Mite math
Three calculations you need standing at a hive, in a form that works on a phone. Nothing is stored and nothing is sent anywhere.
These do arithmetic, not modelling. If you want to plan a season properly — test a treatment schedule before you commit to it and see whether a colony is headed for a crash — use Randy Oliver's varroa model, linked at the bottom. It is the best thing in the hobby and he gives it away.
A level half cup is about 300 bees. The threshold used on this site is 3 mites per 100. Up to 70 percent of the mites in a colony are sealed in brood, so a low count is good news, not a guarantee.
Same colony, same method, same sample size, both times. This is the step almost everybody skips, and it is the only way to tell a treatment that worked from one that did not. A failed treatment feels exactly like a successful one until February.
This one is deliberately crude. It assumes the mite population roughly doubles every 30 days while the colony is rearing brood, and nothing else — no immigration from collapsing hives nearby, no seasonal change in brood area, no resistance. Real mite growth is messier than that in both directions. Use it to see the shape of the problem, not to plan a season.
The shape is the point, though. Two mites per hundred in early August is not fine. Two becomes eight by the time the winter bees are being reared, and those are the bees that have to live six months.
Where to go for the real thing
Randy's Varroa Model at Scientific Beekeeping. Randy Oliver spent a year building it and gives it away. You enter your colony type, starting mite level and mite immigration, then apply treatments on any date and watch what happens to the mite population across the whole year. It is the only way I know to test a mite strategy before you bet a season on it.
Start with the Necessary mite reduction tab — it asks how many months your colonies rear brood and shows how much total control you need to stay ahead. For northern Wisconsin that number is smaller than it is for California, and understanding why is worth more than any calculator on this page.
It runs in Excel. If you do not have Excel, Trish Harness built a simplified version that works on a phone.
Both are free. Both took real work. If you use them, send Randy something — the site is not-for-profit and the research it funds is the reason any of us know what we know about mites.