Take a total varroa population of 400 mites in July which would not be that unusual in my colonies.
80% will be in the sealed brood, ie 320 mites.
In July the brood nest is huge, even in my native type colonies it can be over 30,000 eggs, larvae and pupae. Others may have a brood nest double that size.
If I make another assumption that the queen lays at the same rate every day, capped brood will be about 60% of the total brood area so the 320 mites will be distributed among 30,000 * 0.6 = 18,000 cells. This amounts to 1.8% of cells affected.
In addition it is known that varroa mites are 12 times more likely to be found in drone brood and there is usually lots of drone brood in a colony in July so the percentage of worker brood affected is probably much less than 1.8%
In September the calculations change quite a bit.
The queen is laying less and the brood nest is often down to 10,000 or so.
The varroa population is now 1600 of which 80%, 1280 are under cappings. There is little or no drone brood present.
The varroa now affects 10,000 * 0.6 = 6000 cells ie 21% of the brood affected. (assuming 1 mite per cell rather than multiple occupancy)
If you start with 1000 mites in July the percentage of brood affected is 4.4% in July but rises to 53.3% in September based on these assumptions.
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