Formic was extensively and intensively investigated and employed as a 
tracheal and varroa control in Europe long before North Americans found 
a need to fight mites.

Yep, for example

Koeniger, N.; Rau, C.  Field trials with formic acid [against Varroa jacobsoni] in Hochtaunuskreis 1979/1980.

> Over 2000 colonies in 235 apiaries were treated with formic acid for 3 weeks in September; nearly all the colonies were without brood. Counts of dead mites were made; also a sample of colonies was killed, and the mites were shaken out and counted. Varroa was found in 679 colonies at 95 apiaries; formic acid was shown to be an effective acaricide, and no harmful effects were observed in overwintering bees during the 6 months following treatment.

I also participated in years of field trials of formic acid. Our problem was we couldn't get it on until the end of September, when the mites were too numerous and the temperatures were too low. Treating with formic in early August in our area makes much more sense. Though in the long run, mite resistant bees are the only answer to this problem. 

Pete

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