I only use DE as an insecticide, I still follow a worming program for my dogs, horses and chickens. I also use flea drops on my dogs and horses, (Fly control) I know DE works as an insecticide, as long as it doesn't get wet. I live in an area of NC where the humidity is often high, and droughts are frequent as well. The fluctuations between the two is the reason I put the DE in areas where it is not likely to get wet.
Scientific studies, government or otherwise are sometimes biased, and
it is best to look at the studies and make your own choices based on the results. For me, as an insecticide in the house, barn and coop, I have found it to be superb!
I must be doing something right, I have one of the oldest dogs (19) in the world! I also have a chicken that is 10 years old and still lays eggs! Try this simple test. Sprinkle some DE around the corners of your garage or out building where insects are a problem and watch the critters fall over dead.
I am a believer. Let me know the results.
Good luck!
Bunny