The entrance reducer allows you to control the size of the entrance to the hive. Put it in place between when you unwrap your hive and the 1st of June, and again between the 1st of October and the time you wrap your hive for winter. You can also use it if your hive has swarmed or is recovering.
The smallest possible opening is great when times are cooler, or when the colony is small or weak and incapable of protecting itself against intruders. The medium-sized opening is great when times are getting warmer and the colony is growing.
Important: Always remove the entrance reducer when using formic or oxalic acid treatments.