> Are there papers where usurpation has been observed?

Well, yes. That's what makes Ellis' study so interesting. Why did he not detect it? Is it due to environmental conditions in Florida? Or a different strain of Africanized bee? The Africanization of Florida appears to be a separate event from the takeover of the Southwest. 

> Nest usurpation is a form of reproductive parasitism that may contribute to the ability of African bees to displace European honey bees in the Americas. We examined nest usurpation by African swarms over a two-year period in a southern-Arizona apiary that contained 76 five-frame European colonies. We observed a mean annual usurpation rate of 21% , with strong seasonal trends in usurpation activity. 

> Most usurpations occurred from October–December, with a minor peak of usurpation activity in the spring-summer months. The seasonal patterns of usurpation corresponded with the reproductive swarming season in spring and summer and the absconding season in the fall-winter months. 

> Our results show that nest usurpation is seasonally frequent among honey bees in the southwestern U.S., which suggests that reproductive parasitism contributes to the invasion success of African honey bees and possibly other introduced social insect species.

Schneider, S. S., Deeby, T., Gilley, D. C., & DeGrandi-Hoffman, G. (2004). Seasonal nest usurpation of European colonies by African swarms in Arizona, USA. Insectes sociaux, 51(4), 359-364.

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