This species is the most commonly observed cynipid gall in the eastern US, and readily recognized by its size, placement, and spongy interior. There is substantial taxonomic confusion in the distinction between A confluenta and A quercusspongifica. Osten Sacken described three species--aciculatus, spongifica, and coccinea--which he distinguished based on host species, gall morphology, and emergence timing. Walsh subsequently conducted a large-scale rearing program of galls from Quercus velutina at a single site in Illinois, all fresh in May. He found that the galls differed morphologically between two endpoints, and that the emergence timings clustered clearly into two periods: sexgen adults in June, agamic females from September to November, with some emerging in the subsequent spring. However, he observed that both gall morphologies were found in both emergence periods, and noted a range of intermediate forms suggesting there was no consistent distinction to be drawn between them. He concluded that they were alternating generations of the same species, of which the agamic females in the fall laid unfertilized male eggs and the mated sexgen females in the summer laid female eggs, both of which hatched in the subsequent spring.
Walsh later apparently conducted experiments that he and Beutenmuller took as definitive confirmation of this hypothesis, which he published, synonymizing quercusspongifica as the sexgen form of Amphibolips confluenta. His experiments involved placing agamic females reared in October onto trees he understood to be isolated and not previously host to Amphibolips. While his host trees were not isolated from oviposition by other wasps, if his assumption that no other oviposition is remotely likely is accepted, Walsh's results show that oviposition by agamic wasps in the fall produces identical galls the following spring from which emerge both sexgen females in June and agamic females in the fall, and that these agamic females in turn lay eggs that produce sexgen females in the following spring. It is noteworthy that Walsh's experiments produced no males even from the sexgen galls, though this may be a coincidence given the small numbers of wasps he successfully reared. Further, Walsh was never apparently able to record the results of oviposition by sexgen females (perhaps due to this lack of males?).
Kinsey considered Walsh's research inadequate because no oviposition was documented, and Weld treated spongifica and confluenta as distinct species pending "further study" but noted they are "probably" synonyms.
The hypothesis Walsh put forward is highly unusual relative to every closed life cycle published since. There are several known cases in which sexual generations become facultative or discarded entirely, with agamic females emerge from agamic galls and laying eggs that become agamic females inducing agamic galls. However, in all such documented cases, the sexual generation gall is very different from the agamic gall, sometimes even occurring on a different host section. In Walsh's hypothesis, the sexgen and agamic galls are indistinguishable in host, phenology, and appearance.
Additionally, by analogy with related species, it is surprising that the sexgen females emerging in June would oviposit in tissues during the growing season but not take advantage of the plant's growth to create a new gall until the following spring. Amphibolips quercusinanis, for instance, is a sexual generation gall similar to quercusspongifica that emerges and oviposits in June. Unconfirmed genetic evidence suggests that the eggs it lays immediately cause the subsequent growth of galls of Amphibolips cookii, which produce agamic females in the fall which oviposit in buds before winter. Assuming that Walsh's evidence is true to the extent he was able to confirm, it is conceivable that this species produces another, distinct agamic gall in the fall.
On red-group oaks, A confluenta/quercusspongifica is commonly confused with A quercusinanis, which also occurs on leaves in the spring but which has a uniformly smooth exterior with distinct spots, A quercusostensackenii, which is much smaller and doesn't distort the host leaf, and A cookii, which occurs on buds in the fall and has larger spots and thicker walls. A confluenta typically has a bumpy surface. The most definitive way to distinguish it is to note the spongy interior; A quercusinanis and A quercusostensackenii has a stringy, empty interior, while A cookii has thick, succulent fibers.
Some specimens that otherwise resemble A confluenta do have spots. This may represent a different species but is more likely a host- or generation-related variation (these spotted variants are typically but not definitively or exclusively associated with Quercus buckleyi, shumardii, pagoda, and velutina).
A confluenta/quercusspongifica early in the spring can also be confused with Dryocosmus quercuspalustris, which also forms green, spherical, integral leaf galls that distort leaves. The galls are easily differentiated by their internal structure, which can be observed by shining a bright light through the gall or but cutting it open. D quercuspalustris has a loose larval cell freely rolling around in an otherwise empty chamber. D quercuspalustris galls are much smaller and differ subtly but distinctly in texture. A confluenta/quercusspongifica galls almost always completely deform their host leaves, to the extent that they are often barely visible, while D quercuspalustris galls may seem to almost completely displace the leaf (especially early in leaf out) or have minimal effect on its overall structure (later in the season).
