Peter Borst says:  "Is it going to go tumbling all the way to the
bottom like Alice falling down the rabbit hole? Or simply drop a few
centimeters onto another bee, or get snagged on a piece of comb?" when
questioning how effective screened bottoms will be based on how likely it is
for mites to lose their grip and fall clear to the bottom.

A couple of thoughts come to my mind immediately (well, at least ONE thought
does anyway), which is that I recall reading that smoking bees with tobacco
smoke causes quite a mite drop.  I don't know how significant it is or whether
or not it has been scientifically or statistically studied, but if smoking the
bees causes a significant number of mites to drop off, and if by falling
through a screen they would then be unable to return to the bees and comb, I
would think that every time you smoked your bees you could be "de-lousing"
them, so to speak.  I use dried Perilla in my smoker now, hoping that the
aromatic chemicals released as it is heated and burns (that are purported to
kill Varroa mites) might have some positive effect on the bees and negative
effect on the mites.  Mine isn't a controlled study, but it is a hope.  At
least I believe it won't hurt and might help.  I imagine that there might be
other factors besides old age or accidentally losing its grip that might cause
a mite to fall.  One of the problems with Apistan is the mites sealed in the
cells that will not come in contact with it until they emerge.  I like to think
that I'm giving my bees a treatment each time I smoke them and open the hive to
look at them.  Is it formic acid that can kill them even while they're sealed
in the cells without hurting the developing bees?  Did anyone ever figure out
how long the mites are out of the cells before they go back into a sealed cell?
This would be a factor in determining how frequently one would need to treat
their bees in order to control the mites that are outside the cell.  I also
understand that it is more effective to treat for mites (higher level of
control) when there is no brood present in the colony.  My thoughts keep
wandering on to other related subjects, so I'd better quit here before getting
any further afield.  Later.

Layne Westover, College Station, Texas