Trupanea conjuncta is monophagous on the desert shrub, Trixus californica (Asteraceae), in southern California... [T]his species is a facultative gall former. If flower heads are absent or rare [because of] a lack of local rainfall, gravid females may oviposit in apical buds and the larvae develop to maturity gregariously in the galls so induced ... No galls were formed on Trixis which flowered in February through April, 1984, in response to rainfall ... Thus, gall formation was a localized, facultative activity ...

The galls of T. conjuncta are initiated when a female oviposits in a terminal bud ... Upon hatching, the larvae feed on the surrounding tissues and extend the gall cavity into the pith of the branch tip. If this feeding killed the apical meristem, the branch ceased to elongate and a subspheroidal gall resulted. If the apical meristems remained intact, the gall assumed a spindle shape as the branch continued apical growth ... galls measured ... 6 to 16 [mm long] and ... 3.6 to 5.5 [mm wide] ... The larvae fed on the parenchymatous pith tissue, expanding the gall cavity in length and width until, eventually, two to five shortened internodes were incorporated ... The cavities were ellipsoidal or subspheroidal, smooth-walled, and free of frass ...

[See photos b and e on pg. 286]