What are Those Bumps on the Willows? The arroyo willow is Sausal Creek’s namesake plant; the word sausal means “willow grove” in Spanish. If you’ve been to Dimond Park and other parts of the creek, you have seen many of these water-loving trees. You might have even seen some red bumps on the leaves and wondered “what are those”? If you turn over the leaf, you will find that the bumps extend further on the underside. These are willow apple galls induced by a sawfly Pontania californica. ©May Chen Sawflies are not actually flies but, like ants, bees, and wasps, are members of the suborder Symphyta within the order Hymenoptera. The common name comes from the saw-like ovipositors that the females use to cut into the plants to lay their eggs. The major distinction between sawflies and the other Hymenopterans is that the adults lack a “wasp waist" and instead have a broad connection between the abdomen and the thorax. Female ovipositing on a willow ©Peter J. Bryant Sawflies lay their eggs directly into the leaf of the willow. Each gall holds only a single larva which feeds on the gall tissue. Prepupae drop to the ground via silken threads, where they encase themselves in cocoons. Adults usually emerge in spring, timed with the appearance of new leaves. The gall is itself a complex ecosystem involving inquilines, parasites, and hyperparasites. These can be a wasp, beetle, or even a moth. Note the black pin holes on the galls. They could be the exit holes of the gall Larva emerged from a gall ©Peter J. Bryant makers. Or they could be the portals whereby other insects infiltrate the gall. It’s a jungle in there! Here’s another common gall found on the stems of willows, induced by yet another sawfly, the willow stem sawfly in the genus Euura. Each gall may contain one or more larvae. When the larvae have completed their development, they crawl to the top of the gall and cut an exit hole, which they plug with frass just prior to pupation. Pupation occurs in the gall, and emergence of sawflies occurs in spring as new shoots begin to grow on their host plant. Willow stem sawfly gall ©May Chen --May Chen, edited by Nicki Alexander - 2 - .