What is "pachypsylla"

Pachypsylla is a genus of psyllids. Each of its four species lay eggs on the leaves of the Celtis occidentalis tree. Upon hatching, the young psyllids become encased in a "gall" which the young leaf parts grow in response to the infestation. Species of Pachypsylla include Pachypsylla celtidisgemma (hackberry bud gall maker), Pachypsylla celtidismamma (hackberry nipplegall maker), Pachypsylla celtidisvesiculum (hackberry blistergall psyllid) and Pachypsylla venusta (petiolegall psyllid).