I've had a long drawn out debate with someone and we can't agree when Wormtail switches sides.
The wiki says:
After graduating from Hogwarts, Pettigrew and his friends joined the Order of the Phoenix and fought against Lord Voldemort and the Death Eaters during the First Wizarding War. However, at some point, Pettigrew began working as a double agent for Voldemort, having come to the belief that nothing could be gained from opposing him. Despite being considered a low-ranking soldier for his coward.
I know that he betrayed Lily and James on the night they died but that doesn't say when he starts working for Voldemort.