Population dynamics and gall growth of the gall-forming aphid Forda hirsuta Mordvilko (Hemiptera: Aphididae) on pistachio tree ( Pistacia vera L.) in northeastern Iran

Gall-forming aphids are models for studying insect–plant interactions due to their specialized life history and host association. This study investigated the population dynamics and gall development of Forda hirsuta Mordvilko on pistachio (Pistacia vera L.) in Chelqi village near Mashhad, northeastern Iran, during 2022–2023. Fortnightly sampling from late April to early November recorded galls per leaf, gall size, and aphid number per gall. Forda hirsuta first appeared in late April, and gall abundance peaked in early to mid-June (4.51 ± 0.05 galls per leaf). Gall size increased gradually until mid-June and expanded rapidly thereafter, particularly on male trees, reaching maximum values of 22.80 ± 0.34 mm in length, 4.45 ± 0.10 mm in width, and 103.10 ± 3.96 mm2 in area. Aphid populations peaked in late August to early September (71.88 ± 17.59 aphids per gall). Open galls containing alate aphids first appeared in early August and continued until early November. Gall size and aphid number differed significantly between tree sexes (T-test, p < 0.01), and gall size was strongly correlated with aphid population (r = 0.92, p < 0.01). These findings highlight the relationship between gall growth, aphid dynamics, and host tree sex, with implications for pistachio pest management.