The moths are nocturnal and fly in November and December, rarely into January. Presence of the larvae causes formation of persistent stem galls on Baccharis sarothroides (Asteraceae) that are similar in form to those of Gnorimoschema baccharisella Busck on B. pilularis. Mature galls of G. powelli are more or less uniformly spindle-shaped, ranging in size from 8 x 13 mm to 12 x 17 mm in outside diameter and height ... Frass was tightly packed at the upper end of the chamber, retained by frail silken mesh. At maturity larvae left the galls via an emergence hole about 1 mm in diameter, usually excavated at or below the middle of the gall, and dropped to the ground for pupation ...

Gnorimoschema powelli
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The integral stem galls of this moth develop on the spring growth of desert broom. These bark-colored, monothalamous galls are stout and spindle-shaped.

Host:
Baccharis sarothroides