Dave,
The big issue now is low sperm count due to use of Fluvalinate & in some
cases coumaphos. With even a single use comb contamination happens. Both
have now been shown to cause low sperm count in drones.
You need to use clean comb for rearing drones and increase the number of
drone colonies around mating areas if you suspect low sperm count to be a
problem.
Most queen breeders keep two full drone combs per hive. Many have went to
the new green colored plastic drone comb which is easily seen in the hive.
Also always keep those combs to the very outside of the brood nest because
of varroa.
Bob

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