Hi all
Considering the movement of bees back and forth between regions with Africanized bees and the north, it's pretty well accepted now that there is a high likelihood of US bees having African traits. One of these traits may be poor wintering ability. Ed Southwick wrote about this back in 1990:

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First, all colonies lose progressively more individuals (which die) as the confinement period is extended. This “winter loss” is substantial, even in European colonies (Farrar, 1963). So as the winter temperatures prevent flight, it becomes more and more costly for the group to continue thermal control, with these costs rising faster for the Africanized clusters. 

Second, as Africanized honey bees will tend to enter autumn and winter seasons with relatively small colonies (certainly less than the 30,000 bees recommended for success, Furgala, 1975) from poorly established late summer swarms, they will be under increased thermal stress even at the beginning of the low temperature season. Third, the smaller overwintering colonies will be prevented from the early colony growth considered to be critical for success (Seeley and Visscher, 1985). The stresses associated with the smaller winter clusters of Africanized bees can be expected to exacerbate “stress diseases” such as Nosema.  

A number of investigators have moved African and Africanized honey bees from their native habitats to northern areas for study. None have reported success in maintaining colonies over lengthy cold winters. Woyke (1973) transported Africanized honey bees (Apis mellifera scutellata) from Brazil to Poland and was unsuccessful in overwintering any colonies there. African honey bees kept in Oberursel, near Frankfurt, West Germany, did not maintain their colonies over the winter (unpublished observations). In fact, the standard practice used by the Oberursel Institut fuer Bienenkunde has been to house African queens in large colonies of European honey bees (Apis mellifera carnica or Apis mellifera mellfera) in order to “keep” them over the winter (Koeniger, personal communication). 

The Africanized honey bee is inferior to European honey bees in cold tolerance, as demonstrated by comparative colony energy balance and behavioral characteristics. The distribution of the feral Africanized honey bee colonies, which have by far the most significant impact on beekeeping and the ecology of local plants and animals, will be strongly limited by their relatively inefficient attempt to thermoregulate during extended exposure to low temperatures. 

E. E. SOUTHWICK, D. W. ROUBIK and J. M. WILLIAMS. COMPARATIVE ENERGY BALANCE IN GROUPS OF AFRICANIZED AND EUROPEAN HONEY BEES: ECOLOGICAL IMPLICATIONS

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