Source for information on African bees in the Southwestern USA:

THE AFRICAN HONEY BEE:
Factors Contributing to a Successful Biological Invasion
Written by: Stanley Scott Schneider, Gloria DeGrandi-Hoffman,
and Deborah Roan Smith
Published in: Annual Review of Entomology 49: 351-376.

you can get a full copy of it for personal use at:
Stan Schneider's home page
http://www.bioweb.uncc.edu/Faculty/Schneider/


quoted material:

The African bee arrived in south Texas in 1990 and in the intervening 13
years has spread throughout Texas, New Mexico, and Arizona and is
currently colonizing southern California and the central valleys. As has
occurred in Latin America, the African bee appears to be displacing feral
European colonies (especially east European colonies) in the southwestern
United States; The African bee is now spreading rapidly through the
central valleys of California despite contact with large managed European
populations.

Every year more than a million colonies are moved throughout the United
States for overwintering or to pollinate crops. If a colony loses its
queen during transport and requeens itself in a region where there are
African drones, it will become Africanized. Also, queenless colonies may
be more susceptible to invasions by African swarms. Consequently, the
transportation of colonies from areas with a feral African population back
into apiaries in other parts of the United States could accelerate the
spread of African patrilines and matrilines and move them past natural
barriers that might otherwise slow or stop their progression.

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