One means by which the AHB may supplant the EHB is through colony parasitism. Previous reports have shown that small AHB swarms, no bigger than the size of a softball, can cluster near the entrance of a colony. Over a few days, the workers in the swarm can overcome the defenses of the hive and usurp the resident queen, leaving the colony wide open for the AHB queen in the swarm to take over. Such a coup d’etat can change an EHB colony into an AHB colony literally overnight.
A recent study was published by UNC-Charlotte Professor Stanley Schneider and his collaborators in Arizona at the USDA Carl Hayden Honey Bee Research Facility that addresses this issue. They placed many five-frame nucleus hives in their bee yard, all headed by EHB cordovan queens, and they tracked the progress of these hives over the course of two years. They performed weekly or bi-weekly inspections of each colony and rated its strength (according to the amount of brood and number of adult bees) and queen status (thriving, weak, queenless, or superseded). They witnessed dozens of invading parasitic AHB swarms and the resultant usurpation of the EHB queen.
They found an average take-over rate of 21% over the course of the study, much higher than previously thought. Moreover, there was a seasonal effect of this process, such that most swarms were seen during the months of October, November, and December. There was also a highly significant effect of EHB colony status on the likelihood of being parasitized. Thriving colonies had only a 1.6% chance of being usurped each month, but weak and queenless colonies had a 2.2% and 12.8% chance of usurpation, respectively. These results suggest that the parasitic AHB swarms are somehow able to locate colonies with weakened defenses and take them over at a surprisingly high rate.
Knowing how this usurpation process works may enable beekeepers to take certain measures to minimize how often their colonies can be taken over by Africanized bees. It is for this reason, and many others like it, that make beekeepers the first line of defense against the AHB.
Schneider, S. S.,T. Deeby, D. C. Gilley, and G. DeGrandi-Hoffman. (2004). Seasonal nest usurpation of European colonies by African swarms in Arizona, USA. Insectes Sociaux, 51: 359–364.