Phloem-Feeding Specialists Sharing a Host Tree: Resource Partitioning Minimizes Interference Competition among Galling Aphid Species

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Abstract
This study deals with a guild of specialist, phloem feeding aphids (Homoptera; Pemphigidae; Fordinae) which form galls on Pistacia trees (Anacardiaceae). In Israel, each of two Pistacia species carries five common species of galling aphids, which may occur in large numbers on the same individual tree, shoot, leaf or even leaflet. These Fordinae have very similar life cycles, they all feed on phloem sap, and all of them need very young, unfolding leaves for gall formation. Our data show, however, that most pairs of species are unlikely to compete for galling sites because their niches are separated either temporally (fundatrices arrive at different times and, therefore, occupy different leaves) or spatially, by attacking different sites on the tree, shoot, or leaf. In 1991-1993, we calculated niche breadth (B) of each species, and proportional similarity (PS) between each species pair on shoots colonized by more than one species. PS between different species pairs on leaves within shoots ranged between 0 and 0.48. This means that, on the niche dimension represented by the shoot, the two species may share some of the habitat units. If the two species compete, we expected that niche breadth of at least one of them would be reduced on the cohabited 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 same shoot. In species sharing leaves within shoots, B showed similar trends. On P. atlantica, Smynthurodes betae and Forda riccobonii had the highest PS. Both species make their galls on leaflet margins and occur frequently on the same leaves. Only at this level did we detect negative interactions: the frequency of co-occurrence of galls of both species on the same leaflet (within galled leaves) was significantly less than expected by chance. Taking into account our deliberate non-random selection of trees, shoots, and leaves, where competition was likely to occur, we conclude that interspecific interference competition for galling sites is not a major determinant of the resource partitioning of these closely-related herbivores.

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