Yes, natural acids, more or less, but i don't like to work with them. And the bees don't like'em too. So only if really, really necessary.Around here varroa pressure is quite high; the most gentle approaches such as the sugar one you're using are not sufficient. The vets were even not impressed when I decided to go organic-only (gotta use the really heavy duty synthetic stuff!) but so far (8 years I think) it seems to be working. We do have other problems that may be a show stopper such as the new weather patterns... We'll see.
Both organic acids are found naturally in honey (of couse in lower concentrations) so if there are some leftovers at the time when we collect honey, it's not a foreign element and does not really change the nutrition profile. I apply formic as slow-release several times over the warm part of the year and oxalic as a sugar drip in late December, any day now. Vaporized oxalic seemed to me potentially problematic even for humans.
Sun today, ducks exploring the outer part of the orchard. They really blend in well.
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