starshrike,
  This one of very few areas where Cuda and I disagree.
I ONLY use Diatomaceous  Earth!  I don't believe in too many of the chemicals out there. For my flocks and my family I prefer to keep it as chemical free as possible. I mix DE into all of my feeds at no more than 2% by weight and my flocks stay worm free year round. They also stay free of other pests such as lice and mites because I sprinkle a 50/50 mixture of wood ash and DE in their favorite dusting areas.
To us, (humans), DE may seem like a powder when it is dry and a bit like mud when wet, but to something the size of most parasites it is like a desert of broken glass! when wet, (when ingested), it simply becomes a flowing river of broken glass! LOL It really does those buggers in!
Again, I never use any commercial dewormers, only DE and an array of herbs, including mint and basil, and my fowl stay worm free the year round.
Hope this helps a bit,
Dave