Hi:
 
I may be wrong, but I don't think you can call polyurethane non-toxic.
Maybe after it is fully cured it might qualify, assuming the bees can't
rasp it off and digest it.  But notice the warnings about harmful vapors
and application in well ventilated areas (human hazard).
 
Years ago I was conducting a literature search of carbamate insecticides
and kept finding cross-references between carbamates and urethanes.  If
memory serves me, there were some toxicity tests done with urethanes and
cockroaches.
 
Jerry Bromenshenk
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