Bob Darrell wrote
Last fall(Sept 16) I put propolis mats on 2 hives to collect some for a 
customer.  I had never used the mats before.  When I took them off 2 
weeks later, they were full of wax.  If there was propolis, there wasn't 
much.  I will try it again this spring.

Might I suggest that Fall(Sept 16) is too late, My bees (middle of UK) 
start collecting propolis in July and by the end of August all hatches 
are battened down.
They also collect propolis in March and April and where available from 
Balsam Poplar, they also collect pollen from the catkins. When the brood 
nest is full they seem to stop collecting!
Nothing scientific just 30 years of looking.
Regards Andy Johnston

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