Letting bees build their comb with a natural cell size is one of the tenets of Michael Bush's organic beekeeping method.  I thought I was following his lead, but I think I skipped a step and didn't actually manage to downgrade my bees to natural cells.  Maybe that's part of why my hives kept dying.

Here's the technical information:

• Most foundation that beekeepers put in their hives has cells 5.4 mm in diameter.
• Michael Bush (and others) believe that bees naturally build cells ranging in size from 4.6 mm to 5.1 mm for worker brood.

Putting package bees in a hive with foundationless frames is a good start, but your bees won't build naturally sized cells right away.  Instead, you need to go through a process called regression where you allow the bees to draw out the comb and raise bees in it, then cut that wax out and make the bees draw it again.  Workers that have been raised on modern foundation will generally build 5.1 mm brood cells the first time they're exposed to foundationless frames, then the smaller bees that hatch out of that frame will draw 4.9 mm comb.

For easy regression, keep feeding empty frames into the center of the brood nest and your bees will eventually turn the outer frames into honey storage.  Then you can just cut out that larger celled wax as you harvest honey.  (I wonder if it's a coincidence that this is how top bar hives are often managed?)  Making the brood chamber top bars 1.25 inches wide instead of 1.5 inches wide will also prompt the bees to build smaller cells.

I was such a newbie when we got our first package of bees that I didn't realize I needed to regress them.  As I've been cutting out honey from our dead hives, I decided to measure the cell size in the brood nest to see what my bees were actually doing.  To measure cell size, lay the centimeter side of your ruler down on the comb so that 0 lines up with the edge of one cell, then count ten cells.  The number of centimeters that encompass those ten cells equals the size of one cell in millimeters.

The photo at the top of this post shows some old brood comb that probably dates back to the original package workers.  It looks to me like my bees drew 5.2 mm foundation (once I factor in the way the camera angle offsets the ruler slightly).

The second photo shows younger comb that probably got drawn after the meltdown of 2010.  It actually looks to me like this comb is a hair bigger, with cell size of 5.3 mm.  Perhaps that's because the bees were rebuilding during a nectar flow and were making comb for honey (which is traditionally bigger than comb for worker brood).

The moral of the story is --- don't just assume because you're using foundationless frames that your bees are building at a natural cell size.  Bring your ruler out to the hive and do some quick measurements to see where you're at, then regress until you achieve 4.9 mm cells.