvae, reaches full development before the eggs hatch. The adults emerge in the spring, often to coincide with the emergence of new willow shoots. Adult sawflies have biting-chewing mouth parts that are important for chewing their way out of the woody stem gall following pupation inside the gall.
Since I am, at this point without leaves or other identification aids, unable to specify which willows these galls infected, I will not try to identify the Euura species that induced these willow stem galls. Additionally, although different Euura species are associated with particular willow (Salix) species, they may occur on other willows. The only way to determine definitively which sawfly created the stem gall is to retrieve the sawfly from the gall.
Amy Savage and Merrill Peterson (2007) reported that ants often “farm” (tend) aphids on willow branches with galls. Where there are ants and aphids on the willow branches there appear to be more galls. Aphids have higher populations when both galls and ants are present. The abundance of ants, aphids and galls are all positively correlated with one another. They hypothesize that the occurrence of ants, aphids and galls may benefit all three, but the mechanism behind this correlation is unknown.