A storming first half from Wasps helped them to a 24-20 LV= Cup win over London Irish at the Madejski on Sunday.
Wasps, who took this match seriously by playing a close-to-full-strength team, were 18-10 up at the break thanks to the touchdowns by two of their back three.
Walder had kicked a conversion and two penalties by then and he added another three-pointer after the interval to steer the visitors home.
Peter Hewat went over for a brace of tries for Irish, one either side of half-time, but they could not recover despite late pressure.
The other good news for Wasps was the successful return of England lock Simon Shaw, who played the whole match in his first senior start since the Lions tour this summer.
The 36-year-old, who has missed the start of the season with a metatarsal injury, was hoping to prove his fitness ahead of England's remaining two autumn Tests - against Argentina and New Zealand - and although he looked rusty at times, he held up well to give national team boss Martin Johnson food for thought.
Shaw was yellow-carded late on, however, to blot his copybook.
The fare here was far removed from the attritional stuff that has been served up in the Guinness Premiership this season.
Both sides put some width to their play early on, and it resulted in two smart tries in the opening eight minutes.
The first, scored by Lemi, owed much to the lightning break of fly-half Walder through the Irish midfield. Walder had Lemi on his shoulder, and the nippy Samoan winger scurried over in the left corner.
Irish replied with an impressive try of their own, Richard Thorpe smashing through Wasps' defensive line before the ball was eventually recycled out for Hewat to ground on the right wing.
Hewat slotted over the conversion but missed a relatively simple penalty attempt soon after.
The contest was easy on the eye but fussy South African referee Jonathan Kaplan, who will officiate Ireland's Test against Australia next weekend, was doing his best to make it a stop-start affair. Walder made the most of it, landing two successive penalties, to sandwich one booted by Hewat as Wasps built an 11-10 lead.
Hewat went on to miss two kicks at goal in the space of three minutes, both from around 30 yards, and the visitors made him pay with their second try of the afternoon.
Its origins were not too dissimilar to the first, Jacobs this time piercing the midfield and providing an inside pass to former England full-back Van Gisbergen, who finished the move off for a score converted by Walder.
The 18-10 half-time score was slightly harsh on Irish, who were the dominant force up front despite having, on paper at least, a weaker team out on the field.
Wasps, though, could have furthered their lead had they not butchered an overlap in the 48th minute, Lachlan Mitchell sending his pass outside behind fellow centre Jacobs.
With their first slice of possession in Wasps' 22, the Exiles proved clinical as Tom Homer slipped in Hewat with a short pass that looked slightly forward. The Australian was not complaining as he crashed over under the sticks and Jamie Lennard, who took over kicking duties in the second half, converted.
Walder and Lennard exchanged penalties around the hour mark to leave it a one-point ball game, and Irish brought on veteran back Mike Catt for the final quarter.
However, the former England centre could not inspire a turnaround and Lemi's drop goal sealed the win for Wasps.
Shaw, for a cynical knock-on, and Joe Simpson, for tackling a man without the ball, were sent to the sin-bin late on but Wasps held on.
For London Irish:
Tries: Hewat 2
Cons: Hewat, Lennard
Pens: Hewat, Lennard
Tries: Van Gisbergen, Lemi
Pens: Walder 3
Drop goal: Lemi
London Irish: 15 Jamie Lennard, 14 Peter Hewat, 13 Tom Homer, 12 Charlie Gower, 11 John Rudd, 10 Chris Malone, 9 Peter Richards (c), 8 Richard Thorpe, 7 Jonathon Fisher, 6 Kieran Roche, 5 Gary Johnson, 4 Andy Perry, 3 Faan Rautenbach, 2 David Paice, 1 Dan Murphy.
Replacements: 16 James Buckland 17 Alex Corbisiero; 18 Max Lahiff; 19 Matt Garvey; 20 Steffon Armitage 21 Marcus Watson 22 Mike Catt 23 Alfredo Lalanne.
London Wasps: 15 Mark Van Gisbergen, 14 Tom Varndell, 13 Lachlan Mitchell, 12 Ben Jacobs, 11 David Lemi, 10 Dave Walder, 9 Warren Fury, 8 Hugo Ellis, 7 Joe Worsley, 6 John Hart, 5 Simon Shaw, 4 George Skivington (c), 3 Bob Baker, 2 Joe Ward, 1 Charlie Beech.
Replacements: 16 Chris Whitehead, 17 Gabriel Bocca, 18 Ben Broster, 19 Marty Veale, 20 Will Matthews, 21Joe Simpson, 22 Eoghan Hickey, 23 Mark Odejobi.
Referee: Jonathan Kaplan (South Africa)
Assistant referees: Bruce Robertson (England), Roger Baileff (England)
Television match official: Brian Abrahams (England)