I know that ERISA does not provide a statue of limitations for suing to recover delinquent employer contributions.

Does a state's contract SOL apply automatically?
Does a federal SOL govern?
How is the determination made?

You choose the "most applicable" state limitations period. I think in most Cirucits you are looking at the state's written contract limitations period.

However, watch out for the Third, I think applicable precedent may still mandate loking at wage payment collection statutes which are usually very short