Wasps love grapes. When the fruits ripen in the summer, the bugs chow down and fly back to their nests, where they give the masticated mush to their larvae.
When they eat the fruit, the insects ingest the yeast, and their stomachs provide the perfect environment for it to survive the frosty months. More importantly, when wasps feed their young, they pass the fungus to their babies. That way, when the larvae mature, they reintroduce S. cerevisiae to the vineyards and start the process all over again.