The rate that matters

Under good conditions in worker brood, a varroa population grows about 2.4 percent a day, which sounds trivial until you compound it: that rate doubles the population roughly once a month.

Over a full temperate brood season, something like four months of continuous brood rearing, unchecked growth of that kind works out to about a twelve-fold increase from spring to autumn. A hive that starts spring with a handful of mites can end the season with more than ten times as many, with no single dramatic event marking the change.

This is also why the number that matters is a rate, not a total: mites per hundred bees. A colony's bee population rises and falls through the season, and a raw mite count on a small colony means something different than the same count on a large one.