Based on that description, the full-size image labeled carolinensis here looks more like titillator to me.

The elytral apices feature is small and hard to see on most photos. For example, the images here and here may not be large/clear enough to determine the species with any certainty. Do you agree? Are there any other ways (such as host/habitat or geographical distribution) to separate these two species?

Species IDs in the photos you link look good to me. The elytral apices character seems ambiguous but is fairly distinctive in reality - mostly a matter of seeing enough specimens.

The two species occupy the same habitats/hosts throughout the eastern U.S., although titillator tends to be more prevalent in the southern states.

how Joanne Shuman's photo can be identified by two people as M. titillator and yet the full-size image of my specimen (which shows a larger and directly overhead view) is not identifiable to species.

I don't have Yanega's book so I don't know what the "elytral spines" are in Ethan's comment on Joanne's photo but I notice that her photo and mine both have an apical tuft of hairs on each side of the midline. Are these the "elytral spines" Ethan mentioned? I lightened Patrick's photo and Troy's photo on my hard drive, and neither of those photos have the paired apical tufts. Are the tufts a female characteristic? Are they significant?

Your photo lacks sufficient clarity to see the elytral apices. The elytral "spines" are at the end of the elytral suture (where the elytra meet) - they are more distinct in titillator than carolinensis and, combined with the characters I mentioned earlier, easily separate the two species. I suspect yours is titillator, but without a clear view of the elytral apices it's not possible to be certain.

Thanks for clearing that up. Now that I know what elytral spines are, they can be seen in the cropped enlargement I've added here (they're lying side-by-side and resemble the tips of pointed stakes). Are they distinct or prominent enough to be representative of M. titillator?

The paired hair tufts in the photo appear to be part of the abdomen protruding beyond the tips of the elytra. Their presence/absence is not important for identification, judging from your comments.

I suspect it is M. titillator, but a still clearer view of the elytral spines is needed to be certain. You are correct in that the hair tufts are not diagnostic.

Your suspicion of M. titillator, combined with the observable differences between my close-up and Troy's full-size image, is good enough for me.