Indole-3-acetic acid and ball gall development on Solidago altissima

•  Indole-3-acetic acid (IAA) concentrations are reported in ball galls, stem tissue, and in the gall-forming larvae of the tephritid fly Eurosta solidaginis, to gain insight into the mechanism of ball gall formation on the goldenrod, Solidago altissima. •  IAA was identified and quantified by gas chromatography-mass spectrometry, with [¹³ C₆ ]-IAA as an internal standard. •  Concentrations of IAA were higher in gall tissues than in stem tissues on a weight per stem length basis, but not on a weight per weight basis. First instar larvae of E. solidaginis contained IAA at 9 µg g^-1 f. wt or 1.21 ng per larva, a concentration 33-times higher than in control stem tissues. Gall development was unaffected by the early removal of the apical bud and leaves from galls, and lateral bud release was inhibited by galls in terminal positions in the absence of the apical bud, indicating that ball galls act as sources of auxin. •  The presence of an E. solidaginis larva results in higher amounts of IAA in a given length of stem than are found in its absence. E. solidaginis larvae may act as point sources of IAA in developing ball galls.