It is my understanding that flies carry many of the less desireable species (acetobacter, mold etc.). A good reason not to let them into your wort.
I have heard that before as well, and maybe flies don't usually carry Saccharomyces, but is it true in general that insects only bring stuff we don't want?
Nope. Here's a story about recent research highlighting the role of social wasps in transporting brewer's yeast (Saccharomyces cerevisiae) from place to place:
http://blogs.discovermagazine.com/notrocketscience/2012/07/30/you-can-thank-wasps-for-your-bread-beer-and-wine/
Exerpts:
"The wild strains do grow on grapes and berries, but only found on ripe fruits rather than pristine ones. And theyre usually only found in warm summery conditions. So, where do they go in the intervening months, and how do they move around? They certainly cant go airborne, so something must be carrying them."
"Sampaio thinks that the bark of oak trees is the true natural habitat for S.cerevisiae. He has isolated the fungus from oaks all year round and in three quarters of his samples...'For our conceptual framework, wasps are not needed as habitats although, theyre very interesting as vectors, he says. After all, they may live in trees, but they still need a lift.'"
Here's another article about the discovery that an ancestor of lager yeast is living in infected beech trees:
http://news.sciencemag.org/sciencenow/2011/08/lager-beers-mystery-yeast.html
"He and his colleagues were compiling a genetic directory of different Saccharomyces species, which in nature live in oak trees"
I think there are two main points I'd like to make:
1) It doesn't make much sense to expect to catch an individual microbe species from the air. Think about it -- there are millions of microbe species, perhaps billions. Living microbes can't even travel in the air -- they will dry out and die, so they have to make spores that can make the journey. You're counting on a spore of S. cerevisiae taking off from a fluxing oak tree or rotting fruit, then randomly traveling to your wort on the breeze. The authors of the first study didn't even believe that brewer's yeast could travel in the air by itself. (I am skeptical. There have to be at least
some S. cerevisiae spores in the air. But apparently there are not many.)
2) It also doesn't make munch sense to believe that only certain types of microbes would be transported by insects. Wouldn't you expect an insect to be covered in the microbes from whatever environment it just came from? If an insect were in a vinegar environment, then sure it would be covered in acetobacter (or any other of the many acetic acid producers). But if the bug just visited an oak tree with an infected flux, then it would be covered in some mixture of yeasts (some of which will likely be Saccharomyces).
So, given what scientists know about the natural habitat and transportation of brewer's yeast, how should we go about catching it? I think a good place to start would be to wait for the early spring when oak trees begin to flow their sap. Either find an infected oak flux and collect Saccharomyes directly from the infected sap, or allow sugar-seeking insects like social wasps to bring the yeast from the oak to our wort (just leave wort out at the right time of year without cheesecloth).
A good second option would be to wait until mid-summer to early Fall when fruit lies rotting on the ground, and either collect directly from rotting fruit, or just let insects bring the yeast to our wort. Don't bother collecting in the winter or from fresh fruit -- the white stuff on the outside of grapes and plums is not yeast.
I know contributors to this forum have been cultivating yeast from flasks of wort placed in dusty basements and screened windowsills, but we typically don't do unopened controls to make sure we're not accidentally growing something that was already present in the wort we made (something like WLP001).
It just makes sense that we're most likely to catch wild or feral S. cerevisiae from a completely open jar placed outside in the warm months.