One possible explanation for the findings of "no usurpation" in Florida as
opposed to the findings of 21% usurpation in Arizona, and rare usurpation in
"Coastal Sao Paulo" would be the nesting opportunities in each area.
I've been to all three places.  Lived in Ft. Lauderdale for a decade,
island-hopped on from there through the Caribbean and cruised down the coast
of South America to IIhabela and Southward multiple times.  Been to AZ going
through the desert on a horse with no name, and so on.

The difference is that Florida has very few natural cavities for bees aside
from their famous Live Oak trees, but many man-made cavities, so the
Africanized bees can easily find things like perfectly dry water meter
enclosures, like this one:
http://sswd.org/modules/showimage.aspx?imageid=109

Note the nicely-sized entrance hole, where the water meter reader would
insert his pry bar to remove the cover, and the dry, smaller than a European
swarm trap cavity that AHB tend to favor.

In AZ, bees are kept out in areas that may have zero decent cavities for
swarms, so the only choice for the AHB swarms in the area of the hives are
the man-made structures, beehives.

Coastal Sao Paulo?  Not a lot of opportunities for bees at all.  Steady
stiff winds off the beach blowing salty air, small plants, not a lot of
larger trees.  AHB may have not expanded naturally in that area due to a
lack of suitable nest sites to the West.  Bees only swarm a few miles at
most, even AHB will not swarm 25 miles away in one jump. 

So, one might conclude that less nesting opportunity means more usurpation
attempts, as the AHB swarms have limited options.
Pure speculation, but the question to ask is not about usurpation rates, it
is about where AHB swarms and EHB swarms ARE found to nest in each area.

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