I found a stream of ants heading towards my fermenter the morning after pitching. I had left some splashed wort on the side of the fermenter and they were hard at the ball. Quite a few scouts cruising around the rim and looking for a way into the motherlode too. I just cleaned it up with some alcohol spray.
Ants would probably need the teeniest hole to get in. They probably wouldn't like the CO2 and alcohol fumes after a day or two but just after pitching they would be in ant heaven.