I think the BT must fade over time because I know many people on here who recommend quarantining the plants for a couple of weeks and rinsing them thoroughly several times before planting them. To be fair, I've never had the first problem buying milkweed from Home Depot. Most of my host plants are such specialized plants with a narrow market that I have to mail order them or buy them from clubs. We're lucky here to have such a great chapter of the Florida Native Plant Society. The USF Botanical Gardens hosts two big plant sales (as well as some smaller ones) in the spring and fall. The fall one is coming up in October! The Native Plant Society always has a big booth full of native plants that I think are grown mostly by their members. I've gotten many host plants from them with no problems. They even have signs in front of each plant giving basic info like the name, growing tips, and also if it's a host plant or nectar plant, or if it attracts hummingbirds. So they're well aware of people like me buying the plants for those reasons.
I've also had good luck buying organic parsley from the grocery store (Publix, down here in FL) when I ran out of food for my Black Swallowtails. I knew I was taking a chance the first time I did it, but they would have starved to death otherwise.
I think the best advice is probably to go to local nurseries vs the big box stores simply because the local growers have way more knowledge about their plants. I think everyone on this website has a story about buying plants at HD or Lowe's. The local nurseries can probably answer your questions much more accurately. I don't want to demonize the big box stores; I have made the occasional find of a host plant there and like I said, I've had to restock on milkweed before. My biggest problem is seeing the same old plants time after time, and not being able to find the natives I want to plant. Also, look to see if you have any plant societies or gardening clubs that have sales in your area. You can usually get better deals and you get people who are wildly passionate and knowledgeable about the plants.
