COEXISTENCE AND WEAK AMENSALISM OF CONGENERIC GALL-FORMING APHIDS ON THE JAPANESE ELM

Closely related species of gall-forming aphids are often associated with a single host species. Six Tetraneura species coexist on the Japanese elm, Ulmus davidiana, in Sapporo, northern Japan. This paper describes the probabilities of coexistence on macro- and microgeographic scales (i.e., on host trees and host leaves) and examines whether coexistence with conspecific or heterospecific galls on leaves or shoots has any effect on the fecundity of each aphid species using multiple regression. A Monte Carlo simulation showed that the frequency distribution of the numbers of species on individual host trees differed significantly from that expected from the null model. There were significantly positive or negative associations between species. Tetraneura radicicola and sorini always coexisted with other species on trees they infested. Multiple regression revealed that the coexistence of conspecific or heterospecific galls on individual leaves had no influence on the fecundity of radicicola and sorini, but had a negative influence on that of sp. O. On average, radicicola and sorini produced a smaller number of offspring in galls than sp. O, and obviously consumed only a small part of resources available on the galled leaves. Evidence available suggests that although amensalism does arise between sp. O and other species, its influence is not so strong as to exclude sp. O competitively from the Tetraneura community.