>I worked with them for many years, and never saw any benefit.

I have not noticed any benefits with mite control either.  They do allow the bees to draw in a larger, cooler mass of air through the hive on hot days.  One possible negative: hives on screened boards, tend to have the broodnest center substantially higher.  I personally prefer for the queens to lay eggs as low as possible.

Waldemar


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