Alyson,

I must admit I'm not up on the nutritional requirements of 
earthworms.  Mammals don't do well on corn alone but earthworms have some 
enzymes that mammals do not, e.g. they make omega-3 fatty acids from 
omega-6 fatty acids.  I would think a corn-based diet supplemented with 
whatever amino acids, fatty acids, vitamins, and minerals required for 
earthworms would do the trick, and cheaply.  Best of luck with it.

Tom

At 03:34 PM 6/12/2008, you wrote:
>Thanks for the suggestions, Tom.
>I originally considered using C3 and C4 plants, respectively, to create two
>isotopically distinct diets but concerns were raised over the comparative
>nutritional value of each (I was considering corn and potato). It seems that
>there is a strong precedent for the use of corn in feeding experiments,
>though; any sense as to whether I should worry about its relative lack of
>nutritional value?
>
>Thanks for your help,
>Alyson