Cassius Blue (Leptotes cassius) 
The published range of this species does not include Louisiana, but it does include part of the upper Texas coast and Florida. Butterflies and Moths of North America (BAMONA) lists this species as having been recorded in Saint Helena Parish, but with no further information.  The photos below were taken along the Rio Grande River in Hidalgo County.
Cassius Blue
Frontera, Hidalgo County, TX, November 2, 2013
Cassius Blue
Frontera, Hidalgo County, TX, November 2, 2013