For those of you that don't know, Maricopa County, Arizona is the county that Phoenix is located in. Pardon my speculation but it is likely that Pogonomyrmex maricopa was discovered in Maricopa County, Arizona and would thus make Maricopa County the type locality.
One more thing, regarding the signage, it is obvious to most ZooChatters, but the species name maricopa should be lower case. Maybe I should point that out to relevant staff...
@UngulateNerd92 the type locality was Alamogordo in New Mexico. The name maricopa is probably given for the Maricopa tribe. The author (Wheeler) didn't give an etymology in his paper describing maricopa but it is probably related to another of the genus named P. comanche - although he described it as a subspecies of P. californicus Wheeler also postulated that P. comanche was "the parent species" of maricopa.