Hello Robert & All,
The only information I have seen on the Thymol gel was done by Dr. Keith
Delaplane in Georgia. Not a high kill rate if I remember but at least a
control you could alternate with.
 Food grade mineral oil or perhaps the new folicel might help till a better
idea comes along. I may be going out on a limb here but in the U.S. the
U.S.D.A. does not press the issue of label violation  when *food grade*
controls are used. Some beekeepers put menthol cough drops in their hives to
help control tracheal mites. The label on the box of Smith Brothers cough
drops did not say *for use in beehives* but the FDA told me when I asked
that they were not  greatly concerned unless they had a complaint by a
consumer of contamination. . Smear peanut butter or jelly across the frame
of a hive and little notice will be taken.
 Misuse of chemical is another matter. At least one fine of $10,000 (U.S.)
has been applied for using a chemical against label in beehives.
Sincerely,
Bob Harrison
Odessa, Missouri