Styrofoam will ride high in the syrup and does not have any capillary action 
to wick the syrup. The bees will still have to get to the syrup. Straw will 
allow the bees good footing and capillary action.
The classic method has been a wood block covered with burlap for the same 
reason.

I have always felt that open feeding is a great way to spread bee diseases.

Dan Veilleux
in the mountains of NC
zone 6a

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