Sf21 (officially called IPLB-Sf21AE) is a continuous cell line developed from ovaries of the Fall Army worm, Spodoptera frugiperda, a moth species that is an agricultural pest on corn and other grass species. It was originally developed in the United States at the Henry A. Wallace Beltsville Agricultural Research Center. Sf9 is a substrain (clone) of these cells that was isolated from Sf21 by researchers at Texas A&M University.
This is just one of the over 500 insect cell lines that have been developed from more than 125 insect species.
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Vaughn, J. L.; Goodwin, R. H.; Tompkins, G. J. & McCawley, P. (1977). "The establishment of two cell lines from the insect Spodoptera frugiperda (Lepidoptera: Noctuidae)". In Vitro. 13 (4): 213–217. PMID 68913. doi:10.1007/BF02615077.