Moved from Hackberry Psyllids.

But the systematics of this group is complicated. By 1890 Riley recognized 7 distinct leaf-galling Pachypsylla, which he "described" in (1). However, his new species descriptions detailed only the galls, and not the adult insects. Riley went as far as to say that the adults of these species were so similar that they were difficult if not impossible to differentiate without the galls. Because of this, subsequent authors sunk Riley's species into synonymy, and deemed that all leaf-gallers other than the blister-galler were the same species.

Many current authors still accept this classification, though recently there has been a shift back toward Riley's species concept with the belief that Pachypsylla celtidismamma is a cryptic species complex, and that the galls may indeed be the only way to differentiate species. Indeed, bugguide itself seems to be in agreement with this hypothesis as evidenced by the inclusion of one of Riley's species here. In a 2013 paper by D.B. Thomas (2), the case is made for the validity of Riley's names but urges that substantial work needs to be done to determine which of these are actually distinct species. Because of the current uncertainty in this group, leaf-galling Pachypsylla on bugguide may only be recognizable by galls, (and all adults especially those not associated with galls can only be considered to be members of the Pachypsylla celtidismamma complex, as descriptions simply dont exist for the majority of species).

With that said, the galls you have here are a perfect match for Riley's Pachypsylla celtidisumbilicus, which is new for the guide. It will be up to future authors to determine whether it's a distinct species or not, but it makes sense to me to make a new page for them on bugguide until such work is done on the group. Here is an excerpt from Riley's description:

Upper surface of leaf: circular in outline and abruptly depressed at its outer margin beneath the surface of the leaf. From this outer margin toward the center the gall gradually rises again to about the level of the surface or even above it, the center being furnished with an elongate nipple.... under side of the leaf: distinctly elevated, averaging 2mm in height and 5mm wide, circular in outline, the sides not abruptly elevate but gradually rising, with a ... shallow depression at top which is often furnished with a small central nipple

Moved from Mites and Ticks.