So I have battled this for the past several years in a couple of my out building near farm fields. I have experimented with basically all of it.

I have found the best system to be the PVC home made bait stations placed around the exterior of the property, coupled with bait stations and regular mouse traps in the building to be the most effective.

The outside stations take care of them before they get in the building. I rarely catch one now or see evidence of them eating the bait inside. It is in my opinion a year round deal. Keep the bait stations outside full and try to keep the population down, even in the summer months. YMMV.