[creeper]

I think its moles or voles. I can't remember which one digs tunnels feeding on grubs and worms and the other digs tunnels and feeds on bulbs and plant roots. Both are a mouse like creature. You will see mounding of displaced earth every so often along their tunnel line.

 

[user1007]

I think its moles or voles. I can't remember which one digs tunnels feeding on grubs and worms and the other digs tunnels and feeds on bulbs and plant roots. Both are a mouse like creature. You will see mounding of displaced earth every so often along their tunnel line.

My guess too and the size of the portals you describe would be consistent with them.

I haven't seen a chipmunk since I left the Pacific Coast and don't think I ever saw one in my yards. I don't remember them as being critters that borrowed to go after roots, bulbs and things. Of course in Central Illinois the overpopulated and dumb as rocks squirrels would dig up anything for no particular reason. Groundhogs were a problem too and they would certainly build an underground system 5 feet in diameter in a heartbeat. They wouldn't fit through the portals you mention though.

 

[whammytap]

I read an article in which a woman said she wraps her bulbs loosely in 1/4" hardware cloth before planting them. The bulbs grow through the hardware cloth as a volunteer tree will grow through a chain link fence, and it keeps the critters from eating the bulbs. I haven't tested this, but it sounds like a solid idea. I'd recommend using stainless hardware cloth, since it's going to be kept in a damp place (underground).