Roger Sonnenberg questions whether oil/sugar patties can or should be left
on during the flow or taken off.
Roger, Sue Coby at Ohio State did some work a couple of years ago and
compared the effects of tracheal mite control with just spring and fall
oil/sugar patties and continual treatment.  Her conclusion was that just
spring and fall treatment was not at all effective, but very good results
were obtained with continual treatment.
I think the general conclusion is that continual treatment does not
adversely contaminate, as both products (oil and sugar) are foods we consume
every day.  I personally use continual treatment.
That said, I have observed (as have others) that the bees store the
granulated sugar in cells and cap it with liquid honey.  If you extracting
honey, I suggest you look carefully to see if sugar crystals are in your
jars.  This might be a particular danger if you extract frames from the
brood nest.  Presumably, they will either be strained/filtered out or
liquefied by heating, but I suggest you be certain.  I personally raise only
comb honey and have never observed any sugar crystals in the combs.
Lloyd
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Owner, Ross Rounds  the finest in comb honey production.