Pachypsylla is a genus of psyllids. Each of its four species lay eggs on the leaves of the Celtis occidentalis

Hackberry

Celtis is a genus of about 60-70 species of deciduous trees widespread in warm temperate regions of the Northern Hemisphere, in southern Europe, southern and eastern Asia, and southern and central North America, south to central Africa, and northern and central South America...

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).