We've been seeing these cottony galls
with little white larvae inside them,
sometimes with apparent exit holes,
on rabbit brush pretty much everywhere we go in the desert and, less frequently, these spiky green and purple gall-ish things
and I've finally taken the time to research them a bit, which, admittedly, didn't take a whole lot of effort thanks to the ever-amazing internet. Near the top of the list generated by typing in "rabbitbrush parasite," there was Bulletin 506A published by the Aspen Delineation Project. A botanist's dream journal, not only does Insects that Feed on Colorado Trees and Shrubs tell me what I wanted to know about rabbitbrush gall, it also explains many of the crazily colored and textured plant parasites that thrilled and amazed me when we backpacked in the West Elks.
In case you all aren't as excited as I was to peruse all 176 pages of the document -- even though you should be because where else are you going to learn about things called hackberry nipplegall psyllids, or pinyon spindlegall midges, or stubby needlegall midges, or willow redgall sawflies or pinyon pitch nodule makers (!!!) -- here is what the authors have to say about rabbitbrush gall (p. 61):
RABBITBRUSH GALL MAKERS
(Aciurina bigeloviae (Cockerell), other Aciurina spp., Procecidochares spp.)
Appearance: Adults are small "picture-winged" flies with patterned or generally dark wings. Larvae are pale-colored maggots found within plant galls.
Damage: Several different gall-forming fruit flies make galls on the stems and buds of rabbitbrush. Perhaps the most conspicuous are cottony swellings of the buds. Others produce green flower-like growth of leaf axils or sticky globular swellings on stems. These do not appear to damage the plant but attract attention when plants are in landscape settings. Life History and Habits: The gall makers on rabbitbrush are generally similar in habits to the gall midges. Overwintering typically occurs as a partially grown larva within the gall. They complete development and adults emerge during periods when the plant is actively growing. After eggs are laid, the
developing larvae stimulate gall development by their secretions and feeding activities.
Control: Controls have not been developed nor are needed. If galls are considered unattractive, they may easily be pruned out. In native stands of rabbitbrush, these gall makers are heavily parasitized by several species of wasps. Also, several gall
midges may also develop within the gall, competing with the flies creating the gall.