Most syrphids feeding on gall forming aphids belong to Pipizini tribe. The biology of two species relatively abundant in Mediterranean areas of Southeastern Spain, Pipiza festiva Meigen, 1822 and Heringia heringii (Zetterstedt, 1843), is given after 3 years of detailed survey. In this paper we study the relationship of both species of syrphids with the gall-forming aphids associated with trees of the genus Pistacia (Anacardiaceae). We also establish the relationship syrphid-aphid-fruit tree, the relative abundance in each crop, length of preimaginal stages, aphid-specificity and parasitism. At last we show the seasonal adaptations of these hoverflies and their role in biological control of some fruit tree aphid pests. ; This study was supported in part by a grant from the Research Project GV-C-RN-12-069-96.