Well, we aren't a one-ferret household, but I think I might have a
suggestion: mightn't you take out a small quantity and keep it in a
Tupperware container, and then put the rest of the bag(s) of ferret food
into a Ziplock freezer bag and toss it into a deep freeze?  This is what
I do with flour (which I buy in 25-lb bags) to keep out the bugs.  If you
don't have a deep freeze (or enough space in yours), perhaps a neighbour
would be willing to lend you some space in hers.  Still, I would think
that it would need to be used within maybe 6 months(?)  (I'm just guessing
on the time frame -- an educated guess, but still just a guess).
 
Take care,
Chana
[Posted in FML issue 3460]