GALL USURPATION BY THE GALL-FORMING APHID, TETRANEURA-SORINI (INSECTAHOMOPTERA)

First-instar nymphs of the gall-forming aphid, Tetraneura sorini, freq uently usurp incipient galls of its own and other species of Tetraneur a by fighting the gall initiator. Fights with conspecific nymphs often escalated. The success of T. sorini in usurping a gall was dependent on the difference in size between the initiator and the invader, and t he developmental stage of the gall. Although the probability of succes s decreased with the development of the gall, T. sorini invaders exhib ited a strong preference for the more developed galls. Because of thei r larger body size, T. sorini nymphs were more successful in usurping incipient galls of other species than of their own species. The tenden cy in T. sorini to usurp galls may have resulted from their frequent f ailure to form their own galls. Hypotheses are advanced to explain suc h gall failure in terms of the interactions between I: sorini and its host plants.