John,
I'm not aware of any predatory springtails (Collembola http://en.wikipedia.org/wiki/Springtail ) per se, although they are so very common almost everywhere in leaf litter there probably is some. They are one of those groups of organisms where only a small fraction all the species have been identified.
But its not just springtails in those old nests, there are mites and assorted small insects also, seems a probability that some must eat martin mites.
It would be interesting to look at tree holes, the sort of places where martins originally nested, and see what lives inside of them. If there are whole complexes of species inside there, including mite predators, it might be that tree cavities are less prone to mite outbreaks.
Mike Scully
Purple Martin Forum
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