Cheap sources of burlap are easy to find.  Untreated burlap is less easy these days, especially since much is treated without the subsequent manufacturer (the coffee packer, for example) necessarily knowing about it.  Treated burlap must be registered with the EPA before being imported or sold as burlap but I know of no such requirement when it is imported as the packaging of something else.  Most burlap material traces to India, Pakistan and Bangladesh.
 
Common treatments include:
- copper sulfate (especially when used for root-balls of trees as it slows root growth)
- fungicides such as Compsol
- permethrin (a pesticide)
- various rodenticides
- many different types of flame retardants  (very common because untreated jute is highly flammable)
- dyes (both organic and inorganic - more common on fabric-grade hessians)
- detergents (while not actively left on the material, there may be residues, especially for cheap "industrial-grade" burlap)
 
Some of those chemicals are potentially toxic to bees.  Most would be aerosolized by the smoking process - that is, they would not be detoxified by burning or at least would not fully detoxified.  The dose would be highly variable.

Mike Rossander

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