The question is, what happened?
Kefuss e-mailed me a copy of their classic study with Intermissa and Carniolan bees 1993-2004, which must be the longest lasting TF study ever made. It was published 2004 in ABJ July number.
Since 1985 there had been breeding work done by Wolfgang Ritter in Tunisia with Intermissa black bees. This pre-selected stock was then sent to Kefuss in Toulouse France 1993. John explained that because beekeepers in Tunisia had too little money they were not able to buy stuff to treat their colonies. Ritter realized this opportunity and started selecting. Same story in Siberia, with Primorski and Rinderer…
In the end of ABJ article there is a reference to Apidologie 21:368-370, by Ritter W (1990), I cannot find it for some reason. * Name of the study is supposed to be “Development of the varroa mite populations in treated and untreated colonies in Tunisia.”
By 1996 most of the original queens had superseded. In 1999 they started new study with the offspring of the best queens.
Because both the Intermissa and Carniolan control were originally black, they could confirm the results of free mating. The superseded hives got more yellow colouring. They write: “After 1996 all the remaining colonies probably became genetically similar.”
Interesting for the IPM enthusiasts are their remarks in the discussion part of the study:
“…This suggests that, as infection persisted, colonies reacted more strongly to the presence of varroa. The positive slopes of the regression lines indicate a tendency for both races to eliminate a higher percent of varroa as the infestation level increased. This high elimination may be a simple population threshold reaction. Care must be taken when using high varroa fall as an indicator of susceptibility. Under certain conditions on the contrary, high varroa fall may simply represent lower receptivity… This means that by itself varroa fall is not a valid indicator of susceptibility.”
They gave a definition to receptivity in the introduction: Receptivity is the possibility of an animal to permit the presence of a parasite and the normal development of its life cycle.
In the late part of the study, from year 2000 to 2002, when the colonies had “become genetically similar” the infestation level (varroa on bees) medians varied between 1,72% (April 2002) and 5,39% (October 2001).
Talking to John it seems to me now, that what happened in his beekeeping was duplicate of this study. The only difference is that he managed to maintain viable colonies for a much longer period. In the study free mating did not affect results in the beginning. But then they write: “The accelerated drop in colony numbers after 1999 (after 6 years of tf) might be due in part to the dilution effects of earlier natural mating with susceptible drones.”
*
https://www.apidologie.org/component/issues/