In the early 1900s, beekeepers used 4.9mm foundation as it was what the bees naturally drew out. Thinking that bigger bees would mean increased honey production, they started pressing 5.4mm cells (which we now consider conventional cell size). Sure enough the bees grew bigger and honey production rose. Eighty years later, Varroa destructor comes along and almost completely devastates honeybees by laying eggs in the larger brood cells.
Then in 1995 some scientists noticed that Africanized bees—which are slightly smaller than European bees—had lower mite counts. So the idea was born that the natural (smaller) cell size might impede the growth of Varroa mites. This idea was supported by earlier research that showed that when immature male mites are squeezed between the bee pupae and the cell walls, they often die.
With this in mind, beekeepers started regressing their colonies. That is, they tried to do the opposite of what the beekeepers in the early 1900s did: they provided smaller foundation in the hope that male mites would get squeezed to death in the tight confines of the smaller cells.
You can find out more behind small cell or sometimes called natural cell on Micheal Bush’s website here: Natural Cell Size
Listen to a discussion between treatment free beekeeper and educator Solomon Parker, Marshall Dudley and Joseph Bessetti on Small Cell Beekeeping.
Small Cell Beekeeping