PITTSBURGH --This what the Pittsburgh Pirates were anticipating when they traded for Detroit Tigers closer Joakim Soria last week -- a three-heading bullpen monster to close out a win.

Soria, a right-hander, pitched a scoreless seventh then left-hander Tony Watson worked a scoreless eighth before closer Mark Melancon overcame a rare hiccup to finish off the Pirates' 7-5 victory over the Chicago Cubs on Wednesday night.

Right fielder Gregory Polanco and center fielder Andrew McCutchen both homered and drove in runs in a decisive sixth inning to help the Pirates end the Cubs' season-best six-game winning streak.

"It's always been a mindset here you want your starter to get finish seven innings and then we'll figure out the rest," Pirates manager Clint Hurdle said. "Now if they can go six innings, we can figure out the rest.

"I like those guys in the bullpen. I like their pedigree. You're going to have a special pitcher out there (from the seventh inning on)."

The Cubs got within two runs in the ninth on rookie second baseman Addison Russell's run-scoring ground out, but Melancon notched his 34th save in 35 tries by striking out center fielder Dexter Fowler with a runner on second for the final out.

The Pirates (62-44) won for the 12th time in their last 15 home games at PNC Park. They lead the National League wild card race by a 3 1/2 games over the San Francisco Giants

Another Pirates reliever, left-handed reliever Antonio Bastardo (3-1), got the win by retiring the only batter he faced. He induced pinch hitter Chris Coghlan to ground out on the first pitch with the bases loaded to end the sixth and protect Pittsburgh's 6-4 lead.

"It was the biggest pitch of the night, the absolute biggest of the night," Hurdle said.

Polanco led off the bottom of the first inning with a home run off Dan Haren, making his Cubs' debut, to open the scoring, then hit an RBI-single to break a 4-4 tie in a two-run sixth inning.

McCutchen hit a solo homer in the third inning then added a sacrifice fly in the sixth to make it 6-4.

Polanco had two hits and two RBIs, McCutchen drove in two runs and catcher Francisco Cervelli and third baseman Aramis Ramirez each had two hits

The Cubs (58-48) had tied it 4-4 in the top of the sixth when rookie catcher Kyle Schwarber hit a two-run home run, his fifth and first baseman Anthony Rizzo added a solo blast, his 21st, to chase left-hander Jeff Locke.

Schwarber also doubled and Rizzo extended his hitting streak to seven games. The Cubs dropped four games behind the Pirates in the wild card standings and host the Giants in a four-game series beginning Thursday night.

The Pirates added an insurance run in the eighth when Cervelli tripled and scored on first baseman Sean Rodriguez's single.

Cubs lefty reliever Travis Wood (5-4) gave up two runs while retiring only one of the four batters to get the loss.

Both starters struggled as Locke surrendered four runs in 5 2/3 innings and Haren allowed four runs -- three earned -- in five innings in his debut with his new team after being acquired from the Miami Marlins in a trade last Friday.

Home runs continued to plague Haren as he gave up two more after allowing 21 in his 21 starts with the Marlins. He is winless in his last four starts.

He fell behind 2-0 in the first inning when Polanco connected and first baseman Pedro Alvarez hit an RBI single.

"I made a bad pitch right out of the gates and stuff got going pretty quickly there," Haren said. "After that it was alright. It's over and done with. I've got to be better next time."

Locke has a 5.48 ERA in four starts since the All-Star break after posting a 1.59 mark in his final six outings of the first half. Locke had allowed only one home run in his previous 56 innings before Schwarber and Rizzo took him deep.

NOTES: Cubs RHP Rafael Soriano was placed on the 15-day disabled list, retroactive to July 31, with right shoulder inflammation before the game and LHP Clayton Richard was re-signed as a free agent two days after being designated for assignment. ... Pirates 3B Josh Harrison (torn ulnar collateral ligament in left thumb) and SS Jordy Mercer (sprained left knee) are expected to begin rehabilitation assignments at Triple-A Indianapolis next week. Harrison is expected to start playing Aug. 11 and be joined three days later by Mercer. ... Cubs RHP Jason Hammel (6-5, 3.13 ERA) faces San Francisco RHP Chris Heston (11-5, 3.24) on Thursday night in the opener of a four-game series against the visiting Giants. ... The Pirates are off Thursday and then open a three-game home series against the Dodgers on Friday night. Pittsburgh RHP Gerrit Cole (14-5, 2.29) faces Los Angeles LHP Clayton Kershaw (9-6, 2.37), who has a streak of 37 consecutive scoreless innings, in a matchup of All-Stars.