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Tomoplagia cressoni Aczel is oligophagous in flower heads of Perezia microcephala (deCandolle) Gray and Trixis californica Kellogg (Asteraceae) in southern California. Both hosts are in the tribe Mutiseae. Third instar larvae and puparia, and two new types of sensilla on them, are described for the first time. The larvae feed on the floral tubes and ovules as first and second instars, respectively, but most growth occurs in the third instar. The third instar feeds mainly on sap that collects in a cup-shaped cavity it chews in the receptacle. Most seeds in infested heads were undamaged. Mating behavior is described, which is unique among non-frugivorous Tephritidae for the more posterior, upright position of the male and the back and forth, rhythmic movements of his abdomen during initial phases of copulation. The unusual "butterfly stroke" wing movements of adults when walking also are described. This tephritid has a spring generation on Tr. californica, a summer generation on P. microcephala, and facultatively, a fall generation, again on Tr. californica, depending on the incidence of late-summer, desert rainfall triggering a second flowering.

Eurytoma veronia Bugbee (Eurytomidae), Colotrechnus ignotus Burks (Pteromalidae), and Pteromalus sp. (Pteromalidae) are reported as primary, solitary, hymenopterous parasitoids of To. cressoni.