Pachypsylla celtidisvesicula is the only "blister gall" caused by a named and formally described insect. Blister gall morphology varies substantially, however, and four additional species have been proposed to account for these differences. Three of these, P reticulovesicula, laevovesicula, and rugosa, were given formal names by Riemann 1961, but because this was a PhD thesis and not a refereed publication, the names are not valid species.
These galls are easily distinguished from P celtidisvesicula, and less easily distinguishable from each other, by range, morphology, and host. P celtidisvesicula sensu strictu occurs in the northeastern US, from Minnesota to Quebec, north of DC; forms very small, typically numerous, galls with distinctly raised rings above, and only on Celtis occidentalis. The other three species occur primarily in Texas, with some found in Oklahoma, New Mexico, and Arizona as well, are larger and occur in smaller numbers, occasionally alone; are flat or sunken above; and are found either on Celtis laevigata, reticulata, or both.
So far, blister galls have been reported only rarely from the southeastern US. Observations from North Carolina (2) and Tennessee show galls on Celtis occidentalis but lacking the distinct raised circle of celtidisvesicula and with a size and shape more similar to laevovesicula. These may be related to a fourth proposed species, listed as "? Blister gall with upper central spine" by Yang 1995, noted on Celtis laevigata and tenuifolia in VA and LA, but they apparently lack the central spine, so this may be a regional variant of P celtidisvesicula or a different undescribed species.
