Pesticide is a possibility but I have not known a retailer that sprayed the plants. They just aren't on site long enough to worry about such things. It is possible they were sprayed by the growers but for that plant I also doubt anything so long term as to affect them after you purchased and planted them was used. There are diseases of caterpillars that may have been involved. I suggest just to be sure you spray the plants off several times spraying upward from beneath plants too. I don't think that even if it was a pesticide that the product would be so persistant as to continue to do damage a weed or more later.
Good luck with your plants. I too have a butterfly garden and understand the concern. Thanks for the question. Please stop in again soon!
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