Desert Broom
Baccharis sarothroides
Each of these galls on Desert Broom (Baccharis sarathroides) supports a dozen or more cecidomyiid fly larvae.
On fresh new growth these galls may be abundant. They are soft and easy to
break open where the small orangish larvae of gall midges can be found. 
A fresh gall may contain 20 or more. Eventually
adult midges, similar to the one at left, emerge. Later the galls harden and
may remain on the plant for many seasons, but without occupants.
The flies release an irritating substance into the plant tissue and in response the plant produces a durable home containing the fly's food. But the gall midges are heavily parasitized by small wasps whose search for prey may be aided by the swelling?
Many insects live upon desert broom especially during periods of rapid vegetative growth - i.e. after especially summer rain. Boldt et. al 1989 (Proceedings of the Entomological Society of Washington, Vol. 90, no. 2, pp. 207-215.) recorded 64 species of herbivores species in Arizona and New Mexico.
Dictyna spiders take advantage of this prey abundance. The silk has a very white appearance due to its hackled structure and the web is typically constructed near the top growing points where prey are most abundant.
Desert Broom's flowering period in mid October offers an abundance of nectar and pollen: Read More.