No reliable scientific data exists that supports the claim of using diatomaceous earth (DE) as a feed additive to control livestock flies.
Diatomaceous earth is comprised of the mineralized "shells" of one-cell aquatic plants called diatoms. Over millions of years, these diatoms died and settled to the bottoms of bodies of water. Now, these diatom "skeletons" are mined or dug to yield diatomaceous earth.
An insect's body covering, the cuticle, contains fat layers making the cuticle nearly waterproof and preventing water loss. It is known that sorptive dusts absorb fats, disrupting the cuticle's waterproof nature. Abrasive dusts damage the insect's water barrier by actually scratching or cutting the cuticle, resulting in "dehydration," usually causing the insect's death. The most effective sorptive dusts have been the silica aerogels (ammonium fluosilicate) and acid-activated clay. Dri-die, a silica aerogel, has been used in urban pest control against ants, roaches and stored grain pests with some success. However, DE is far less sorptive than the silica aerogel with its effectiveness primarily from abrasiveness.
Although DE is labelled as a livestock feed-through for fly control, there is no scientific data to support efficacy regarding control of internal parasites and specifically fly larvae in animal manure. Nevertheless, adult horn flies and face flies could move in from surrounding untreated herds and wild animals. DE may control some of the ectoparasitic lice, fleas and some mites if applied thoroughly and repeatedly.