I'm not sure that this is the larvae of the alligator weed flea beetle. I have seen these beetle larvae furled in alligator weed leaves. I have not been able to determine what happens after the eggs are laid on the underside of the leaves.

When I saw limp leaves at the top instead of defoliated plants, and I found a hole in the nearest hollow stem, I opened them up to see what was in them. I have been looking for indications of the thrip, another bio-control agent against alligator weeds introduced at the same time as the flea beetles.

Not sure, but this was in the hollow stem of alligator weed with beetle nearby and with hole in the stem. I've taken photos of eggs, larvae, adults (is there a pupa stage for this beetle? -- if so what may be a pupa), but don't know how the beetle transitions from one stage to another. I've been checking hollow stems of alligator weeds with withered tops and which have holes in the stem. This had a withered top and a hole.