[The galls] are found commonly on a main stem, but rarely on a branch. The flower cluster is sometimes entirely aborted, but usually only partly so, the lower pedicels in the cluster remaining normal.
In the majority of cases the gall is terminal, but in a few instances the stem ... project[s] a short distance beyond it. The gall has the relatively simple structure of a spindle-shaped enlargement of the stem. In length it varies from 10 to 15 mm, and in greatest width from 5 to 8 mm.
It is roughened on the outside by the stumps of the aborted branches. On account of the shortening of the stem axis and the consequent crowding of the nodes, these branches are more numerous on a gall than on a corresponding length of normal stem. This gives the gall a gnarled surface and forms a strongly-protected case for the larva. The gall in some cases is surmounted by a tuft of leaves growing from its apex.
The aperture through which the moth escapes from the gall is made always near the upper end.
The insect passes the winter in the larval condition.

[N]ewly hatched larvae move directly into the terminal growth where they penetrate the buds and inflorescences to cause the swellings or galls ... Before pupation begins, the larva cuts an exit hole partially through the side of the gall to leave only a thin film of bark which covers the entrance ... The galls may vary greatly in abundance, ranging from only an occasional gall to 20 or more per lineal foot of branch and twigs in heavy infestation ... [T]he gall is somewhat spindle-shaped and averaged about 13 mm in length by 6 mm diameter. This is about three times the diameter of the twig ...

Examination of Ceanothus collections at [several Southern California arboreta and botanic gardens] showed varying degrees of non-susceptibility to the stem galls. [Table 2 shows] Heavy occurrence [on] Ceanothus griseus [and] C.g. horizontalis ... Moderate occurrence [on] C. cyaneus, C. thyrsiflorus [and two cultivars] and Light occurrence [on] C. arboreus, C. oliganthus, C. diversifolius, C. lemmonii, C. integerrimus, C. leucodermis, C. lobbianus [and several cultivars].

[click below to see several grayscale photos]

Stem Gall Moth
Perploca ceanothiella

This moth induces integral, spindle-shaped galls on stems of several species of ceanothus, especially deer brush.

Hosts:
Ceanothus integerrimus
Ceanothus velutinus
Ceanothus cuneatus

Range: CA, KS, NY, OR, TX