Caveats
About 75% of the readership of The Apiarist are from the UK and USA. Although our bees are all the same it's inevitable that our beekeeping practices differ. The equipment is different and so is the beekeeping season.
Local bees
Beekeeping is a profoundly local activity. Your bees forage within 4-5 km of your hives, a tiny little patch of less than 80 km2 of countryside (or town). When I'm shivering during the late frosts of mid-May, a beekeeper in the south of England - or southern California - will probably already have newly mated queens.
I live in north-west Scotland at a latitude of 56°N. Our climate is temperate and oceanic, which means relatively mild and wet. Temperature minima and maxima vary from about -8°C to 24°C during a hard winter or warm summer. We get about 2 metres of rain a year.
Remember this when reading the posts or commenting. The calendar does not dictate your beekeeping, the local conditions do.
Varroa
Varroa mites are a problem for bees globally. However, the need to deal with them depends upon your locality, and the timing of any interventions you make depends upon your beekeeping season.
Furthermore, the chemicals (miticides) you have available for mite control depends upon local or national legislation ... some of which is poorly documented or apparently illogical.
I don't know where you live or what your local rules and regulations are. I only know what I'm allowed to use, and when to use it.
When - with regard to the local beekeeping season not the calendar - is critical.
What you use is often dictated by local conditions, but the comments and advice you'll read on this site will still be largely relevant as many of the same miticides are used globally.
Legacy posts
Despite what some 80 year old beekeepers might suggest, beekeeping is different now from the end of the last century, and very different from before Varroa became globally distributed. With further globalisation and its consequences (e.g. the Asian hornet) the only certainty is continued change.
This site has existed for over a decade. Even during that period beekeeping has changed. New miticides have been introduced, other treatment options have been withdrawn or outlawed. Changes in what is considered 'good beekeeping practice' have occurred. Our understanding of the biology of bees and their diseases has dramatically improved.
Remember this when you read legacy posts. All posts carry the date they first appeared. What I wrote 5 years ago might no longer be sensible, good practice or legal.
Legacy posts - those published before 2024 - may have minor formatting issues. I've tried to fix the most egregious, but many get viewed so infrequently it's probably better use of my time to write new stuff.
Oxalic acid
I use the term oxalic acid generically to mean:
Miticides in which the active ingredient is oxalic acid dihydrate that are approved for use.
During the last decade 'pure' oxalic acid - a chemical used successfully for over 20 years - has largely been outlawed. In its place there are now commercially available approved compounds such as Api-Bioxal. These or similar should be used where approved.