One of the most valuable tools in the beekeeper's arsenal is the simple entrance reducer. If you're wondering what it is, it is exactly what the name of it suggests, it reduces the size of the hive entrance.
So when exactly should a beekeeper use an entrance reducer?
One of the most important times to use this tool is when you have a newly established colony. When you order your nuc colony and set it up in the Spring you will want to reduce the entrance until they get established and grow to a decent population size.
When a bee colony is new and trying to develop it doesn't have adequate workforce to allocate to foraging activities, comb building, brood rearing while defending a large open entrance. Keeping the entrance small will allow the bees to focus on other key growth areas without having to dedicate as many guard bees to keeping out other potential threats to the colony like hornets or wasps, or other robbing bees.
You also want to use an entrance reducer on established colonies that may be weaker in population size. Of course, you'll want to diagnose why the colony is weak to begin with but you want to give them a smaller entrance to defend as well while they work to get stronger after you have resolved whatever the problems may be in that colony.
There's an old saying that goes, "an ounce of prevention is worth a pound of cure" well in beekeeping that is definitely a true statement. Many new beekeepers often fail to anticipate when issues will arise and only take actions to try and rectify those issues after the events have begun. As you become a more advanced beekeeper you will realize there are distinct events that are more prone to happen at certain times of the year, or when performing certain jobs and you will know what to do to prevent these events before they get a chance to occur in the first place.
One of those major events is robbing. When natural resources are plentiful and the Summer is in its full glory most bees don't have issues with robbing as the other bees and insects are more interested in nature's bounty.