7 C.F.R. § 301.1
(a) Under section 436 of the Plant Protection Act (7 U.S.C. 7756), a State or political subdivision of a State may not impose prohibitions or restrictions upon the movement in interstate commerce of articles, means of conveyance, plants, plant products, biological control organisms, plant pests, or noxious weeds if the Secretary has issued a regulation or order to prevent the dissemination of the biological control organism, plant pest, or noxious weed within the United States. The only exceptions to this are: