Be careful where you get your burlap!
I got some from a friend who said he had some that had weathered  behind his
barn for a year. It looked OK to me so I used it on two hives for smoker
fuel.
After a week I started to see some dead bees in front of the hive, so of
course, I fired up the smoker to take a look what might be wrong.-- a
pesticide kill?
Well this finished off the colonies.
When I questioned my friend in depth, he said that his son had dumped the
burlap with the other poly feed sacks in back of the barn.
It seems that the burlap I used was used to ball and burlap nursery plants
and undoable was treated with copper naphthlate preservative. I then took
some of the left over and placed it on a small fire-alas, I got a nice blue
flame which is characteristic of copper.
I now stick to natural materials or old denim.
Dan Veilleux
Vilas, NC