FWIW,
I have had no luck when trying to rid one of my hens of worms. I have always used it in their food/coop/nesting boxes and one hen developed worms in spite of this. Then, I tried to separate her and treat with garlic/pumpkin seeds/DE and this did not work either. So, I have no idea . . . . . This is the only bird that has developed worms for me before or since, so I'm not sure what the underlying causes may or may not have been. We just culled her after many weeks of trying to help her.
Many in the chicken world argue that DE will NOT work as a wormer because once they have eaten it it becomes wet. The characteristic that makes DE effective against bugs is its ability to "cut" the exoskeletons of bugs with its sharp edges. (It does not prevent bugs, simply kills them on contact) Some say that once the DE is wet, it is not "sharp" any more.
I have had NO bug trouble in my feed when I mix lots of DE into the food for preservation purposes. (I use DE when I buy individual grains in bulk for my own feed mix.) It is used "dry" in this method.
HTH
Food grade is supposed to be useful for internal parasite cleansing for humans and animals. Some claim it helped them lose weight--I'm guessing from the loss of the parasites...
Please post results if you try DE for internal parasite control. I am skeptical because I was using it in the goats for parasite control and it did nothing ( as shown by fecal counts) During that time (about 7 months) we did have fewer flies, but am not sure that it was connected with the DE use. Still intrigued by all the positive claims, but less than enthusiastic at this point. 
