Re: Possums, I got possums...
I've used mothballs (napthalene) when I've stashed food in the woods. Has worked perfectly if I've buried it, but for things on the surface birds have not been too discouraged.
Damned things will work and worry until they get at "it". Really determined. And I don't know how they can smell well enough to go for freeze-dried food inside the factory packaging and not be put off by the mothballs.
I'd bet they'd work with possums, but might drive the dog nuts.
Arf