Jillian Michaels -- 'I Guess I Need Cara to Train Me' ... 'Biggest Loser' Star Hits Back At Co-Star After Triathlon Diss

More smack talk in the war between two former "Biggest Loser" co-stars -- this time Jillian Michaels is firing back at Cara Castronuova with some good ol' fashioned venom-laced sarcasm.

We broke the story ... Cara -- who was a trainer on the show -- ripped Jillian apart over Michaels' poor showing at the Malibu triathlon earlier this month, calling her an embarrassment.

This weekend, Michaels returned to Malibu -- where she offered up some brand new excuses for her abysmal showing (bad wet suit, bike problems, a horse?) and vowed to do better in the race next year.

Then, it was open season on Cara.