One might best use propolis traps to gather propolis, to keep it clean.
However, the bees will only plug the slots in a propolis trap when they
want them plugged.  I was told that light above the trap would induce them
to propolize, but my outer covers are very translucent and no propolis has
been deposited. This year I'm  using propolis traps as inner covers to make
it easier for the bees to evaporate their nectar, and they deposit not a
milligram of propolis in the trap slots, but only around the edges where it
sits on the box.  I expect that when it turns cooler, the bees will adjust
their ventilation by plugging slots with propolis.  Later, I'll use a
propolis trap under a comb honey super as a shavings box for winter.  The
bees are not apt to comply with the insertion of a propolis trap by
producing propolis unless they want it plugged.

Two colonies that sit side by side and have always produced equally are now
producing unequally.  The one with a propolis trap for an inner cover is
making more honey than the one with a regular inner cover.  Less bees are
hanging out in hot weather with the greater ventilation, and the bees seem
very content.  Not a large test yet, but indicative.

Bill Morong