Differential colonization of host trees by galling aphids: Selection of hosts or selection by hosts?

The pattern of adaptive diversification of galling aphids (Pemphigidae: Fordinae) on Pistacia (Anacardiaceae) is different from other gall-inducing insects. The Fordinae are host-specific, and radiated mainly by using different sites on one plant organ. Other gallers, including other galling aphids (Pemphiginae), share hosts and attack different plant organs. Species richness is very different among the three Pistacia hosts of the Fordinae in Israel. Pistacia palaestina and Pistacia atlantica each have 6-9 host-specific galling aphids. In contrast, P. lentiscus is colonized by only one species, Aploneura lentisci, throughout the Mediterranean and Middle-Eastern distribution of the host tree. Attraction to specific volatiles emitted by the leaves, or host-tree apparency, are unlikely to be effective mechanisms of selection of hosts in the Fordinae. The pattern is more likely due to selection by the hosts. Different response of the plant tissues to stimulation by fundatrices of different aphid species is possible. The presence of resin in the phloem of Pistacia may require special adaptation of the phloem-feeding Fordinae. P. lentiscus in particular produces commercially exploitable quantities of resin. A. lentisci may have evolved a unique mechanism to exploit this host. Several host shifts are suggested by the phylogeny of the Fordinae. None of these shifts involved P. lentiscus. The difference in species richness between hosts may have been conserved phylogenetically.