California Aster Cotton Bud Gall
Rhopalomyia sp.

This bud gall begins formation in spring and matures by late summer and fall. As the gall matures, it develops an outer webbing with a texture similar to wool or cotton. Locally common in Southern California.

A full series of adults, larvae, and pupae were sent to Raymond Gagné for inspection. He initially surmised that this gall was caused by an undescribed Asteromyia species with unusual life habits, but requested more samples of the galls. Luann Wright (@nature4lzw on iNaturalist) sent those samples.

Wright reports on iNaturalist that Dr. Gagné wrote as follows in a private communication on July, 20, 2024:

"When cottony flower head galls were first noticed on Corethrogyne [sent July 19, 2021 by James Bailey (@silversea_starsong )], they contained many blister galls on bracts that were made by a species determined as an Asteromyia by R.J. Gagné. Adult gall midges were then emerging in some numbers, so it appeared they were possibly the cause of the cottony flower head gall. Upon closer examination of the galls, R.J Gagné found conical galls attached to the pedicels that appeared to be a more likely culprit for gall formation. Such galls are caused by various species of Rhopalomyia on other plants also, e.g.. sagebrush. A more recent collection of galls sent to R.J. Gagné showed the exclusive presence of early-developing Rhopalomyia galls on pedicels beneath new cottony galls and the absence of any Asteromyia anywhere else on the galls. It appears then that the Rhopalomyia causes the galls and that Asteromyia is a secondary occupant of the flower heads."

- Gallformers Contributors: (2024) Gallformers ID Notes^©