Just read an abstract that I found interresting:
"Old honey bee brood combs are more infested by the mite Varroa destructor than are new brood comb", Piccirillo,-Giancarlo-A [Author]; De-Jong,-David [Author,-Reprint-Author], Apidologie-. 2004; 35(4): 359-364
Abstract - Varroa destructor preferentially invades larger honey bee brood cells. Consequently, it was expected that brood in old combs with reduced-size cells would be less infested than the brood in new comb cells. An old brood comb was placed in each of eight Africanized honey bee colonies, along with a new, naturally constructed comb (without comb foundation). The mean percentage of brood cells infested with V. destructor was significantly higher in the old combs (22.6%), than in the new combs (9.75%), even though the inside width of the cells was significantly smaller in the old (4.58 mm) than in the new combs (4.85 mm). Within the range where there was an overlap in the width of brood cells between old and new combs, which was from 4.5 to 4.9 mm, the old comb cells were over four times more frequently infested with mites than were the new comb cells. Some factor other than cell size makes old brood comb cells much more attractive to V. destructor than newly constructed brood comb.
Unfortunately I haven't got a full version of this article, and I don't know what their definition of what "old combs" is. And even though it is not new old comb could be a problem I must admit I had no idea that old comb was four times more likely to be infested with mites. But a very important thing is not mentioned: What if the old comb is removed? Will the mite population drop, or would they remain the same and the mites living just as well using their second choice? The abstract doesn't say (maybe the full version does).
The research are presented in a journal called Apidologie. It is a honeybee research journal that unfortunately costs about $300 a year, and my university do not subscribe :-( Old articles (from around 2002 and earlier) are freely available online though.