>I have long wondered if bees exude any substances, chemicals, or acids that
might, if produced in larger quantities, repel mites.
Propolis?

Nice thought, but if propolis was the sine qua non then I (and I suspect you) would have no varroa mites at all!  Unfortunately, that is not so.  But... despite limited varroa treatment we seem to have very few mites at the moment.

This winter is going to be the acid test, as for many reasons I have not been able to remove over 50% of my crop so far and have, of course, only treated those colonies where the crop has been removed.  I suspect that we will go into winter with only 75% of the crop removed and most colonies having probably a third of their normal treatment (which was already below recommended levels).

Next spring could prove interesting...

Best wishes

Peter 
52°14'44.44"N, 1°50'35"W

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