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Photo#277492
Hackberry Petiole Gall Psyllid - Pachypsylla venusta

Hackberry Petiole Gall Psyllid - Pachypsylla venusta
Stephens State Forest, Lucas County, Iowa, USA
May 18, 2009
Size: ~1 cm diameter
Gall on terminal bud of hackberry (Celtis occidentalis) twig in upland oak-hickory forest. (Orange spots are splatters of paint from tree harvest marking, so disregard that.)

Possibly...
...an atypical gall of Pachypsylla venusta that consumed the whole leaf. A typical gall is visible in the upper right corner of this image:

Did you see any like that?

 
Good eye!
I saw only this one that I photographed, and it looks like an older, weathered specimen compared to the one in the photo link you provided. So it is not a bud gall, but rather a leaf gall? With the leaf blade missing or consumed?

 
I think so
Now that I look at it again, it appears that there was a leaf attached (upper right) that has broken off. I think these galls would normally fall to the ground with the leaf, but apparently this one formed far enough down on the petiole that it was fused with the twig. But yes, Pachypsylla venusta galls are usually petiole swellings, with a mostly unmodified leaf blade protruding.