Yesterday and today I have been treating my hives with FormicPro. I also specifically choose this week because of the milder temperatures, which slow the formic acid vapor release. Usually the bees just buzz a little louder when the pads are put in and give the pads a wide berth. Yesterday I saw an unusual reaction. About 15 minutes after I treated a hive it seemed that all of the bees exited the hive and then formed a beard beneath the outer cover. Its now been 24 hours and the majority still haven't gone back in. I've treated about 40 hives in the last two days and no other hive reacted this way.
UPDATE--The bees went back in the hive after 3 days. A week later I checked to verify if the queen was OK. She wasn't in the hive. In fact there was no eggs, uncapped or capped brood in the hive. That means this hive had gone queenless at least 3 weeks ago; prior to the formic acid treatment. Maybe since the bees were queenless they had no reluctance to abandon the queen in the hive.