Hello Jean-Pierre I've consulted with a few researchers from Europe with experience over several years, with formic acid treatments. Dr. Ingemar Fries from Sweden has been very helpful. As I've said, the queen rejection problem hasn't been enough to measure in the trials (about 500 colonies over 4 years) or in the cooperating operation I work with (1700 hives treated over 3 years). A good answer to your question about how much queen rejection to expect from formic acid treatment, would start with a discussion about the details of the treatment regime. Different frequencies, and especially dose size and concentration, can be used to get effective control of several mite species. Ingemar says 5 % queen loss from 4 treatments with a commercial source of pre-moistened blotter material, which is labelled as having "14.2 g waterfree formic acid", is a typical expectation. (Since waterfree formic is very expensive, I imagine the plates actually contain some standard dilution ( 85 % perhaps) with the equivalent of the labelled amount. His 1989 paper, "Short Interval Treatments with formic acid for control of varroa .. in cold climates", had 5 of 91 queens lost (all after the first treatment, 3 were found dead), with that sort of treatment. He says losses of queens are avoided with the extended or prolonged-release plates (Kramer). Two other cases from Canada I could mention: ( 30-40 ml 65% liquid applied on absorbent paper on top bars) 10 of 30 queens were lost when hives were split in the middle of a 14 day treatment period. The queens lost were some of the ones being introduced in cages. Not surprising. In a small group of treated hives, 1 queen (marked) was found on the bottom board the day after the first treatment. It was found that there had been 2 queens in the colony. Generally it sounds not serious, but it's worth finding out how to avoid things like the case of 150 queens lost of 500. regards Kerry Clark, Apiculture Specialist B.C. Ministry of Agriculture 1201 103 Ave Dawson Creek B.C. V1G 4J2 CANADA Tel (604) 784-2225 fax (604) 784-2299 INTERNET [log in to unmask]