Disholcaspis quercusglobulus is most commonly confused with Disholcaspis quercusmamma. The simplest way to tell them apart is by the identity of their host plant. However, when host ID is uncertain, they can also be fairly reliably distinguished by morphology (and even used to ID their host oak).
Disholcaspis quercusglobulus is almost always found at the nodes of Quercus alba twigs, typically in groups of 1-4 arranged around the circumference of the node. It has a smooth, hairless texture often roughened by raised gooseflesh points, becoming minute splits as the gall matures. Its shape is typically almost perfectly spherical except for its attachment to the twig, but nipple-like points do occasionally occur, so this trait is not definitive. It occasionally softens or wrinkles, especially in galls that don't develop fully, but more typically holds a firm spherical shape after wasps have exited. Infrequently, galls have been seen to fuse with adjacent galls.
Disholcaspis quercusmamma, conversely, is often found in groups of a dozen or more arranged along the length of Q bicolor or more often Q macrocarpa twigs, often dense enough to deform adjacent galls. The distinctive ridges of Q macrocarpa twigs surrounding these galls are often the easiest way to identify the species. Its texture is velvety when young and leathery when old, often rough but never showing the minute splits found on Disholcaspis quercusglobulus. Galls typically but not universally have a pronounced nipple. Many clusters of galls, especially early in the season, include small undeveloped galls that are more conical than globular, something never seen with D quercusglobulus. After wasps have exited galls, frequently wrinkle or collapse to varying degrees.
On Quercus alba, Disholcaspis quercusglobulus can be confused with Disholcaspis globosa. Weld lists this species as a "root" gall, and it is typically found in clusters where young trees meet the surface of soil, often concealed by leaf litter. But it is apparently found on nodes or along the stems of young sprouts and occasionally at the base of lateral branches of mature trees as well, where it might be confused with Disholcaspis quercusglobulus. Aside from its location, it can be distinguished by its deep red color (though both species are yellow/pink when immature) and more haphazard distribution around stem nodes in larger numbers than is typical for Disholcaspis quercusglobulus. Disholcaspis globosa also seems to be more velvety in texture and lack the raised points/splits of Disholcaspis quercusglobulus.
Quercus austrina added based on unpublished specimens COI barcoded by the Forbes lab.
