Darren Anderson

First understand the lifecycle of the flea.  (I tried to share a link but... no go...)
If your gonna do this in as natural a way as possible.  You have to understand this is a long battle.
What we often do first (We are a foster for a cat rescue) Is bathe the critter in dawn dish soap. The blue unscented stuff. This will do a job on the living pests and wash some eggs a such away.
DE will work. But it isn't something that is just gonna kill everything. You have to apply and re-apply. (See the link for understanding the lifecycle of a flea)
It will be hard work, and you must be diligent.
Otherwise....
the only alternative is what the vet recommends. PLEASE IF YOU GO THE CHEM WAY! DO NOT BUY STUFF FROM YOUR PET STORE! PLEASE PLEASE! have seen too much, paid too much in vet bills and witnessed too much pain to not say anything about the substandard consumer goods at the pet store.  

Hi Ian
My experience is minimal I have a half built storm/root cellar that has been sitting nearly two years. (I know I'm a slacker... but I'm not... that is the problem... lol)
So the bags after sitting exposed that long have held up quite well actually. Although it is not in direct sun after the trees leaf out.
I would guess they would last quite a while if you were to fill them with soil good enough to grow a low ground cover something not too needy perhaps thyme or perhaps a moss? Pending on light situations.  You may get some extra life out of the plastics. I dunno..
I do know they have held up this long at my place with no protection, they are getting patches of something black? (hope its not mold) and that the mullein and other poor soil critters are moving into the packed bags. (soil inside is sandy on sight less than desirable soil as  to growing veg)

Heya Michiganders!
Was happy to see all the reply's in this thread! Lol That means I'm not alone!
I live in the Montrose MI along the M57 corridor and if you have ever driven that route you may get why I may feel that way. (Lotsa well medicated and abused soil microbes along that route.)
Anyhoo. I think this is my first post on permies!  You guys got me excited enough jump out of my lurking on this webs!
I am working to turn my little bit here into a permainspired homestead.  As I slave along in my regular job that pays the bills and all the purchases of those perennial veg, trees and tools that allow me to create that idea and work efficiently as I go (not machines lol I want to rid myself of those critters)... so ... its slow going.  I cant jump in head first and full on but I'm in up to about where your damn sure the water is cold! The next step is a shocker! haha if ya get my jest?

I just want to say hello! And express how happy I am to see that even in Michigan these ideas are gaining ground so to speak in our area