Yesterday Cheryl Harleston of Yelapa, Mexico showed me these photos she had recently taken, and asked if I had any thoughts about them. She called the subject a “corral made of grains of sand.”
My immediate reaction was “I’ve never seen anything like it, but I love it!” She provided a few more details: “The size of the circle was 6 mm on the outside… There were several of these on several Xanthosoma leaves. I’ll be posting more photos later, as I went to see them today, and found no signs of the larva or the ‘beads’… Only dark rust-like circles where they had been…”
Xanthosoma is a member of the arum family (Araceae), and is one of several similar-looking plants known as “elephant ears” (Alocasia and Colocasia being other examples, which, I’m obliged to mention, have species of Marmara that mine their leaves).
As I waited to see the additional photos, I wondered, “Did each circle have a larva? And was there always feeding sign within the circle?” I looked at the photos a little more closely, and then asked, “Are you sure there was sand there, or could these all be beads of milky latex (does Xanthosoma have sticky sap)? If so, this could be a trick the larva uses to cut off the flow of sap so it can eat the leaf tissue within the ring. Some insects that feed on milkweed have similar strategies, though I don’t know of any that make a ring like this.”
Thomas Eisner’s book For Love of Insects has great photos of a red milkweed beetle (Cerambycidae: Tetraopes tetrophthalmus) biting into the midrib of a milkweed leaf to stop the flow of sticky latex, then eating the portion of the leaf beyond this without any problem. This feeding sign is a very common sight in milkweed patches.
There is a photo in my book of a milkweed weevil doing something similar, but in the middle of the leaf. Anyway, Cheryl responded: “There is sand around, but I’m not sure that’s what it was…”
Her additional photos included two more views of the original ring (the second of these is backlit)…
…as well as these other examples:
It sure looks to me like my hypothesis was correct! I believe there is no foreign material here, just little beads of sap forming where the caterpillar made little nibbles in the leaf surface. When the caterpillar had made enough of these nibbles to bleed out the disc of leaf tissue within the ring, it then got to have a meal. Seems like it would be easier just to find a different host plant!
As for what kind of moth this larva will turn into, I have no idea. Anyone else?
BugTracks 
Amazing. The bits look like tiny shells. Guess you need a video trained long time to solve the mystery but I love your photos and commentary.
Fascinating!
I came to the same conclusion before reading yours: the ‘grains of sand’ are really drops of fluid – obvious where one has a lot of concave parts – and as they dry up, they get smaller. Yes it does remind of milkweed sap walls that can be seen close to very young Danaus cats. Great story!
Coincidence: I was reading Publication E-100 “Squash Beetles on Cucurbits” from Purdue University shortly after reading your post. Figure 8 on Page 3 shows a circular trench snipped out by a squash beetle larva to isolate its feeding area.
Interesting! I’ll add a link to that publication here.
This is ridiculously cool!
Very nice, very interesting, thank you; I don’t live near you folks, but I agree with your ideas on what it is about.