When I took beekeeping 1 and 2 with an instructor locally, he taught us to use oxalic acid in the spring. In our area March 1 is when we applied it. The sugar trickle method. I trust this advice from my instructor. He runs about 1,200 hives, has an incredible "love" of his bees, not just a soul that is working his bees for money. His care and love shines through in everything that he does around his farm, very obvious. I can only say good things about this man who was born in the Far East and has been in Canada for probably not more than 15 years. Don't know why I went on about him, but that is what he/we use in spring. In the fall we use formic acid, beginning of September after the honey is pulled off. As stated before, formic acid treats many pest of the bees. i.e., varroa mite, trachael mite, wax moth and chalkbrood. It does quite a bit.
I had an experience with the actual inventor of the formic acid pad, his name his Bill Ruzick. He lives in a city that is about 4 hours travel from our home, about 1/2 km from my daughter's house, whom we visit frequently. Bill is an older man, a retired professor of university. He travels worldwide extensively (and I mean north, south, east, west) lecturing and teaching about the use of the formic acid treatment for honeybee health issues (and maybe lectures about other topics as well, I don't know). Right now he is somewhere is Africa I believe. I had the opportunity to go to his farm in August of this past summer and be taught about how to use formic acid with the bees. I left his farm feeling somewhat enlightened and have a firm believe in the power of formic acid. Having only a few hives myself, I use the prefilled pads. They are attached to the inside wall of the hive, the acid fumes are heavier than air, which drop to the bottom and pool, the bees are irritated by the smell and fan the fumes throughout the hives, killing the varroa and doing good stuff in the hive. For the larger beekeeper, the prefilled pads would be probably too expensive, the unfilled pads can be filled at the apiary in a very short time and applied. The cost per hive is very low, maybe 80 cents, I am not sure, but in Canada it is cheap.
Yes, oxalic acid has been approved for use in Canada, we sometimes take so much longer in Canada for approval for stuff that has been in use in the U.S. for a long time. We are somewhat behind the times I guess.
Do no take anything I have said as gospel, it is only my opinion on some very serious matters. I am on a fight with the varroa mite, one day we will have rid ourselves of this pest.
Oh ya, by the way, there is another fellow that lives in a neighbouring community that runs a farm of honeybees too. I have taken mini courses with him as well. There has been funding approval for research to be done with breeding of varroa-resistant honeybees. He is involved very deeply with this breeding program and should have his results in a couple of years. I know nothing more of this at this point in time as I have not spoken with him recently. But it sounds like they are doing great work. This varroa is worse than a thorn in the side of beekeepers. I have witnessed the damage personally with hive deprecation, loosing now probably 6 in total.
My advice to any new beekeeper (and maybe seasoned ones as well), is to really be aware of the symptoms of this evil doer, I was not aware strongly enough personally, it was too late to save my hives before they entered their stage of doom, where I could do nothing to stop the damage done. Learn about how to keep the hives healthy, #1 point in beekeeping in my opinion. Great day all. Cindi