The heads just after flowering frequently show excessive and un-symmetrical swelling at the base, with spreading apart of the involucral scales, and exposure of the white pappus through the opening. This is due to the development of galls upon some of the flowers of the head, and these galls result from infestation by the gall midge, Asphondylia bea Felt...

The ovary, shortened and thickened, will contain no seed, but solid greenish tissue that is apparently glandular on its upper surface next the larva. The hairs of the pappus crowning the ovary will be shortened and thickened and very irregularly crinkled at the base. The thickened walls of the lower part of the corolla tube surround the larva and form the gall cavity, within which there is more or less upgrowth of a living glandular tissue arising from the top of the ovary. The outer layer becomes thick and firm, and finally brown and brittle, splitting away in bits under a dissecting needle. Pistils and stamens are quite obliterated, and the withered tip of the unopened corolla persists at the top of the gall.

These galls are very inconspicuous, if only one or two are present—not enough to split the involucre by their swelling—for they are completely overtopped and covered up by the normal flowers.

- J. G. Needham: (1925) Observations on a flower gall of the chaparral^©