America’s hottest singing hottie TAYLOR SWIFT went all green-eyed and bared her girlie-fangs when she spotted JULIA ROBERTS’ cute niece EMMA ROBERTS making goo-goo eyes at her “Twilight” werewolf boyfriend TAYLOR LAUTNER – who’s co-starring with both girls on the new movie “Valentine’s Day” – and swiftly yanked Wolf Boy’s chain, bringing him sharply to heel.

“Everyone on the set could tell Emma was crushing on Taylor’s teenage toyboy – she kept making goo-goo eyes whenever she saw him, even though they have no scenes together,” said an insider.

Then wolf-stalker Emma pushed too hard, unexpectedly showing up on-set to peeping-tom a love scene between Taylor & Taylor (known on-set as “Swifty” and “Taylor” to avoid confusion).

“Swifty got really creeped out because Emma kept staring at Taylor, so just to show her that the race really does go to the swift, she suddenly kissed her boyfriend like he was the last guy on Earth,” said the source.

“Her kiss was so long and so passionate that everyone held their breath, knowing it was an in-your-face warning to Emma that she’d better back off fast.”

No question Emma got the meee-ow message: She’s now steering clear of Lautner – who also got the message when his ladylove purred: “You cheat on me, and I’ll write a very nasty song about you!”

(Dude, beware – ’memba that mean ditty she penned when JOE JONAS dumped her?)