If you started the new hive a month ago...I would assume they are now on drawn comb (or have drawn the foundation installed on) and they should be doing well enough by now to defend their entrance. If so, and the bees are building up and getting strong, I would recommend removing the entrance reducer or certainly at least opening it up to the next larger opening. If by last month, you mean just last week, then you should probably wait a bit yet.
"My child, eat honey, for it is good." (Proverbs 24:13)