Scarlets fly-half Rhys Priestland tests the Treviso defence
The Scarlets kept themselves in the mix for qualification from Pool Five with a bruising Heineken Cup win at Treviso.
Dominic Day struck first for the Welsh from a ruck, while tries from Dan Newton and Gareth Maule underlined the visitors' slicker back play.
Jonny Fa'amatuainu sealed a perfect afternoon as the replacement lock went over to seal a bonus point.
Treviso's powerful mauls often troubled the Scarlets and earned tries for Robert Barbieri and Marco Filippucci.
The Scarlets were put under immediate pressure from the kick-off as fly-half Rhys Priestland dawdled and had his attempted clearance kick charged down.
Tavis Knoyle was able to gather the loose ball but was forced to hang on in the tackle and Treviso fly-half De Waal kicked the resulting penalty.
Scarlets regathered themselves after that early set-back and began to boss possession, with a couple of charging runs from wing Sean Lamont catching the eye.
There was a scare for the visitors when Joe Maddock intercepted a pass from Knoyle and a breakaway try seemed on.
But the overeager Treviso wing, a recruit from Bath, had strayed offside and instead Priestland was able to kick Scarlets level.
The Welsh attacked the Treviso line but the Italians were organised in defence and kept each foray at bay, until Edoardo Gori was too slow rolling away after a tackle and conceded a second penalty for Priestland to slot.
Morgan Stoddart then broke through, putting a grubber in behind the defensive line only to be thwarted by a fine piece of covering play from Gori as the scrum-half made amends for his earlier indiscipline.
But the pressure finally told as the Scarlets pack put together a series of pick and drives, with lock Day finally going over for a converted try after Josh Turnbull had been stopped just short.
Treviso came back strongly - with Brendan Williams to the fore after replacing Luke McLean at full-back - but spoiled their efforts with knock-ons and misplaced passes.
With half-time looming the Scarlets grabbed a second try thanks to Stoddart, who cut between Tommaso Benvenuti and Alberto Sgarbi to send Newton clear, although this time Priestland missed the conversion.
But Treviso hit back just before the break with an impressive forward effort that rumbled a maul fully 20 metres to the Scarlets try line for Barbieri to score, with De Waal's conversion reducing the deficit to 18-10 at the break.
That was far less than the 22-9 lead the Scarlets had let slip at the Stadio di Monigo in September, when Magners League newcomers Treviso introduced themselves by coming back to beat the Welsh side 34-28.
So the warning was there for the Scarlets that they could not afford to slacken if they hope to qualify ahead of Leicester and Perpignan from Pool 5.
Priestland suffered a knock soon after the restart but although the fly-half stayed on the field he had to temporarily hand over the kicking duties to Newton, who comfortably slotted the penalty chance that came his way.
Treviso almost grabbed a second try when Williams raced away down the right wing, only to be dragged down just short as Scarlets forwards Ken Owens and Day got across just in time.
The forward power of the Italians continued to cause trouble for the Scarlets, who were forced to drag down another maul before it built up a head of steam.
Treviso opted to go for the attacking line-out rather than kick at goal and the Scarlets again infringed, as lock Vernon Cooper was sin-binned for taking out a lifting player.
The Italians again went for the five-metre line-out and this time got their rewards as the forwards splintered the Welsh defenders to pop replacement back-row Filippucci over.
De Waal missed the conversion but Treviso now trailed just 15-21 with 26 minutes remaining.
Despite being down to 14 men the Scarlets hit back swiftly as replacement scrum-half Martin Roberts jagged across field to pick out Maule.
The centre straightened the line beautifully to stretch over, although the try needed confirmation from the video referee before JP Doyle could award it.
Priestland proved he had recovered from his earlier injury by kicking the tricky conversion into the glare of the setting sun and then adding a penalty to stretch the lead to 31-15.
With the win seemingly wrapped up, the Scarlets had eight minutes to find a fourth try that would add a bonus point to top off the afternoon.
Ben Morgan was held up over the line, scrum-half Roberts also went close with a dart from close range, but finally replacement lock Fa'amatuainu crashed over to complete the Scarlets' joy.
Benetton Treviso: L McLean; J Maddock, T Benvenuti, A Sgarbi, A Vilk; W de Waal, E Gori; I Fernandez Rouyet, E Ceccato, L Cittadini, A Pavanello (capt), C van Zyl, R Barbieri, A Zanni, M Vosawai. Replacements: Williams for McLean (29), Semenzato for Gori (77), Muccignat for Rouyet (77), Sbaraglini for Ceccato (32), Filippucci for Cittadini (50), Bernabo for Zanni (77), Di Santo for Vosawai (50). Not Used: Burton.
Scarlets: Dan Newton; Morgan Stoddart, Gareth Maule, Regan King, Sean Lamont; Rhys Priestland, Tavis Knoyle; Iestyn Thomas, Ken Owens, Simon Gardiner, Vernon Cooper, Dominic Day, Josh Turnbull, Johnathan Edwards (capt), Ben Morgan. Replacements: Emyr Phillips for Gardiner (53), Rhodri Jones I. Thomas (65), Phil John for Owens (53), Jonny Fa'amatuainu for Cooper (63), Rob McCusker, Martin Roberts for Knoyle (53), Scott Williams for Priestland (77), Lee Williams for Newton (60).
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