Cytokinins in the Ball Gall of Solidago altissima and in the Gall Forming Larvae of Eurosta solidaginis

• The identities and concentrations of cytokinins in goldenrod ball galls, normal goldenrod stems, and in the gall-forming larvae of the tephritid fly Eurosta solidaginis were determined in order to gain insight into the mechanism of ball gall formation on Solidago altissima. • Zeatin, zeatin riboside, isopentenyladenine, and isopentenyladenosine were identified and quantified by gas chromatography-mass spectrometry in ball galls, goldenrod stems, and in larvae of E. solidaginis. • Concentrations of the four cytokinins were higher in gall tissues than in stem tissues when expressed on a weight per stem length basis but not when expressed on a weight per weight basis. Isopentenyladenine was the most abundant cytokinin in larvae from developing galls and in larvae from fully formed galls exhibiting 'green islands'. In first instar larvae, isopentenyladenine was present at a much higher concentration (c. 50 fold) than in control stem tissues. • The presence of an E. solidaginis larva results in higher cytokinin amounts in a given length of stem than are found in its absence. Larvae of E. solidaginis may act as point sources of isopentenyladenine in developing ball galls.