Regarding climate:
 
Apparently, alfalfa flowers are harder to trip in humid weather.  Where the
summers are humid, the leaf cutter bees are practically a necessity to get a
good seed set.  (Leaf cutters are too dumb to figure out how to avoid getting
bonked by the alfalfa flower... they always trip the flower, unlike our
lovable intelligent friends, the honey bees.)
 
Where the weather is dry, the trip mechanism on the alfalfa flower is much
more sensitive,  so honey bees are not as successful at avoiding getting
bonked.  Where the summers are dry (like California and Arizona), honey bees
are generally preferred.  The leaf cutter bees would still set more seed per
bee than the honey bees, but the honey bees are much easier to manage, hence
more economical.
 
Shawna