Gall usurpation by the gall-forming aphid, Tetraneura sorini (Insecta Homoptera)

First-instar nymphs of the gall-forming aphid, Tetraneura sorini, frequently usurp incipient galls of its own and other species of Tetraneura 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 the developmental stage of the gall. Although the probability of success decreased with the development of the gall, T. sorini invaders exhibited a strong preference for the more developed galls. Because of their larger body size, T. sorini nymphs were more successful in usurping incipient galls of other species than of their own species. The tendency in T. sorini to usurp galls may have resulted from their frequent failure to form their own galls. Hypotheses are advanced to explain such gall failure in terms of the interactions between I: sorini and its host plants.