You're confusing two different organizations.

Stephen M. Routh didn't serve under Wirt Adams, but joined the "Natchez Cavalry." Raised by Capt. Will T. Martin at Natchez MS, this company became Co. "A", Jeff Davis Legion. Routh's enrollment date is reported as May 18, 1861. This should explain why he went to Virginia rather than waiting on Wirt Adams to organize a regiment in Mississippi. Serving as courier to J.E.B. Stuart from April 12, 1862, Routh received a commission as 1st Lieutenant in the Signal Corps on Oct. 8, 1862. He then reported as ordered to General Richard Taylor in Louisiana.