This method was reportedly developed by the Ontario Beekeepers
Organization. It is not legal in the U.S.A., and I have no experience with
it. Users do so at your own risk.
 
Cut an 8 x 9 1/2 inch piece of 1/2 inch homasote board. Homasote is a
building material that looks like is is made out of recycled newspaper pulp
and is very similar to suspended acoustic ceiling tile.
 
Mix a 65% percent solution of formic acid by diluting 2 parts 100% acid
with 1 part water. Be careful the stuff is very nasty.
 
Place the homasote piece in a ziplock bag and pour in 250 ml of the 65% FA.
Seal it up. It is reccomended that you double bag the board for safety. Let
the FA soak in.
 
To treat, put the saturated homasote in a perforated ziplock vegatable
storage bag. Use in the spring or fall when daytime temparatures are 54 to
77 F, with night temparature above 45 F. Place the perforated bag
containing the homasote on top of the brood chamber, using a 1 inch hive
body riser to make room. Treat for 15 to 21 days.
 
I have heard of some beekeepers in the Southeastern U.S. who have tried
this are less than fully satisfied. Personally, it seems like a massive
dose of FA to me.
 
I have heard of another method which uses wooden toungue depressors soaked
in pig miticide, which is also no doubt illegal in the U.S.A. No further
details on this method were available.
 
Good Luck.