PHLOEM-FEEDING SPECIALISTS SHARING A HOST TREE - RESOURCE PARTITIONING MINIMIZES INTERFERENCE COMPETITION AMONG GALLING APHID SPECIES

This study deals with a guild of specialist, phloem feeding aphids (Ho moptera; Pemphigidae; Fordinae) which form galls on Pistacia trees (An acardiaceae). In Israel, each of two Pistacia species carries five com mon species of galling aphids, which may occur in large numbers on the same individual tree, shoot, leaf or even leaflet. These Fordinae hav e very similar life cycles, they all feed on phloem sap, and all of th em need very young, unfolding leaves for gall formation. Our data show , however, that most pairs of species are unlikely to compete for gall ing sites because their niches are separated either temporally (fundat rices arrive at different times and, therefore, occupy different leave s) or spatially, by attacking different sites on the tree, shoot, or l eaf. In 1991-1993, we calculated niche breadth (B) of each species, an d proportional similarity (PS) between each species pair on shoots col onized by more than one species. PS between different species pairs on leaves within shoots ranged between 0 and 0.48. This means that, on t he niche dimension represented by the shoot, the two species may share some of the habitat units. If the two species compete, we expected th at niche breadth of at least one of them would be reduced on the cohab ited shoots. This was not the case. B was not negatively affected when pairs of species occupied the same shoot, compared with shoots where only one species was present. B was positively correlated with density : the chance of niche overlap increased when more galls occupied the s ame shoot. In species sharing leaves within shoots, B showed similar t rends. On P. atlantica, Smynthurodes betae and Forda riccobonii had th e highest PS. Both species make their galls on leaflet margins and occ ur frequently on the same leaves. Only at this level did we detect neg ative interactions: the frequency of co-occurrence of galls of both sp ecies on the same leaflet (within galled leaves) was significantly les s than expected by chance. Taking into account our deliberate non-rand om selection of trees, shoots, and leaves, where competition was likel y to occur, we conclude that interspecific interference competition fo r galling sites is not a major determinant of the resource partitionin g of these closely-related herbivores.