I'm not sure which species this is, but you may actually have it correct. The genus Olycella is usually treated as a synonym of Melitara, but not always. They are also closely related to South American Cactoblastis, which have orange to red larvae instead of blue. I have not learned to tell the species apart yet, but there aren't many, and they are probably not difficult, once you know them. Location helps, because some regions only have one species, but I think there are at least a few in that part of Texas. I did find an interesting web page discussing the genus here.