I came across a couple of California Croton plants that were hosting many galls.
I searched @nancyasquith's collection and lists, https://www.inaturalist.org/projects/california-plants-with-mystery-galls, as well as the Russo guide, but couldn't find anything.
I'm posting this observation for the host plant, hoping we can find the gall inducing insect...

This trunk was washed down in a flash flood on 10 September 2004, over 18 years ago, and it is just starting to decay at the end next to the trail. See:

http://tchester.org/sd/plants/guides/anza_borrego/borrego_palm_canyon.html

Thanks to Mark Fisher at the Philip L. Boyd Deep Canyon Research Center for finding and sharing this population in bloom this year! This species was last recorded in Deep Canyon in 1978: https://www.cch2.org/portal/collections/individual/index.php?occid=1490992

Collected gall from Ceanothus verrucosus on 4/11/21.
https://www.inaturalist.org/observations/73583630

Gall cut open to find larvae inside on 4/21/21 and placed in container with a cup of soil.
https://www.inaturalist.org/observations/74768171

7 midges emerged today.

Last photo is to document the host plant. 4th photo is a cropped version of the last photo which has the insect gall in question in the lower left while A. trixa is found in the top half of the photo.