Some beekeepers use Oxalic Acid when brood is present to remove varroa mites with multiple applications at 5 day periods. This study from the US says that it is not that effective.
https://academic.oup.com/jinsectscience/article/22/1/15/6523139?login=false
"However, we did not find evidence that frequent periodic application of oxalic during brood-rearing periods is capable of bringing V. destructor populations below treatment thresholds".
https://academic.oup.com/jinsectscience/article/22/1/15/6523139?login=false
"However, we did not find evidence that frequent periodic application of oxalic during brood-rearing periods is capable of bringing V. destructor populations below treatment thresholds".
May your bees read the same books as you do.