Quote Originally Posted by abou
Indeed. Anytime you see "Ala", it means auxiliary even though the literal translation is "wing". By the time of the Imperial era (and possibly Marian) the change in economy and recruitment method meant that anyone who was wealthy enough to own a horse and the land wouldn't serve in the military.
More likely beginning of the Marian era. I wouldn't be surprised that one of the reasons they stopped recruiting citizen cavalry was because their ability to recruit had all but disappeared. Those types would only serve if they were interested in a future in the Senate, in which case they'd try to get in as a contubernales or tribune.