Return flight of sexuparae of galling aphids to their primary host trees: Implications for differential herbivory and gall (Aphidoidea: Pemphigidae: Fordinae) abundance

The life cycle of the Fordinae (galling aphids on Pistacia) requires annual colonization of the host trees. Gall abundance often differs greatly in adjacent Pistacia trees. We investigated the effect of variable numbers of winged aphids (sexuparae) arriving at different trees in the spring on the abundance of galls on the same trees in the following year. Sticky traps were set on marked trees in 1992 and 1994 at 2 sites, and they were replaced weekly throughout the spring flight period. All trapped migrants were counted (separately by species in 1994). We also sampled shoots on each of the marked trees in 1993, 1994, and 1995 to estimate gall abundance of the most common aphid species on P. palaestina, Geoica sp. (probably G. wertheimae Brown and Blackman), Forda formicaria von Heyden, and Baizongia pistaciae (L.). Catches on different trees varied greatly. The trees with the largest total seasonal catches were located together at one end of the study area, indicating a possible common source (secondary hosts of the aphids). The numbers of sexuparae of F. formicaria, and of B. pistaciae at one site, in 1994 were correlated significantly with the gall abundance of these species in 1995. Gall abundance of G. wertheimae in 1993 was correlated significantly with abundance in 1995, but not in 1994 (nor were numbers in 1994 and 1995 correlated). This suggests a 2-yr periodical cycle of gall abundance, possibly driven by the 2-yr holocycle of the Fordinae. F. formicaria and B. pistaciae, however, may have anholocyclic populations on some nearby secondary hosts that produce sexuparae every year.