the main purpose of a entrance reducer is to help the bee's protect their hive from being robbed out by other hives or bumble bee's, wasp, yellow jackets, ect. . the smaller the amount of bee's the smaller the entrance should be, as the hive grows the entrance should be enlarged, once the hive entrance is causing a traffic jam then it should be removed, by that time the hive should have enough population to take care of itself without being robbed out, but like understudy said above you can put in on in the winter to help the hive stay warmer.