The Scarlets moved top of the league with a bonus-point win at Netherdale.
Borders (8) 8
Scarlets (12) 41
Tries: Evans, Taylor (2), Watkins, Finau, Thiel
Cons: Bowen (4)
The Llanelli side stunned the Scots with tries in the first six minutes from Garan Evans and Matthew Watkins.
Borders rallied before half-time, Tanner Vili and Gary Armstrong going close before Dodd Weir squeezed over for a short-range try.
The Welsh were merciless in the second period, though, shutting out the Scots and scoring tries from Mark Taylor (2), Salesi Finau and Jon Thiel.
Evans opened the scoring with his touchdown in the right-hand corner,
before Watkins also took advantage of the hesitant Borders defence.
The home side worked their way back into the game through Weir's try and a Vili penalty, and the game's flow was disrupted by two sin-binnings.
Borders skipper Andy Rennick was first to depart for deliberate obstruction, then Scarlets wing Salesi Finau was cautioned for throwing a punch.
But the home side's hopes were killed off by another two-try burst just after the restart.
First, Mark Taylor glided past his markers for a simple score converted by Gareth
Bowen, who also added a penalty.
Finau made sure of the bonus point after ripping the ball from Vili's grasp,
before the referee had to bring out his card again, Iestyn Thomas and Semo Sititi dismissed for their part
in a mass bust-up.
Taylor and Jon Thiel bagged late touchdowns for the Scarlets, with Bowen adding
both goals to round off the rout.
Borders: T Vili, C Murray, G Morton, J Stuart, C Moir, C Laidlaw, G Armstrong, C Dunlea, S Scott, B Douglas, D Weir, C Stewart, S MacLeod, A Rennick (capt), S Sititi.
Replacements: R Ford, G Cross, A Davidson, J Dalziel, C Cusiter, I Fairley, I Bethinnusen.
Scarlets: B Davies, G Evans, M Taylor, M Watkins, S Finau, G Bowen, D Peel, I Thomas, R McBryde, J Davies, V Cooper (capt), C Wyatt, D Jones, S Easterby, S Quinnell.
Replacements: J Thiel, A Gravelle, A Jones, D Hodges, M Phillips, R Rees, T Selley.
Referee: Marshall Kilgore (Ireland).