Mengarelli scored on runs of 13 and 37 yards in the second quarter and added a 10-yard scamper in the third as the Jackrabbits (2-0) notched their fifth straight home-opening win. He led all rushers with 143 yards on 17 carries.
Zach Lujan was 19-for-27 and 314 yards passing, including two touchdowns, and Dalton Douglas tossed a 77-yard TD pass to Dallas Goedert. Jake Wieneke hauled in both of Lujan's touchdown throws and finished with 205 yards on 11 receptions.
Trailing by a touchdown late in the first quarter, South Dakota State took control with 48 unanswered points ignited by Jay Carlson's 40-yard field goal.
Ammon Olsen led Southern Utah (0-2) with 137 yards passing and a touchdown.
AP College Football website: www.collegefootball.ap.org