Scarlets showed their intent early, putting plenty of width on the ball and making early yardage before taking a third-minute lead after Treviso were pinged for coming in at the side.
Owen Williams made no mistake but Treviso, intent on taking their win count into double figures for the first time in RaboDirect PRO12 history, hit back soon after.
The Scarlets' lineout misfired inside their own half and while No.8 Robert Barbieri failed to gather the loose ball, Alessandro Zanni snaffled it, swiftly off loaded to Antonio Pavanello, who burst towards the line.
The second row couldn't quite make it to the try-line but he looked inside and found the supporting Barbieri who stormed over the line for his eighth try of the season on eight minutes.
Kristopher Burton was on target with the conversion in his final appearance before joining Newport Gwent Dragons to hand Treviso a 7-3 lead.
Back came the Scarlets however and Williams' 13th-minute penalty trimmed the gap to just one and took the young fly-half into triple figures in his first season in the RaboDirect PRO12.
Williams soon undid his good work however, tackling the man without the ball with Treviso well placed and Burton extended his side's lead from the tee on 15 minutes.
But halfway through the first half, Williams notched his third penalty of the night from more than 40 metres after Treviso were penalised at the scrum.
And eight minutes before the break, the Scarlets retook the lead when Williams was again on target after the home side were again awarded a penalty from a scrum.
Scarlets were proving their own worst enemy however and almost immediately, Samson Lee infringed and Treviso edged back ahead 13-12, Burton once more with the kick.
It was more of the same after the break and Burton extended the visitors' lead with another penalty on 42 minutes but three minutes later Treviso found themselves down to 13 men when first Valerio Bernabo and then Bees Roux were sent to the sin bin.
Scarlets endeavoured to make their advantage count but it was Treviso who were next on the scoresheet with another penalty from Burton and against all odds, the Italians extended their lead with their second try of the match.
Rhys Priestland came off the replacements bench but his first action was a knock-on and after a punt down field, Treviso worked Manoa Vosawai over in the corner, Burton adding a superb conversion for a 26-12 lead.
The Scarlets had to respond and almost did immediately after a break down the middle from Priestland but Treviso wing Christian Loamanu made a superb covering tackle to deny Andy Fenby.
Burton then missed two drop goals in quick succession but despite concerted pressure from the Scarlets, composure eluded them and the clock ticked into the closing 20 minutes and Luca Morisi caught the hosts cold, bursting through a lacklustre Scarlets defence to put the game beyond reach.
And Loamanu rubbed salt into the wounds of the Scarlets with a fine finish in the right hand corner to seal the bonus point before Treviso moved through the 40-point mark with a penalty from the halfway line after a clumsy tackle by Gareth Edwards on the try-scorer.
Scarlets' night got even worse when British & Irish Lion George North limped off injured and while Liam Williams grabbed a try right at the death for the hosts, it was far too little too late.
|Scarlets Score Card|
|Benetton Treviso Score Card|